MODIFIED ENZYMES AND THEIR USE FOR PEPTIDE SYNTHESIS

Abstract: The present invention relates to modified enzymes with one or more amino acid residues from an enzyme being replaced by cysteine residues, where at least some of the cysteine residues are modified by replacing thiol hydrogen in the cysteine residue with a thiol side chain to form a modified enzyme, wherein the modified enzyme has high esterase and low amidase activity. Also, a method of producing the modified enzymes is provided. The present invention also relates to a method for using the modified enzymes in peptide synthesis.

Description: - 1 - MODIFIED ENZYMES AND THEIR USE FOR PEPTIDE SYNTHESIS CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application Serial No. 60/072,351, filed January 23, 1998, and U.S. Provisional Patent Application Serial No. 60/072,265, filed January 23, 1998, and which are hereby incorporated by reference. FIELD OF THE INVENTION The present invention relates to modified enzymes with one or more amino acid residues being replaced by cysteine residues which are modified by replacing thiol hydrogen in at least some ofthe cysteine residues with a thiol side chain to form a modified enzyme. The modified enzyme has high esterase and low amidase activity. The present invention also relates to the use of modified enzymes in peptide synthesis. BACKGROUND OF THE INVENTION Modifying enzyme properties by site-directed mutagenesis has been limited to natural amino acid replacements, although molecular biological strategies for overcoming this restriction have recently been derived (Cornish et al., Angew. Chem., Int. Ed. Engl., 34:621-633 (1995)). However, the latter procedures are difficult to apply in most laboratories. In contrast, controlled chemical modification of enzymes offers broad potential for facile and flexible modification of enzyme structure, thereby opening up extensive possibilities for controlled tailoring of enzyme specificity. Changing enzyme properties by chemical modification has been explored previously, with the first report being in 1966 by the groups of Bender (Polgar et al., J. Am. Chem. Soc. 88:3153-3154 (1966)) and Koshland (Neet et al., Proc. Natl. Acad. Sci. USA, 56:1606-1611 (1966)), who created a thiolsubtilisin by chemical transformation (CH OH -> CH 2 SH) ofthe active site serine residue of subtilisin BPN' to cysteine. Interest in chemically produced artificial enzymes, including some with synthetic potential, was renewed by Wu (Wu et al., J. Am. Chem. Soc. 111 :4514-4515 (1989); Bell - 2 - et al., Biochemistry, 32:3754-3762 (1993)) and Peterson (Peterson et al., Biochemistry, 34:6616-6620 (1995)), and, more recently, Suckling (Suckling et al., Bioorg. Med. Chem. Lett, 3:531-534 (1993)). Enzymes are now widely accepted as useful catalysts in organic synthesis. However, natural, wild-type, enzymes can never hope to accept all structures of synthetic chemical interest, nor always be transformed stereospecifically into the desired enantiomerically pure materials needed for synthesis. This potential limitation on the synthetic applicabilities of enzymes has been recognized, and some progress has been made in altering their specificities in a controlled manner using the site-directed and random mutagenesis techniques of protein engineering. However, modifying enzyme properties by protein engineering is limited to making natural amino acid replacements, and molecular biological methods devised to overcome this restriction are not readily amenable to routine application or large scale synthesis. The generation of new specificities or activities obtained by chemical modification of enzymes has intrigued chemists for many years and continues to do so. U.S. Patent No. 5,208,158 to Bech et al. ("Bech") describes chemically modified detergent enzymes where one or more methionines have been mutated into cysteines. The cysteines are subsequently modified in order to confer upon the enzyme improved stability towards oxidative agents. The claimed chemical modification is the replacement ofthe thiol hydrogen with Cι -6 alkyl. Although Bech has described altering the oxidative stability of an enzyme through mutagenesis and chemical modification, it would also be desirable to develop one or more enzymes with altered properties such as activity, nucleophile specificity, substrate specificity, stereoselectivity, thermal stability, pH activity profile, and surface binding properties for use in, for example, detergents or organic synthesis. In particular, enzymes, such as subtilisins, tailored for peptide synthesis would be desirable. Enzymes useful for peptide synthesis have high esterase and low amidase activities. Generally, subtilisins do not meet these requirements and the improvement ofthe esterase to amidase selectivities of subtilisins would be desirable. However, previous attempts to tailor enzymes for peptide synthesis by lowering amidase activity have generally resulted in dramatic decreases in both esterase and amidase activities. Previous strategies for lowering the amidase activity include the use of water-miscible organic solvents (Barbas et al., J. Am. Chem. Soc. 110:5162-5166 (1988); Wong et al, J. Am. Chem. Soc. - 3 - 112:945-953 (1990); and Sears et al., Biotechnol. Prog.. 12:423-433 (1996)) and site- directed mutagenesis (Abrahamsen et al., Biochemistry, 30:4151-4159 (1991); Bonneau et al., J. Am. Chem. Soc, 113:1026-1030 (1991); and Graycar et al., Ann. N. Y. Acad. Sci., 67:71-79 (1992)). However, while the ratios of esterase-to-amidase activities were improved by these approaches, the absolute esterase activities were lowered concomitantly. Abrahamsen et al.. Biochemistry, 30:4151-4159 (1991). Chemical modification techniques (Neet et al., Proc Nat. Acad. Sci.. 56:1606 (1966); Polgar et al., J. Am. Chem. Soc. 88:3153-3154 (1966); Wu et al., J. Am. Chem. Soc. 111 :4514-4515 (1989); and West et al., J. Am. Chem. Soc. 112:5313-5320 (1990)), which permit the incorporation of unnatural amino acid moieties, have also been applied to improve esterase to amidase selectivity of subtilisins. For example, chemical conversion ofthe catalytic triad serine (Ser221) of subtilisin to cysteine (Neet et al., Proc. Nat. Acad. Sci., 56:1606 (1966); Polgar et al., J. Am. Chem. Soc. 88:3153-3154 (1966); and Nakatsuka et al., J. Am. Chem. Soc. 109:3808-3810 (1987)) or to selenocysteine (Wu et al., J. Am. Chem. Soc. 111 :4514-4515 (1989)), and methylation of the catalytic triad histidine (His57) of chymotrypsin (West et al., J. Am. Chem. Soc. 112:5313-5320 (1990)), effected substantial improvement in esterase-to-amidase selectivities. Unfortunately however, these modifications were again accompanied by 50- to 1000-fold decreases in absolute esterase activity. The present invention is directed to overcoming these deficiencies. SUMMARY OF THE INVENTION One aspect ofthe present invention relates to modified enzymes with one or more amino acid residues from an enzyme being replaced by cysteine residues, where at least some ofthe cysteine residues are modified by replacing thiol hydrogen in the cysteine residue with a thiol side chain to form a modified enzyme, where the modified enzyme has high esterase and low amidase activity. Another aspect ofthe present invention relates to a method of producing a modified enzyme. This method involves providing an enzyme with one or more amino acids in the enzyme being replaced with cysteine residues and replacing thiol hydrogen in at least some ofthe cysteine residues with a thiol side chain to form a modified enzyme. The modified enzyme has high esterase and low amidase activity. - 4 - The present invention also relates to a method of peptide synthesis. This method includes providing a modified enzyme with one or more amino acid residues in the enzyme being replaced by cysteine residues, where at least some ofthe cysteine residues are modified by replacing thiol hydrogen in the cysteine residue with a thiol side chain, where the modified enzyme exhibits high esterase and low amidase activity. An acyl donor, an acyl acceptor, and the modified enzyme are then combined under conditions effective to form a peptide product. The modified enzymes ofthe present invention provide an alternative to site-directed mutagenesis and chemical modification for introducing unnatural amino acids into proteins. In addition, these modified enzymes more efficiently catalyze peptide synthesis as a result of an increased esterase-to-amidase ratio compared to wild-type enzymes. Further, the modified enzymes ofthe present invention can incorporate D- amino acid esters as acyl donors in peptide synthesis and α-branched amides as acyl acceptors in peptide synthesis to form a variety of dipeptides which cannot be produced with wild-type ("WT") enzymes. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows peptide coupling catalyzed by an enzyme. Figure 2 shows the chemical modification of subtilisin Bacillus lentus mutants to generate chemically modified mutant enzymes. Figure 3 shows the ratio of k^ Ku constants for esterase-to-amidase activity. Esterase and amidase activity was determined with succinyl-alanine-alanine- proline-phenylalanine-thiobenzyl ester ("suc-AAPF-SBn") and succinyl-alanine-alanine- proline-phenylalanine-pαrα-nitroanalide ("suc-AAPF-pNA") substrates, respectively. All chemically modified mutants had the structure enzyme-CH -S-R, where the structure of the various R groups investigated is shown. In the N62C family, the straight-chain alkyl group of intermediate length was hexyl (e) and in the L217C family it was pentyl (d); n. d. = not determined. For comparison, the ratio for the WT enzyme was 17. Figure 4 shows the active site of subtilisin Bacillus lentus with sucAAPF (heavy black) bound. The catalytic triad and the four active site residues investigated are shown. Residue 62 is part ofthe S 2 pocket, residue 217 is at the mouth ofthe ST (leaving - 5 - group) pocket, residue 166 is at the bottom ofthe Sj pocket, and residue 222 is between the Si and Si' pockets. Figure 5 shows the peptide ligation of L-amino acids using subtilisin Bacillus lentus modified enzymes. Figure 6 shows the peptide ligation of D-amino acids using subtilisin Bacillus lentus modified enzymes. Figure 7 shows the proposed binding ofthe Z-protecting group of Z-D- Phe-OBn with subtilisin Bacillus lentus. The large hydrophobic carbobenzoxy protecting (Z) group is binding in the Si pocket instead ofthe D-phenylalanine side chain. Figure 8 shows the chemical modification of S166C mutants of subtilisin Bacillus lentus to generate modified enzymes. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to modified enzymes with one or more amino acid residues from an enzyme being replaced by cysteine residues, where at least some of the cysteine residues are modified by replacing thiol hydrogen in the cysteine residue with a thiol side chain to form a modified enzyme. The modified enzyme has high esterase and low amidase activity. Preferably, the enzyme is a protease. More preferably, the enzyme is a Bacillus subtilisin. Subtilisins are alkaline serine proteases that are finding increasing use in biocatalysis, particularly in chiral resolution, regioselective acylation of polyfunctional compounds, peptide coupling, and glycopeptide synthesis. The latter two applications are of particular interest, because they provide an alternative to site-directed mutagenesis and chemical modification for introducing unnatural amino acids into proteins. As shown in Figure 1 , subtilisins can catalyze peptide bond formation starting from an ester substrate, by first forming an acyl enzyme intermediate which then reacts with a primary amine to form the peptide product. This application thus requires high esterase activity to promote acyl enzyme formation and then low amidase activity to minimize hydrolysis ofthe peptide bond ofthe desired product. Generally, subtilisins do not meet these requirements and the improvement ofthe esterase to amidase selectivities of subtilisins has been a long sought after goal. Also, preferably, the amino acids replaced in the enzyme by cysteines are selected from the group consisting of asparagine, leucine, methionine, or serine. More preferably, the amino acid to be replaced is located in a subsite ofthe enzyme, preferably, the Si, Si', or S subsites. Most preferably, the amino acids to be replaced are N62, L217, M222, and SI 66 where the numbered position corresponds to naturally-occurring subtilisin from Bacillus amyloliquefaciens or to equivalent amino acid residues in other subtilisins, such as Bacillus lentus subtilisin. In a particularly preferred embodiment, the enzyme is a Bacillus lentus subtilisin. In another particularly preferred embodiment, the amino acid to be replaced by cysteine is N62, L217, SI 66, or M222 and the thiol side chain group is selected from the group consisting of: -SCH 3 ; -SCH 2 CH 3 ; -SCH 2 CH(CH 3 ) 2 ; -S(CH 2 ) 4 CH 3 ; -S(CH 2 ) 5 CH 3 ; -S(CH 2 ) 9 CH 3 ; -SCH C 6 H 5 ; -SCH 2 CH 2 NH 3 + ; and -SCH 2 CH 2 SO 3 " ; or the amino acid to be replaced by cysteine is SI 66 or M222 and the thiol side chain group is selected from the group consisting of: -SCH 2 C 6 H ; -SCH 2 (p-COOH-C 6 H 4 ); -SCH 2 C 6 F 5 ; and -SCH 2 CH 2 NH 3 + . Preferably, the modified enzymes ofthe present invention have an esterase activity of from about 3.5 s "1 mM "1 to about 1110000 s "1 mM "1 and an amidase activity of from about 0.056 s "1 mM "1 to about 35500 s "1 mM "1 . Most preferably, the modified enzymes ofthe present invention have an esterase activity from about 350 s "1 mM "1 to about 11100 s "1 mM "1 and an amidase activity of from about 5.6 s "1 mM "1 to about - 7 - A "modified enzyme" is an enzyme that has been changed by replacing an amino acid residue such as asparagine, serine, methionine, or leucine with a cysteine residue and then replacing the thiol hydrogen of at least some ofthe cysteine with a thiol side chain (e.g., -SCH 3 , -SCH 2 CH 3 , -SCH 2 CH(CH 3 ) 2 , -S(CH 2 ) 4 CH 3 , -S(CH 2 ) 5 CH 3 , - S(CH 2 ) 9 CH 3 , -SCH 2 C 6 H 5 , -SCH 2 CH 2 NH 3 + , -SCH 2 CH 2 SO 3 " , -SCH 2 (p-COOH-C 6 H 4 ), and -SCH 2 C 6 F 5 ). After modification, the properties ofthe enzyme, i.e., activity or substrate specificity, may be altered. Preferably, the activity ofthe enzyme is increased. The term "enzyme" includes proteins that are capable of catalyzing chemical changes in other substances without being changed themselves. The enzymes can be wild-type enzymes or variant enzymes. Enzymes within the scope of the present invention include pullulanases, proteases, cellulases, amylases, isomerases, lipases, oxidases, and reductases. The enzyme can be a wild-type or mutant protease. Wild-type proteases can be isolated from, for example, Bacillus lentus or Bacillus amyloliquefaciens (also referred to as BPN'). Mutant proteases can be made according to the teachings of, for example, PCT Publication Nos. WO 95/10615 and WO 91/06637, which are hereby incoφorated by reference. Several types of moieties can be used to replace the thiol hydrogen of the cysteine residue. These include -SCH 3 , -SCH 2 CH 3 , -SCH 2 CH(CH 3 ) 2 , -S(CH 2 ) 4 CH 3 , -S(CH 2 ) 5 CH 3 , -S(CH 2 ) 9 CH 3 ;-SCH 2 C 6 H 5 , -SCH 2 CH 2 NH 3 + , -SCH 2 CH 2 SO 3 " , -SCU 2 (p- COOH-C 6 H 4 ), and -SCH 2 C 6 F 5 . The terms "thiol side chain group," "thiol containing group," and "thiol side chain" are terms which are can be used interchangeably and include groups that are used to replace the thiol hydrogen of a cysteine used to replace one ofthe amino acids in an enzyme. Commonly, the thiol side chain group includes a sulfur through which the thiol side chain groups defined above are attached to the thiol sulfur ofthe cysteine. The binding site of an enzyme consists of a series of subsites across the surface ofthe enzyme. The substrate residues that correspond to the subsites are labeled P and the subsites are labeled S. By convention, the subsites are labeled Si, S 2 , S 3 , S 4 , Si', and S 2 '. A discussion of subsites can be found in Siezen et al., Protein Engineering. 4:719-737 (1991) and Fersht, Enzyme Structure and Mechanism. 2 ed., Freeman: New York, 29-30 (1985), which are hereby incorporated by reference. The preferred subsites are Si, Si', and S 2 . - 8 - Another aspect ofthe present invention relates to a method of producing a modified enzyme. This method involves providing an enzyme with one or more amino acids in the enzyme being replaced with cysteine residues and replacing thiol hydrogen in at least some ofthe cysteine residues with a thiol side chain to form a modified enzyme. The modified enzyme has high esterase and low amidase activity. The amino acid residues ofthe present invention can be replaced with cysteine residues using site-directed mutagenesis methods or other methods well known in the art. See, for example, PCT Publication No. WO 95/10615, which is hereby incorporated by reference. One method of modifying the thiol hydrogen ofthe cysteine residue is set forth in the Examples. The present invention also relates to a method of peptide synthesis. This method includes providing a modified enzyme with one or more amino acid residues in the enzyme being replaced by cysteine residues, where at least some ofthe cysteine residues are modified by replacing thiol hydrogen in the cysteine residue with a thiol side chain, where the modified enzyme exhibits high esterase and low amidase activity. An acyl donor, an acyl acceptor, and the modified enzyme are combined under conditions effective to form a peptide product. Enzymatic peptide coupling is an attractive method for preparation of a variety of peptides, because this method requires minimal protection ofthe substrate, proceeds under mild conditions, and does not cause racemization. Wong et al., Enzymes in Synthetic Organic Chemistry, Pergamon Press: Oxford, 41-130 (1994), which is hereby incorporated by reference. In spite of these advantages, two major problems have limited the use of serine proteases in peptide synthesis. One is their efficient proteolytic (amidase) activity which causes hydrolysis ofthe coupling product, and the other is their stringent structural specificity and stereospecificity. The modified enzymes ofthe present invention have altered esterase-to- amidase activity as compared to the precursor enzyme. Increasing the esterase-to- amidase ratio enables the use ofthe enzyme to more efficiently catalyze peptide synthesis. In particular, subtilisins can catalyze peptide bond formation starting from an ester substrate (i.e. an acyl donor), by first forming an acyl enzyme intermediate which then reacts with a primary amine (i.e. an acyl acceptor) to form the peptide product, as shown in Figure 1. This reaction thus requires high esterase activity to promote acyl enzyme formation and, then, low amidase activity to minimize hydrolysis ofthe peptide - 9 - bond ofthe desired product. Modified enzymes ofthe present invention show an increased esterase-to-amidase ratio, without reducing the absolute esterase activity ofthe enzyme. In addition, certain modified enzymes ofthe present invention even show a concomitant increase in the absolute esterase activity. Further, the modified enzymes ofthe present invention present a significant enlargement ofthe applicability of chemically modified mutants of subtilisin Bacillus lentus in peptide synthesis. The chemically modified mutant enzymes ofthe present invention can incorporate D-amino acid esters as acyl donors in peptide synthesis or an α-branched amino acid amide as acyl acceptor in peptide synthesis to give a variety of dipeptides. These reactions are not possible with subtilisin Bacillus lentus- ύά type (WT). Therefore, the modified enzymes ofthe present invention can be used in organic synthesis to, for example, catalyze a desired reaction and/or favor a certain stereoselectivity. See e.g., Noritomi et al. Biotech. Bioeng. 51 :95-99 (1996); Dabulis et al. Biotech. Bioeng. 41 :566-571 (1993), and Fitzpatrick et al. J. Am. Chem. So 113:3166-3171 (1991), which are hereby incorporated by reference. The modified enzymes ofthe present invention can be formulated into known powdered and liquid detergents having a pH between 6.5 and 12.0 at levels of about 0.01 to about 5% (preferably 0.1% to 0.5%) by weight. These detergent cleaning compositions or additives can also include other enzymes, such as known proteases, amylases, cellulases, Upases, or endoglycosidases, as well as builders and stabilizers. The modified enzymes ofthe present invention, especially subtilisins, are useful in formulating various detergent compositions. A number of known compounds are suitable surfactants useful in compositions comprising the modified enzymes ofthe present invention. These include nonionic, anionic, cationic, anionic, or zwitterionic detergents, as disclosed in U.S. Patent No. 4,404,128 to Anderson and U.S. Patent No. 4,261,868 to Flora et al., which are hereby incorporated by reference. A suitable detergent formulation is that described in Example 7 of U.S. Patent No. 5,204,015 to Caldwell et al., which is hereby incorporated by reference. The art is familiar with the different formulations which can be used as cleaning compositions. In addition to typical cleaning compositions, it is readily understood that the modified enzymes ofthe present invention may be used for any purpose that native or wild-type enzymes are used. Thus, these modified enzymes can be used, for example, in bar or liquid soap applications, - 10 - dishcare formulations, contact lens cleaning solutions or products, peptide synthesis, feed applications such as feed additives or preparation of feed additives, waste treatment, textile applications such as the treatment of fabrics, and as fusion-cleavage enzymes in protein production. The modified enzymes ofthe present invention may achieve improved wash performance in a detergent composition (as compared to the precursor). As used herein, improved wash performance in a detergent is defined as increasing cleaning of certain enzyme-sensitive stains such as grass or blood, as determined by light reflectance evaluation after a standard wash cycle. The addition ofthe modified enzymes ofthe present invention to conventional cleaning compositions does not create any special use limitation. In other words, any temperature and pH suitable for the detergent is also suitable for the present compositions as long as the pH is within the above range and the temperature is below the described modified enzyme's denaturing temperature. In addition, modified enzymes in accordance with the invention can be used in a cleaning composition without detergents, again either alone or in combination with builders and stabilizers. In another aspect ofthe present invention, the modified enzymes are used in the preparation of an animal feed, for example, a cereal-based feed. The cereal can be at least one of wheat, barley, maize, sorghum, rye, oats, triticale, and rice. Although the cereal component of a cereal-based feed constitutes a source of protein, it is usually necessary to include sources of supplementary protein in the feed such as those derived from fish-meal, meat-meat, or vegetables. Sources of vegetable proteins include at least one of full fat soybeans, rapeseeds, canola, soybean-meal, rapeseed-meal, and canola- meal. The inclusion of a modified enzyme ofthe present invention in an animal feed can enable the crude protein value and/or digestibility and/or amino acid content and/or digestibility coefficients ofthe feed to be increased, which permits a reduction in the amounts of alternative protein sources and/or amino acids supplements which had previously been necessary ingredients of animal feeds. The feed provided by the present invention may also include other enzyme supplements such as one or more of β-glucanase, glucoamylase, mannanase, α-galactosidase, phytase, lipase, α-arabinofuranosidase, xylanase, α-amylase, esterase, oxidase, oxido-reductase, and pectinase. It is particularly preferred to include a xylanase as a further enzyme supplement such as a subtilisin derived from the genus Bacillus. - 11 - Such xylanases are, for example, described in detail in PCT Patent Application No. WO 97/20920, which is hereby incoφorated by reference. Another aspect ofthe present invention is a method for treating a textile. The method includes providing a modified enzyme with one or more amino acid residues from an enzyme being replaced by cysteine residues, wherein the cysteine residues are modified by replacing thiol hydrogen in at least some ofthe cysteine residues with a thiol side chain to form a modified enzyme, where the modified enzyme has high esterase and low amidase activity. The modified enzyme is contacted with a textile under conditions effective to produce a textile resistance to certain enzyme-sensitive stains. Such enzyme- sensitive stains include grass and blood. Preferably, the textile includes a mutant enzyme. The method can be used to treat, for example, silk or wool as described in publications such as Research Disclosure 216,034, European Patent Application No. 134,267, U.S. Patent No. 4,533,359, and European Patent Application No. 344,259, which are hereby incoφorated by reference. EXAMPLES Example 1 - Producing the Cys-Mutants The gene for subtilisin from Bacillus lentus ("SBL") was cloned into the bacteriophage M13mpl9 vector for mutagenesis (U.S. Patent No. 5,185,258, which is hereby incoφorated by reference). Oligonucleotide-directed mutagenesis was performed as described in Zoller et al., Methods Enzymol., 100:468-500 (1983), which is hereby incoφorated by reference. The mutated sequence was cloned, excised, and reintroduced into the expression plasmid GG274 in the B. subtilis host. PEG (50%) was added as a stabilizer. The crude protein concentrate obtained was purified by first passing through a Sephadex™ G-25 desalting matrix with a pH 5.2 buffer (20 mM sodium acetate, 5 mM CaCl 2 ) to remove small molecular weight contaminants. Pooled fractions for the desalting column were then applied to a strong cation exchange column (SP Sepharose™ FF) in the sodium acetate buffer (above), and SBL was eluted with a one step gradient of 0-200 mM NaCl acetate buffer, pH 5.2. Salt-free enzyme powder was obtained following dialysis ofthe eluent against Millipore purified water, and subsequent lyophilization. The purity ofthe mutant and wild-type enzymes, which had been denatured by incubation - 12 - with 0.1 M HC1 at 0°C for 30 minutes, was ascertained by SDS-PAGE on homogeneous gels using the Phast™ System from Pharmacia (Uppsala, Sweden). The concentration of SBL was determined using the Bio-Rad (Hercules, CA) dye reagent kit which is based on the method of Bradford, Analytical Biochemistry. 72:248-254 (1976), which is hereby incoφorated by reference. Specific activity ofthe enzymes was determined in pH 8.6 buffer using the method described in Example 3 below. Example 2 - Preparation of Certain Moieties Preparation of 2, 3, 4, 5, 6-pentafluorobenzyl methanethiosulfonate 2, 3, 4, 5, 6-pentafluorobenzyl methanethiosulfonate was prepared according to the general procedure in Examples 1 and 3 from -bromo-2,3,4,5,6-pentafluorotoluene in 88% yield, m.p.: 64.2 - 64.7°C (95% EtOH); IR (KBr): 3030, 3009, 2961, 2930, 2920, 1514, 1314, 1132, 980, 880, and 748 cm "1 ; 1H NMR (200 MHz, CDC1 3 ): δ 4.46 (br s, 2H, SCH 2 ), 3.36 (s, 3H, CH 3 SO 2 ); MS (El): 292 (M + ), 212 ( + S=CHC 6 F 5 ); HRMS (El): 291.9648 (M + , calc'd for C 8 H 5 F 5 O 2 S 2 : 291.9651). Preparation of 4-carboxybenzyl methanethiosulfonate -C0 2 H 4-carboxybenzyl methanethiosulfonate was prepared according to the general procedure in Examples 1 and 3 from α-bromo-p-toluic acid in 60% yield after recrystallization from 95% EtOH. m.p.: 187.6 - 187.8°C; IR (KBr): 3300-2200, 1683, - 13 - 1608, 1577, 1422, 1301, 1180, 1121, 957, 863, 750, 716, and 551 cm "1 ; Η NMR (200 MHz, DMSO-d 6 and 10% D 2 O): δ 7.90 (d, J = 8.0 Hz, 2H, aromatic), 7.51 (d, J = 8.0 Hz, 2H, aromatic), 4.47 (s, 2H, SCH 2 ), 3.23 (3H, CH 3 SO 2 S); 13 C NMR (50 MHz, DMSO-d 6 and 10% D 2 O): δ 167.87, 141.55, 130.56, 130.45, 130.07, 51.01, 39.37; MS (El): 246 (M + ), 229 (M + )-OH), 166 (base peak, + S=CH-Ar); HRMS (El): 246.0031 (M + , calc'd. for C 9 HιoO 4 S 2 : 246.0021). Example 3 - Peptide Synthesis Using Modified Enzymes with Alkyl Moieties Materials Succinyl-alanine-alanine-proline-phenylalanine-pαrα-nitroanalide ("suc- AAPF-pNA") and succinyl-alanine-alanine-proline-phenylalanine-thiobenzyl ester ("suc- AAPF-SBn") were both from Bachem Inc. (Torrance, CA), Ellman's reagent (5,5'- dithiobis-2,2'-nitrobenzoic acid, DTNB) and phenylmethanesulfonyl fluoride ("PMSF") were from Sigma- Aldrich Inc. (Milwaukee, WI). Sources and syntheses of methanethiosulfonate ("MTS") reagents were as described in Berglund et al., J. Am. Chem. Soc. 119:5265-5266 (1997), which is hereby incoφorated by reference. Buffers, 2-[N-cyclohexylamino]ethanesulfonic acid (CHES), 4- moφholineethanesulfonic acid (MES) and tris hydroxymethylaminomethane (Tris) were from Sigma- Aldrich Inc. (Milwaukee, WI). Wild type SBL and cysteine mutants N62C, S 166C, L217C, and M222C were provided by Genencor International Inc., Rochester, New York and purified as described in Example 1 and Stabile et al., Bioorg. Med. Chem. Lett.. 6:2501-2506 (1996), which is hereby incoφorated by reference. Chemical modification Chemical modification with alkyl MTS reagents was carried out as described in Berglund et al., J. Am. Chem. Soc. 119:5265-5266 (1997) and DeSantis et al., Biochemistry. 37:5968-5973 (1998), which are hereby incoφorated by reference. Briefly, 200 μL of a 1 M solution of MTS reagent in a suitable solvent was added to a solution (5-10 mg/mL, 3.5 mL) ofthe cysteine mutant in 70 mM CHES, 5 mM MES, 2 mM CaCl 2 pH 9.5. The MTS reagent was added in two portions over 30 minutes. Reaction mixtures were kept at 20 °C with continuous end-over-end mixing. Reactions - 14 - were monitored by following the specific activity with suc-AAPF-pNA and by tests for residual free thiol with Ellman's reagent. Once the reaction was complete, the reaction mixture was loaded on a Sephadex PD-10 G25 column with 5 mM MES and 2 mM CaCl 2 , pH 6.5. The protein fraction was dialyzed against 1 mM CaCl 2 , and the dialysate was lyophilized. Characterization of Modified Enzymes The molecular mass of each modified enzyme ("ME") was determined by electrospray ionization mass spectrometry (Berglund et al., J. Am. Chem. Soc. 119:5265- 5266 (1997); DeSantis et al., Biochemistry. 37:5968-5973 (1998), which are hereby incoφorated by reference). The purity ofthe MEs was ascertained by native PAGE on 8- 25% gels using the Phast system from Pharmacia (Uppsala, Sweden). The extent of chemical modification ofthe cysteine mutants was determined by thiol titration with DTNB for the 62, 217 and 166 mutants and with I 2 (Cunningham et al., J. Biol. Chem.. 234:1447-1451 (1959), which is hereby incoφorated by reference) for the more sterically hindered 222 mutants which do not react with DTNB. Active site titrations were performed on all enzymes by monitoring the burst of fluoride released upon addition of phenylmethanesulfonyl fluoride to the enzyme, as described in Hsia et al., J. Anal. Biochem.. 242:221-227 (1996), which is hereby incoφorated by reference. Rapid screen on microtiter plates Detailed procedures and validation of this assay have been described in Plettner et al., Bioorg. Med. Chem. Lett.. 8:2291-2296 (1998), which is hereby incoφorated by reference. Briefly, enzyme solutions were prepared in 5 mM MES with 2 mM CaCl 2 , pH 6.5 at about 10 "7 M for amidase and 10 "8 M for esterase. Substrate solutions in DMSO were 1.6 mM (amidase) and 1.0 mM (esterase). The assay was performed at pH 8.6 in the same buffer used for kinetics (see below). Enzyme solutions were arranged on a microtiter plate (loading plate) along columns, with the last well in each column as a buffer blank. On a separate plate (assay plate), 10 μL of substrate and 180 μL of buffer was added to each well. Reactions were initiated by transferring 10 μL of enzyme from an appropriate column on the loading plate to the assay plate. Reactions were monitored on a Multiscan MCC 340 96-well reader programmed in the kinetic mode at 414 nm, with no time lag, at 5 second intervals for a total time of 1 minute (amidase) - 15 - and 30 seconds (esterase). Background hydrolysis was subtracted automatically. The k c JKu was estimated from the rate of substrate hydrolysis (v) using the low-substrate approximation: v « k c JK M [E][S] where, [S] « K M - Kinetics Assays were done in 0.1 M Tris pH 8.6 containing 0.005 % Tween. Substrate solutions were prepared in DMSO. In the esterase assay, substrate solutions also contained 0.0375 M DTNB (Bonneau et al, J. Am. Chem. Soc. 113:1026-1030 (1991), which is hereby incoφorated by reference). Concentrations of substrate stock solutions ranged from 0.013 to 0.3 M for amidase and 0.0015 to 0.3 M for esterase, and 9- 10 different concentrations were tested in duplicate for each enzyme. Enzyme solutions were prepared in 20 mM MES, 1 mM CaCl 2 , pH 5.8, at a concentration of 10 "6 M for amidase and 10 " M for esterase. Reactions were monitored spectrophotometrically on a Perkin Elmer Lambda 2 instrument equipped with a thermostatted cell compartment. Prior to an assay, 980 μL of Tris buffer in a cuvette was equilibrated to 25 °C. Substrate stock solution (10 μL) was added to the buffer and the reading set to zero. Reactions were initiated by addition of 10 μL of enzyme solution and were monitored at 410 nm (amidase) and 412 nm (esterase). Extinction coefficients for the chromophores were 8800 M "1 cm "1 for p-nitroaniline (Bonneau et al., J. Am. Chem. Soc, 113:1026-1030 (1991), which is hereby incoφorated by reference) and 13470 M "1 cm "1 for 3-carboxylate- 4-nitrothiophenolate in 0.1 M Tris pH 8.6 with 0.005% Tween. Initial rates were obtained by linear fitting up to 5% conversion; r values exceeded 0.9996. In the case of esterase, rates in the presence of enzyme were corrected for uncatalyzed background hydrolysis ofthe thiobenzyl ester. Kinetic constants were obtained by fitting the rate data to the Michaelis-Menten equation using Grafit. ® (Erithacus Software Ltd., Staines, Middlesex, United Kingdom) - 16 - Reaction ofthe cysteine mutants with DTNB Since [DTNB] » [enzyme] and [DTNB] « constant over 30 seconds (time for 5% conversion), the pseudo-first order rate constant for the reaction of N62C, L217C and S 166C mutants with DTNB was determined under the same conditions as used in the assay, using enzyme concentrations from 10 "6 to 10 "4 M. The pseudo first-order rates constant of reaction of N62C, L217C, and S166C with DTNB under the esterase assay conditions were 1.8 x 10 " s " (0.5 % = maximum amount of cysteine mutant reacted with DTNB over the time of the esterase assay), 1.4 x 10 "3 s "1 (4.2 % reacted), and 1.4 x 10 "4 s "1 (0.4 % reacted), respectively. The M222C mutant did not detectably react with DTNB. Results Each ofthe N62C, L217C, S166C, and M222C mutants of SBL were prepared and purified, and the introduced -CH SH side-chain specifically and quantitatively chemically modified with the MTS reagents with alkyl moieties -SCH 3 , -SCH 2 CH 3 , -SCH 2 CH(CH 3 ) 2 , -S(CH 2 ) 4 CH 3 , -S(CH 2 ) 5 CH 3 , -S(CH 2 ) 9 CH 3 , -SCH 2 C 6 H 5 , - SCH 2 CH 2 NH 3 + , -SCH 2 CH 2 SO 3 " (-a-i), as described previously (Berglund et al., Bioorg. Med. Chem. Lett.. 6:2507-2512 (1996); Berglund et al., J. Am. Chem. Soc. 119:5265- 5266 (1997); DeSantis et al., Biochemistry. 37:5968-5973 (1998); and DeSantis et al., I Am. Chem. Soc, 120:8582-8586 (1998), which are hereby incoφorated by reference). The purities ofthe MEs generated were established by native polyacrylamide gel electrophoresis (PAGE), which showed only one band in each case, thereby demonstrating that the MEs were pure and that dimerization had not occurred. Mass analyses ofthe MEs by electrospray mass spectrometry were consistent (± 6 Da) with the calculated masses for single-site modifications. Berglund et al., J. Am. Chem. Soc, 119:5265-5266 (1997) and DeSantis et al., Biochemistry. 37:5968-5973 (1998). Titration ofthe N62C, S166C, and L217C MEs with Ellman's reagent showed a residual thiol content of less than 2 % in all cases, confirming that the MTS reactions were virtually quantitative. Ellman et al., Biochem. Pharmacol.. 7:88-95 (1961), which is hereby incoφorated by reference. The residual free thiol content for the more sterically hindered M222C MEs, which did not react with Ellman's reagent, was determined with I 2 (Cunningham et al., J. Biol. Chem.. 234:1447-1451 (1959)). The M222C MEs contained - 17 - < 2 % free thiol, except for M222C-SCH 2 CH 2 SO 3 " (-i) which contained 3 % residual thiol groups. The concentration of active enzyme was determined by active site titration with phenylmethanesulfonyl fluoride (PMSF). Hsia et al., J. Anal. Biochem., 242:221-227 (1996). All ofthe MEs were 60-80 % active by weight, except for M222C- SCH 2 CH 2 SO 3 ~ (-i) which contained only 4 % active enzyme and was, therefore, not investigated further. Initially, a rapid screen on microtiter plates (Plettner et al., Bioorg. Med. Chem. Lett.. 8:2291-2296 (1998), which is hereby incoφorated by reference) was used to generate estimates of k 0 JKu for amidase and esterase for the enzymes outlined in Figure 2. Of 36 MEs and four cysteine mutants screened, 25 enzymes were chosen for further kinetic analyses. These included all the promising esterases, as well as a few mutants with severely damaged esterase activity for comparison. The results ofthe kinetic analyses with suc-AAPF-pNA and suc-AAPF-SBn as standard amide and ester substrates respectively, are presented in Table 1 , below. It was recognized that the cysteine thiol of the unmodified cysteine mutants N62C, L217C, S 166C, and M222C could react with DTNB, which is used in the kinetic assay to detect the thiol benzyl hydrolysis product of the esterase reaction. This possibility was discounted by studying the rates of reaction of DTNB with N62C, S166C, L217C, and β-mercaptoethanol as a model for a non-hindered thiol which established that these did not react at a rate sufficient to interfere with the assay at the concentrations used. 18- Table 1: Kinetic constants of chemically modified mutants for amidase and esterase activities Amidase" Esterase" Enzyme cat (S ) K M (mM) c Kca ^M "'at ( s ) K M (mM) c ^ca ^M (s'mM 1 ) (s ' M ') WT 153 ± 4 073 ± 005 209 ± 15 1940 ± 180 054 ± 007 3560 ±540" N62C 163 ±8 19 ±02 86 ± 10 2370 ± 90 054 ± 006 4380 ±510 N62C-S-a 73 ±2 055 ± 004 133 ± 10 3130±90 031 ±003 10100 ±1000 N62C-S-b 97 ±2 055 ± 004 177±13 2220 ±110 02 ±004 11100 ±2300 N62C-S-C 139±4 075 ± 006 185±16 2180 ±80 025 ± 004 8700 ±1430 N62C-S-e 146 ±7 063 ± 008 230 ±30 2330 ± 150 026 ± 006 8970 ±2150 N62C-S-f 124 ±4 036 ± 004 344 ± 40 1000 ± 47 039 ±006 2570 ±410 N62C-S-g 121 ±3 034 ± 003 355 ±33 1840 ± 110 029 ± 006 6330 ±1360 N62C-S-h 96 ±5 10 ±01 98 ± 11 2660 ± 80 048 ± 004 5540 ± 490 N62C-S-i 111±4 093 ± 007 120± 10 3190± 110 061 ±006 5230 ± 540 L217C 38 ± 1 080 ± 004 48 ±3 3160 ±120 057 ± 006 5540 ± 620 L217C-S-a 47 ±2 062 ± 007 76 ±9 2520 ± 120 056 ± 007 4500 ± 600 L217C-S-C 93 ±2 061 ±003 152 ±8 2450 ± 70 031 ±003 7900 ± 800 L217C-S-d 87 ±3 052 ±005 167± 17 2280 ± 80 039 ± 004 5840 ± 640 L217C-S-f 120±3 054 ± 003 223 ± 13 1840 ±100 050 ± 008 3690 ± 620 L217C-S-h 36 ±1 064 ± 006 56±6 3070 ± 90 041 ±004 7490 ± 760 L217C-S-i 83 ±6 18 ±02 47 ±6 5060 ±130 10 ±01 5060 ± 520 S166C 42 ± 1 050 ± 005 84 ±9 600 ±70 17 ±04 350 ± 90 S166C-S-a 46 ±2 034 ±005 135 ±20 2320 ±50 038 ±003 6100 ±500 S166C-S-g 23 ±05 12 ±01 20 ± 1 1530± 110 031 ±008 4900 ±1300 S166C-S-h 50 ±1 068 ± 004 74 ±5 1350 ±50 061 ±007 2200 ± 270 S166C-S-i 25 ±1 13 ±01 19±1 1950 ±90 19 ±02 1030 ±120 M222C 61 ±2 081 ±007 75 ±6 3080 ± 140 058 ± 007 5300 ± 680 M222C-S-a 56 ±2 091 ±007 62 ±6 2090 ± 120 13 ±02 1610 ±270 M222C-S-h 50±02 091 ±008 56±09 1970 ±140 04 ±01 4920 ±1280 (a) substrate suc-AAPF-pNA, (b) substrate sucAAPF-SBn, (c) determined by the method of initial rates, (d) mean standard three (esterase) experiments The broad applicability ofthe chemical modification approach for achieving the goal of improved esterase-to-amidase selectivity without reducing absolute esterase activity is evident from the Table 1 data since of 25 MEs and cysteine mutants evaluated, fully 19 displayed improved esterase to amidase selectivity. Furthermore, 20 displayed esterase activity that was higher than WT (See Figure 3). Ofthe N62 MEs, all except N62C-S(CH 2 ) 9 CH 3 (-f) exhibited improved esterase activity relative to WT. Even the N62 mutation to cysteine itself created a better esterase and poorer amidase than WT. Chemical modification of N62C enhanced the absolute esterase activity still further, to «3-fold greater than WT for N62C-S-CH 3 , (-a) and N62C-SCH 2 CH 3 , (-b). In fact, N62C-SCH 2 CH 3 , (-b) with its k c JK M of 11100 ± 2300 s "1 mM "1 had the highest absolute esterase activity of all the MEs investigated. However, the larger R groups of N62C-SCH 2 CH(CH 3 ) 2 (-c) to N62C-SCH 2 C 6 H 5 (-g) - 19 - caused decreases in k cat and k^ Ku for esterase catalysis, and steady increases in both k cat and k c JKu for amidase. Consequently, the ratio of k c JKu for esterase to amidase activity decreased 10-fold as the chain length of -R increased from N62C-S-CH 3 (-a) to N62C- S(CH 2 ) 9 CH 3 (-f) (Figure 3). The positively and negatively charged MEs, N62C- SCH 2 CH 2 NH 3 + (-h) and N62C-SCH 2 CH 2 SO 3 " (-i) respectively, both exhibited higher esterase and lower amidase activity than WT, with the improvement in the esterase-to- amidase ratio being «3-fold regardless ofthe sign ofthe charge introduced. In addition, the larger R groups of N62C-SCH 2 CH(CH 3 ) 2 to N62C-SCH 2 C 6 H 5 (-c to -g) elicited reduced K^ for both ester and amide substrates. This demonstrates that hydrophobic interactions at the 62 site are beneficial to binding. All ofthe L217C CMMs generated also exhibited improved esterase k c JKyβ compared to WT. At this site, mutation to cysteine alone again generated a superior catalyst having 1.5 -fold better esterase and 4-fold poorer amidase activity than WT. However, its modification to L217C-S-CH 3 (-a) caused a decrease in both esterase fc cat and k cat /K M compared to L217C itself. L217C-SCH 2 CH(CH 3 ) 2 (-c) was the most active 217 esterase and exhibited a k c JK M of 7900 ± 800 s "1 mM "1 . While all ofthe 217 MEs exhibited greater than WT esterase activity, further increases in the chain length of - R from -S(CH 2 ) 4 CH 3 to -S(CH ) CH 3 , (-d to -f) caused further decreases in k cat and k c Ku- This is in contrast to the trend observed for amidase k cat and k c Ku, values for the same MEs. Berglund et al., J. Am. Chem. Soc. 119:5265-5266 (1997), which is hereby incoφorated by reference. As a result, all ofthe L217C MEs except L217C- S(CH 2 ) CH 3 (-f) had higher than WT esterase to amidase selectivity (Figure 3). The positively charged L217C-SCH 2 CH 2 NH 3 + (-h) and negatively charged L217C- SCH CH 2 SO 3 ~ (-i) MEs also displayed higher than WT esterase activities, with L217C- SCH 2 CH 2 SO 3 ~ (-i) having a 2.6-fold higher than WT esterase A: cat . Furthermore, at 5060 s "1 , this was the highest esterase & C at of all the MEs studied. The L217C- SCH 2 CH 2 NH 3 + (-h) had a (h KM^si hJKfT)^^ ratio of 134, compared to 17 for WT. The correlations between decreased esterase and increased amidase activities with increasing chain length of -R, and improved esterase and decreased amidase for charged modifications, paralleled each other for both the L217C and N62C MEs. These equivalent trends are consistent with residues 217 and 62 being equidistant from His64 of the catalytic triad (See Figure 4). - 20 - Modification ofthe S166C residue ofthe Si pocket, which is quite remote from the catalytic triad and from the Si' leaving group site of both the ester or amide substrates, exerted large effects on esterase-to-amidase selectivity. The S166C mutant itself, with a k ca _/K M of 350 s "1 mM "1 , had the lowest esterase activity of all the MEs evaluated. However, it also had somewhat decreased amidase activity, giving an esterase- to-amidase selectivity ratio of four, compared to 17 for WT. Apart from having the lowest ; ca t for esterase, S166C had a significantly higher K M for esterase than the WT and was one of few mutants for which K M (esterase) > K M (amidase). In contrast, modification of S166C to generate S166C-S-CH 3 (-a) increased esterase-to-amidase selectivity to 45, a «3-fold improvement relative to WT. The large hydrophobic benzyl group of S166C-S-CH 2 C 6 H 5 (-g) increased esterase-to-amidase selectivity still further to 245, which was 14-fold higher than WT, while the charged hydrophilic groups of S166C- SCH 2 CH 2 NH 3 + (-h) and S166C-SCH 2 CH 2 SO 3 ~ (i) induced little improvement in the esterase-to-amidase ratio. That the esterase K M decreased, while the amidase K M increased significantly, relative to WT for the S166C-S-CH C 6 H 5 (-g) ME, implied long- range interactions between its Si and Si' pockets and different rate-determining steps. These results complement those previously observed for the more hydrophilic G166N and G166S mutants of subtilisin BPN', both of which effected improved esterase and esterase-to-amidase activity relative to WT. Bonneau et al., J. Am. Chem. Soc, 113:1026- 1030 ( 1991 ), which is hereby incoφorated by reference. At the Met222 site, both M222C-SCH 2 CH 2 NH 3 + (-h) and M222C exhibited an improved esterase k c K M of up to 1.5, while all of M222C-S-CH (-a), M222C-SCH 2 CH 2 NH 3 + (-h), and M222C displayed up to 37-fold reduced amidase activity. The esterase-to-amidase activity ofthe cysteine parent, M222C, with its 4-fold improvement, was itself significantly higher than WT. The M222C mutant has a Si' leaving group site that is less sterically congested than WT. This may enhance the rate of acyl-enzyme hydrolysis, which is often the rate-determining step for ester substrates. M222C-S-CH 3 (-a), which differs from WT only in the replacement of one ofthe methionine side-chain methylenes (CH 2 ) by sulfur, had the same k cat as WT, but an increased K M - At this site, the most improved ME was M222C-SCH 2 CH 2 NH 3 + (-h), which exhibited an esterase-to-amidase selectivity of 879, compared to 17 for the WT. This 52-fold improvement in esterase-to-amidase ratio ofthe series arose largely from a - 21 - 31 -fold lowered amidase k caX , but with the WT level of esterase k cat being retained. This result was consistent with the observation that the M222K mutant of subtilisin BPN' caused improved esterase activity and severely decreased amidase activity, thus, generating an enzyme with greatly improved esterase-to-amidase specificity. Graycar et al., Ann. N.Y. Acad. Sci., 672:71-79 (1992), which is hereby incoφorated by reference. With 19 of 25 MEs evaluated achieving the goal of better-than-WT esterase-to-amidase selectivity without diminishing the absolute esterase-rate, the ME approach was clearly broadly applicable. Overall, esterase-to-amidase specificity varied from 4-fold lower than WT for S166C to 52-fold higher than WT for M222C- SCH 2 CH 2 NH 3 + . At least one member from each ofthe four families of mutants studied met both criteria of excellent esterase activity and high esterase-to-amidase selectivity, with: N62C-SCH 3 , (-a) being 3-fold, L217C-SCH 2 CH 2 NH 3 + (-h) and L217C- SCH 2 CH 2 SO 3 " (-i) 6 to 8-fold, S166C-SCH 2 C 6 H 5 (-g) was 14-fold, and M222C- SCH CH 2 NH 3 + (-h) was 52-fold improved in terms of esterase-to-amidase ratio relative to the WT enzyme. With up to 880-fold esterase to amidase selectivity achievable by the ME approach, the potential of chemically modified mutant subtilisins for peptide synthesis was expanded considerably. Example 4 - Peptide Synthesis using S166C-SCH 2 C 6 H 5 , S166C-SCH 2 (p-COOH- C 6 H 4 ), S166C-SCH 2 C 6 F 5 , S166C-SCH 2 CH 2 NH 3 + , and M222C- SCH 2 CH 2 NH 3 + . General Methods. WT-subtilisin Bacillus lentus and mutant enzymes, S166C and M222C were purified (Stabile et al., Bioorg. Med. Chem. Lett.. 6:2501-2506 (1996); Berglund et al., Bioorg. Med. Chem. Lett.. 6:2507-2512 (1996); and DeSantis et al., Biochemistry. 37:5698-5973 (1998), which are hereby incoφorated by reference) and prepared as previously reported in DeSantis et al., Biochemistry. 37:5698-5973 (1998), which is hereby incoφorated by reference. Protected amino acids were purchased from Sigma or Bachem and were used as received. All solvents were reagent grade and distilled prior to use. Thin layer chromatography analysis and purification were performed on pre-coated Merck Silica gel (60 F-254) plates (250 μm) visualized with UV light or iodine. 1H and 13 C NMR spectra were recorded on a Varian Gemini 200 (200 MHz for Η and 50.3 MHz - 22 - for 13 C) or Unity 400 (400 MHz for Η and 100 MHz for 13 C) and spectrometer and chemical shifts are given in ppm (δ) using CDC1 3 or DMSO-d 6 as an internal standard. High resolution mass spectra (HRMS) were recorded using Micromass ZAB-SE (FAB + ). Optical rotations were measured with a Perkin-Elmer 243B polarimeter. General procedure for peptide ligation. To a solution of amino acid acyl donor (0.1 mmol) in DMF (0.4 mL) and water (0.4 mL), glycinamide hydrochloride (0.3 mmol) or alaninamide hydrochloride (0.2 mmol) and Et 3 N (0.083-0.125 mL, 0.3-0.4 mmol) was added, followed by the addition of a solution of 1 mg of active enzyme (0.0037 mmol, 0.037 eq.), as determined by titration with phenylmethanesulfonyl fluoride (PMSF) (Hsia et al., Anal. Biochem.. 242:221-227 (1996), which is hereby incoφorated by reference), in buffer solution (10 mmol MES, 1 mmol CaCl 2 , pH 5.8). The resulting total volume of reaction was 1.0-1.2 mL. The reaction was left stirring at room temperature for the period of time indicated in Tables 2- 4, below. Where D-amino acids were used as acyl donors, after 24 hours, 1 mg more of active enzyme as well as an equal amount of DMF were added. After the reaction was finished, the mixture was then concentrated in vacuo and subjected to purification using preparative TLC (5-10% MeOH in CH 2 C1 2 ). Peptide Ligation ofL-Amino Acids Acyl donors Z-L-Phe-OBn, Z-L-Ala-OBn, Z-L-Glu-OMe, and Z-L-Lys- SBn (1-4) and acyl acceptors Gly-NH 2 and L-Ala-NH 2 (5, 6) were used for the coupling reaction as shown in Figure 5. The acyl donors Z-L-Phe-OBn, Z-L-Ala-OBn, Z-L-Glu- OMe, and Z-L-Lys-SBn (1-4) provided representative examples of large and small hydrophobic, negatively charged and positively charged Pi side chains, respectively and allowed a broad evaluation ofthe affinity ofthe Si pocket of these enzymes for various amino acids. The small amino acid amides Gly-NH 2 and L-Ala-NH (5, 6) were chosen as the acyl acceptors since the Si' pocket of subtilisins is narrow (Moree et al., J. Am. Chem. Soc. , 119:3942-3947 (1997); Betzel et al, J. Mol. Biol.. 223:427-445 (1992); Sears et al, J. Am. Chem. Soc. 116:6521-6530 (1994); and Jackson et al. Science. 266:243-247 (1994), which are hereby incoφorated by reference), and, therefore, it accepts α-branched amino acids only poorly. The narrow nature ofthe Sj' pocket is attributed to the bulky side chain of M222, this residue being a conserved residue - 23 - amongst subtilisins. Siezen et al. Protein Eng., 4:719-737 (1991), which is hereby incoφorated by reference. This has limited their use in peptide ligation applications. The coupling of Z-L-Phe-OBn (1), containing the preferred phenylalanine Pi residue of SBL, with Gly-NH 2 (5) yielded dipeptide Z-L-Phe-Gly-NH 2 (7) in excellent yields using both SBL-WT and all four S166-MEs (Table 2, below) as catalysts after 1 hour. No reaction was observed in the absence ofthe enzyme. M222C-SCH 2 CH 2 NH 3 + which had the highest esterase/amidase ratio gave the product in only 33% yield after 5 hours and starting material, Z-Phe-OBn (1), was recovered in 41% yield. Since M222C is located at the boundary between Si and Si' pockets, its modification apparently causes steric hindrance at the active site precluding substrate binding in the Si pocket. Table 2 : SBL-WT and SBL-ME-catalyzed coupling of L-amino acids (1-4) and glycinamide (5) a MEs Z-L-Phe-Gly-NH 2 Z-L-Ala-Gly-NH 2 Z-Glu -Gly-NH 2 Z-Lys-Gly-NH 2 (7) % yield (8) % yield (9) ' % yield (10) % yield lh lh 5h lh 5h 5h WT 92 68 91 64 62 83 S166C-SCH 2 H 5 92 59 95 68 96 93 S166C-SCH 2 F 5 93 42 94 61 61 71 S166C-SCH 2 (p-COOH- 100 38 82 30 62 99 C 6 H 4 ) S166C-SCH 2 CH 2 NH 3 + 95 63 94 69 100 86 M222C-SCH 2 CH 2 NH 3 + 33 (5h) - - - 33 - a conditions : 0.1 mmol of acyl donor, 0.3 mmol of glycinamide hydrochloride, 0.4 mmol of Et 3 N, 1 mg of enzyme, 1 : H 2 0 : DMF. The total volume ofthe reaction is 1.0-1.2 mL. High yields were also obtained using Z-L-Ala-OBn (2) as the acyl donor with WT and each of MEs but required a longer reaction time of 5 hours. The yields obtained after running the reaction for 1 hour were all lower and in all cases starting material was recovered. The requirement for a longer reaction time in this case, compared to using Z-L-Phe-OBn (1) as the acyl donor, is consistent with SBL-WT's preference for substrate binding of large hydrophobic over small group Pi substituents in the Si subsite. Gron et al. Biochemistry. 31 :6011-6018 (1992), which is hereby incoφorated by reference. When Z-Glu-OMe (3) with its negatively charged Pi residue, was used as the acyl donor, only moderate yields of Z-Glu-Gly-NH 2 (9) were obtained after 1 hour in all cases. Unlike the reaction of acyl donor Z-L-Ala-OBn (2), prolonging the reaction - 24 - time led to an improved yield in the case of S166C-SCH 2 C 6 H 5 , -SCH 2 (p-COOH-C 6 H 4 ), and -SCH 2 CH 2 NH 3 + . The yields using SBL-WT and S166C-SCH 2 C 6 F 5 as catalysts after 5 hours were virtually the same as for 1 hour. However, it was possible to isolate Z-L- Glu-Gly-NH 2 (9) in quantitative yield using S166C-SCH 2 CH 2 NH 3 + with its complementary charged Si pocket. When using M222C-SCH 2 CH 2 NH 3 + as the catalyst, no enhancement of yield was observed with respect to the SBL-WT catalyzed reaction. This reaction yielded only 33% of Z-L-Glu-Gly-NH 2 (9) after 5 hours in addition to 32% ofthe recovered starting material Z-L-Glu-OMe (3). As mentioned above, the M222 residue seems to inhibit the binding substrates Pi residue in Si pocket ofthe enzyme. This may be the cause for the low yield when using this ME and correlates with the recovery of starting material. Z-L-Glu-Gly-NH (9) was also obtained in excellent yield (96%) using S166C-SCH 2 C 6 H 5 as the catalyst. Enhanced turnover ofthe positively charged Z-L-Lys-SBn (4) acyl donor with the charged ME, S166C-SCH 2 (p-COOH-C 6 H 4 ), was observed resulting in 99% of Z- L-Lys-Gly-NH2 (10), the best result ofthe series. Again, a good yield of 93% of Z-L- Lys-Gly-NH 2 (10) was also observed using S166C-SCH 2 C 6 H 5 as the catalyst. This may be due to the high esterase/amidase ratio of this enzyme. The reaction using S166C- SCH C 6 F 5 gave only 71% yield of product which was lower than the reaction using WT as the catalyst. A higher yield, with respect to WT, was also obtained using S 166C- SCH 2 CH 2 NH 3 + in spite ofthe potential electrostatic repulsion between the modified enzyme and the side chain of Lys. The synthetic ability ofthe selected MEs using other acyl acceptors other than Gly-NH 2 was further investigated. Since the Si' pocket of subtilisin is small and restricted, the smallest α-branched amino acid, Ala-NH 2 (6), was used to probe this subsite (See Table 3). - 25 - Table 3 : SBL WT and ME-catalyzed coupling of L-amino acids (1-4) and L-alaninamide (6) a MEs Z-L-Phe-Ala-NH 2 Z-L-Ala-Ala-NH 2 Z-L-Glu-Ala-NH 2 Z-L -Lys-Ala- (11) % yield (12) % yield (13) % yield NH 2 (14) % yield ~"" wf ~" ~""" """ 57 0 O " 0 S166C- 51 0 0 0 SCH 2 C 6 H 5 S166C-SCH 2 C 6 F 5 33 0 0 0 S166C-SCH 2 (p- 48 0 0 0 COOH-C 6 H 4 ) S166C- 88 16 14 0 SCH 2 CH 2 NH 3 + M222C- 22 0 0 0 SCH 2 CH 2 NH 3 + a conditions : 0.1 mmol of acyl donor, 0.2 mmol of alaninamide hydrochloride, 0.3 mmol of Et 3 N, 1 mg of enzyme, 1: 1 H 2 0 : DMF, 24h. The total volume of the reaction is 1.0-1.2 mL. In all cases, the reaction of L-Ala-NH 2 (6) with Z-L-Phe-OBn (1) was slower than Gly-NH 2 (5) with Z-L-Phe-OBn (1). Further, after 24 hours, Z-L-Phe-Ala- NH 2 (11) was obtained in moderate yield (33-57%) using WT, S166C-SCH 2 C 6 H 5 , - SCH 2 C 6 F 5 , and -SCH 2 (p-COOH-C 6 H 4 ) as the catalysts. However, Z-L-Phe-Ala-NH 2 (11) was obtained in 88% yield in the case of S166C-SCH 2 CH 2 NH 3 + . Unlike S166C- SCH 2 CH 2 NH 3 + , the use of M222C-SCH 2 CH 2 NH 3 + did not improve the yield ofthe dipeptide product as compared to the WT-catalyzed reaction; only a low 22% yield of Z- L-Phe-Ala-NH 2 (11) was obtained. This was possibly due to the steric interaction of this residue at the binding site, Si pocket, with the Pj substrate as mentioned above. When Z-L-Ala-OBn (2) or Z-L-Glu-OMe (3) were used as acyl donors, no reaction was observed with L-Ala-NH (6) for five out ofthe six enzymes used. However, when S166C-SCH CH NH 3 + was used as the catalyst, the yield of dipeptides Z-Ala-Ala-NH 2 (12) and Z-Glu-Ala-NH 2 (13) were formed in 16% and 14%, respectively. While these yields were low, they represented a dramatic improvement over WT. No reaction was observed by treatment of Z-L-Lys-SBn (4) and L-Ala- NH 2 (6) with WT and all MEs, including using the negative charged ME, S166C-SCH 2 (p- COOH-C 6 H 4 ), in which case the complementary electrostatic interaction was expected. - 26 - These results contrast to the previously reported preference for Ala over Gly in the Si' pocket of subtilisin Bacillus lentus. Gron et al. Biochemistry. 31 :6011- 6018 (1992), which is hereby incoφorated by reference. This preference was not observed: the yields obtained using glycinamide (5) as the acyl acceptor were higher in all cases, and the reaction times were shorter (Table 2) compared to using alaninamide (6) (Table 3) as the acyl acceptor. D-Amino Acid Ligation Next, the scope of application of SBL catalyzed peptide ligation was extended to include D-amino acid esters Z-D-Phe-OBn, Z-D-Ala-OBn, Z-D-Glu-OMe, Z- D-Lys-OBn, and Ac-D-Phe-OBn (15-19) as the acyl donors (Figure 6) by the ME methodology, which was not possible with SBL-WT. The results are shown in Table 4. Table 4 : SBL WT and ME-catalyzed coupling of D-amino acids (15-19) and glycinamide (5) a ___ ________ Ac >Phe- Gly-NH 2 Gly-NH 2 Gly-NH 2 Gly-NH 2 Gly-NH 2 (20) % yield (21) % yield (22) % yield (23) % yield (24) % yield "wf ~ ™ " - - -- - - - - --- -- - - - 0 - Q - - - --- - ------ S166C-SCH 2 C 6 H 5 66 50 3 0 15 S166C-SCH 2 C 6 F 5 39 49 3 0 27 S166C-SCH 2 (p-COOH- 35 48 6 0 34 C 6 H 4 ) S166C-SCH 2 CH 2 NH 3 + 43 45 10 0 38 a conditions : 0.1 mmol of acyl donor, 0.3 mmol of glycinamide hydrochloride, 0.4 mmol of Et 3 N, 1 mg of enzyme, 1 : 1 H 2 0 : DMF, 48h. After 24h, another 1 mg of enzyme was added. The total volume of the reaction is 1.5-3.0 mL. For accurate comparison, the D-isomers Z-D-Phe-OBn, Z-D-Ala-OBn, Z- D-Glu-OMe, Z-D-Lys-OBn, and Ac-D-Phe-OBn (15-19) ofthe representative L-amino acids Z-L-Phe-OBn, Z-L-Ala-OBn, Z-L-Glu-OMe, and Z-L-Lys-SBn (1-4) examined in the previous ligation examples were used. The stereoselectivity of SBL-WT for L-amino acids was clear (Table 4), because none ofthe D-amino acid esters evaluated gave dipeptide products with WT as the catalyst. All ofthe S166C-MEs yielded dipeptide products containing D-amino acids Z-D-Phe-Gly-NH 2 , Z-D-Ala-Gly-NH 2 , Z-D-Glu-Gly- NH 2 , Z-D-Lys-Gly-NH 2 , and Ac-D-Phe-Gly-NH 2 (20-24). While each of these enzymes - 27 - still showed a preference for L-amino acids, yields of up to 66% of Z-D-Phe-Gly-NH 2 , using S166C-SCH 2 C 6 H 5 , over 0% for WT, demonstrated a dramatic improvement in SBL's acceptance of D-amino acids. Similar yields of Z-D-Ala-Gly-NH 2 (21) were obtained using Z-D-Ala- OBn (16) as the acyl acceptor from all four S166C-MEs catalyzed reactions. This demonstrated that chemical modification at this residue broadened stereospecificity ofthe Si pocket in general manner. When Z-D-Glu-OMe (17) was used as the acyl donor, only a low yield of dipeptide Z-D-Glu-Gly-NH2 (22) was obtained in all ME catalyzed reactions. The best yield, 10% of Z-D-Glu-Gly-NH 2 (22), resulted using S 166C-SCH 2 CH 2 NH 3 + . This may be accounted for by complementary electrostatic interaction between the side chain of S166C-SCH 2 CH 2 NH 3 + and the side chain of glutamic acid acyl donors. In contrast, no product was observed when Z-D-Lys-OBn (18) was used as the acyl donor for all WT and ME catalyzed reactions, including the use of S166C-SCH 2 (p-COOH-C 6 H ) as the catalyst in which case a great improvement in yield in the reactions of Z-L-Lys-SBn was observed. Since Z-D-Phe-OBn (15) as the acyl donor gave 35-66% of product with each ofthe MEs evaluated and 0% yield with WT and furthermore, since replacing the carbobenzoxy (Z) group of Z-D-Phe-OBn (15) with the acetyl group in the acyl donor Ac-D-Phe-OBn (19) resulted in lower yields (Table 4), it is speculated that the carbobenzoxy group in Z-D-Phe-OBn (15) may direct binding in the SI pocket as shown in Figure 7, a process not observed in Ac-D-Phe-OBn (19). Dipeptides Produced Z-Phe-Gly-NH 2 (7) (Moree et al, J. Am. Chem. Soc. 119:3942-3947 (1997) and Morihara et al, Biochem. J.. 163:531-542 (1977), which are hereby incoφorated by reference): 1H NMR (CDC1 3 ) δ 3.10 (m, 2H, CH 2 Ph), 3.85 (2 x d, J= 2, 5 Ηz, 2Η, NHCH 2 CO), 4.40 (m, 1Η, NΗCHCO), 5.05 (s, 2Η, OCH 2 Ph), 5.50, 5.70, 6.25, 6.90 (4 x brs, 4Η, NH), 7.20-7.40 (m, 10Η, 2 x Ph); 13 C NMR (CDC1 3 ) δ 38.2, 42.7, 56.6, 67.3, 127.2, 128.1, 128.3, 128.6, 128.8, 129.2, 135.8, 136.0, 156.3, 171.2, 171.6. ΗRMS (FAB + ) MR + calcd 356.1610, found 356.1613. [α] 30 D = -4.3 (c 0.81, MeOΗ). - 28 - Z-Ala-GIy-NH 2 (8) (Bodanszky et al. Int. J. Peptide Protein Res.. 26:550-556 (1985), which is hereby incoφorated by reference): Η NMR (CDC1 3 ) δ 1.40 (d, J= 7 Hz, 3H, CH 3 ), 3.85 (dd, J= 1.7, 5Hz, 2H, NCH 2 CO), 4.20 (m, 1Η, NΗCHCO), 5.10 (dd, J= 1.6, 2 Ηz, 2Η, OCH 2 Ph), 5.80, 6.60, 7.20 (3 x brs, 4Η, NH), 7.30-7.40 (m, 5Η, Ph); 13 C NMR (CDC1 3 ) δ 17.7, 42.2, 50.6, 66.2, 127.6, 128.0, 136.0, 155.9, 171.3, 172.8. HRMS (FAB + ) MYCcalcd 280.1297, found 280.1310. [α] 26 D = -8.44 (c 0.97, MeOH); lit. [α] 23 D = -8.5 (c 2, MeOH). Z-Glu-GIy-NH 2 (9) (Schon et al. Int. J. Peptide Protein Res.. 22:92-109 (1983), which is hereby incoφorated by reference): ! H NMR (DMSO-d 6 ) δ 1.75-1.95 (m, 2H, CH 2 CH 2 COOH), 2.30 (m, 2H, CH 2 CH 2 COOH), 3.65 (dd, J= 0.5, 1.7 Hz, 2H, NHCH 2 CO), 4.10 (m, 1Η, NΗCHCO), 5.00 (s, 2Η, OCH 2 Ph), 7.20-7.40 (m, 5Η, Ph), 12.40 (brs, IH, COOH); 13 C NMR (DMSO-d 6 ) δ 27.2, 30.3, 41.9, 54.2, 65.6, 128.4, 128.5, 128.7, 128.8, 136.9, 156.2, 170.8, 171.7, 174.0 ΗRMS (FAB + ) MYtcalcd 338.1352, found 338.1332. [α] 28 D = -10.2 (c 1.16, MeOΗ); lit. [α] 25 D = -10.2 (c 1.0, MeOΗ). Z-L-Lys-Gly-NΗ 2 (10): 1H NMR (DMSO-d 6 ) δ 1.50 (m, 2H, CH 2 (CH 2 ) 3 NH 2 ), 1.60-1.85 (m, 4H, CH 2 CH 2 CH 2 CH 2 NH 2 ), 2.00-2.18 (m, 2H, (CH 2 ) 3 CH 2 NH 2 ) 3.30 (m, 2H, NHCH 2 CO), 4.40 (m, 1Η, NΗCHCO), 5.10 (s, 2Η, OCH 2 Ph), 6.05, 6.20 (2 x brs, 2Η, NH), 7.20-7.40 (m, 5Η, Ph); 13 C NMR (DMSO-d 6 ) δ 28.1, 29.0, 32.2, 42.3, 45.9, 53.8, 66.7, 128.1, 128.6, 136.7, 155.6, 172.2, 175.3. HRMS (FAB + ) MH + calcd 337.1876, found 337.1842. [α] 28 D - +6.97 (c 0.55, MeOH). Z-L-Phe-L-Ala-NH 2 (11) (Morihara et al, Biochem. J.. 163:531 -542 ( 1977) and Brubacher et al. Can J. Biochem., 57:1054-1072 (1979), which are hereby incoφorated by reference): Η NMR (DMSO-d 6 ) δ 1.20 (d, J= 1 Hz, 3H, CH 3 ), 2.90 (m, 2H, CH Ph), 4.30 (m, 2Η, 2 x NHCHCO), 4.90 (s, 2Η, OCH 2 Ph), 5.75, 6.10, 6.45 (3 x brs, 4Η, NH), 6.90-7.40 (m, 10Η, 2 x Ph); 13 C NMR (DMSO-d 6 ) δ 18.5, 37.4, 48.1, 56.2, 66.2, 126.3, 127.4, 127.7, 128.1, 128.3, 129.2, 137.1, 138.2, 155.9, 171.2, 174.1. ΗRMS (FAB + ) MH + calcd 370.1766, found 370.1753. [α] 29 D = -8.86 (c 0.57, MeOΗ). - 29 - Z-L-Ala-L-Ala-NH 2 (12) (Katakai et al, Macromolecules. 6:827-831 (1973), which is hereby incoφorated by reference): Η NMR (DMSO-d 6 ) δ 1.20 (2 x d, J= 7 Hz, 6H, 2xCH 3 ), 4.10, 4.20 (m, 2H, 2 x NHCHCO), 5.00 (s, 2Η, OCH 2 Ph), 7.20-7.40 (m, 5Η, Ph); 13 C NMR (DMSO-d 6 ) δ 18.1, 18.4, 47.9, 50.1, 65.4, 127.7, 128.4, 137.0, 155.8, 172.0, 174.1. HRMS (FAB + ) U\2calcd 294.1454 found 294.1457. [α] 25 D = -20.4 (c 0.77, MeOH). Z-L-Glu-L-Ala-NH 2 (13): Η NMR (DMSO-d 6 ) δ 1.20 (d, J= 7 Hz, 3H, CH 3 ), 1.82-2.00 (m, 2H, CH CH 2 COOH), 2.30 (m, 2H, CH 2 CH 2 COOH), 4.00, 4.20 (m, 2H, 2 x NHCHCO), 5.00 (s, 2Η, OCH 2 Ph), 6.20-7.40 (m, 5Η, Ph); 13 C NMR (DMSO-d 6 ) δ 18.4, 26.2, 30.2, 47.9, 53.1, 65.4, 126.5, 127.7, 127.8, 128.1, 128.4, 137.0, 156.2, 173.6, 173.8, 174.1. HRMS (FAB + ) MR + calcd 352.1509, found 352.1478. [α] 25 D = -16.7 (c 0.76, MeOH). Z-D-Phe-Gly-NH 2 (20): Η and 13 C NMR data are identical to (7). HRMS (FAB + ) MU + calcd 356.1610 found 356.1608; [ ] 30 D = +4.12 (c 1.17, MeOH). Z-D-Ala-Gly-NH 2 (21) (Richman et al. Int. Peptide Protein Res., 25:648-662 (1985), which is hereby incoφorated by reference): 1H and 13 C NMR data are identical to (8). HRMS (FAB + ) MH + calcd 280.1297 found 280.1298; [α] 27 D +10.5 (c 0.72, MeOH); lit. [α] D = +10.5. Z-D-Glu-Gly-NH 2 (22): 1H and 13 C NMR data are identical to (9). HRMS (FAB + ) MirΫcalcd 338.1352 found 338.1348; [α] 28 D = +10.77 (c 1, MeOH). Ac-D-Phe-Gly-NH 2 (24) (Thompson et al, j. Med. Chem.. 29:104-111 (1986), which is hereby incoφorated by reference): 1H NMR (DMSO-d 6 ) δ 1.90 (S, 3H, CH 3 ), 3.05 (m, 2H, NHCH 2 CO), 3.65 (2 x d, J= 1, 15 Ηz, 2Η, CHCH 2 Ph), 4.40 (q, J = 7Ηz, IH NHCHCO), 7.15-7.25 (m, 5Η, Ph); 13 C NMR (DMSO-d 6 ) δ 22.4, 29.1, 45.9, 54.7, 126.1, 127.9, 128.9, 137.3, 155.2, 171.2, 171.5. HRMS (FAB + ) UYCcalcd 264.1348, found 264.1321. [α] 30 D = -4.38 (c 0.80, MeOH). - 30 - Example 5 - Peptide Synthesis Using Chemically Modified Mutant Enzymes with Polar Substituents, Such As Oxazolidinones, Alkyl Amino Groups with Positive Charge, and Saccharides General Methods 1H and 13 C NMR spectra were measured on a Varian Unity (400 MHz for 1H and 100 MHz for 13 C) spectrometer with DMSO-d 6 as internal standard. High resolution mass spectra ("HRMS") were recorded using Micromass ZAB-SE (FAB + ). Optical rotations were measured with a Perkin-Elmer 243B polarimeter. ALUGRAM® SIL G UV254 Art.-Nr. 818 133 (Macherey-Nagel GmbH & Co, Duren, Germany) was used for analytical TLC. Preparative TLC was performed on pre-coated Silica gel plate Art.5744 (Merck, Gibbstown, NJ) visualized with UV light. WT-subtilisin Bacillus lentus and mutant enzymes were purified and prepared as reported in Stabile et al, Bioorg. Med. Chem. Lett.. 6:2501-2506 (1996) and DeSantis et al. Biochemistry. 37:5968-5973 (1998), which are hereby incoφorated by reference, and as described in Example 1. Protected acids were purchased from Sigma- Aldrich Inc. (Milwaukee, WI) or Bachem Inc. (Torrance, CA) and were used as received. All solvents were reagent grade and distilled prior to use. General Procedure for Peptide L igation To a solution of Z-L-Phe-OBn (25, 19.2 mg, 0.05 mmol) in DMF (0.25 mL) and water (0.144 mL), glycinamide hydrochloride (31, 17 mg, 0.15 mmol) and Et 3 N (0.15 mmol, 0.0625 mL) were added, followed by the addition of S 166C-S-inden- oxaz(S,R) (ME-n, 0.106 mL, 0.5 mg of active enzyme in 10 mM MES buffer (pH 5.8) including 1 mM CaCl 2 ). The reaction was stirred for one hour at room temperature. The mixture was diluted with AcOEt and washed with 1 M KHSO (lmL x 1) and brine (1 mL x 1), and the organic layer was dried over MgSO 4 . After evaporation, the residue was purified by preparative TLC (CH 2 C1 2 / MeOH = 90/10) to afford Z-L-Phe-Gly-NH 2 (33, 17.8 mg, quantitative). Peptide ligations of other substrates using other enzymes were carried out following the same procedure except for reaction time. In the case of D-amino acids as acyl donors, 0.5 mg more active enzymes was added to the reaction vessel after 24 hours, and then the mixture was stirred for another 24 hours. - 31 - Peptide Ligation ofL-Amino Acids First, the coupling reaction of L-amino acid, Z-L-Phe-OBn (25), Z-L-Ala- OBn (26), and Z-L-Glu-OMe (27), with glycinamide (31) were investigated as standard reactions (See Figures 1 and 8, Table 5). Table 5. WT and MEs of SBL Catalyzed Peptide Coupling"' yield/ % acyl donor acyl acceptor product time/h WT ME-j -k -1 -m -n -p -q -r -s -t Z-L-PheOBn (25) GlyNH 2 HCl (31) Z-L-PheGlyNH 2 (33) 1 92 92 6 S8 82 100 74 75 95 93 91 95 Z- -AlaOBn (26) 31 Z-L-AlaGlyNH, (34) 5 91 8 2 8 7 88 91 95 99 91 8 5 77 92 83 Z-L-GluOMe(27) 31 Z-L-GluGlyNH 2 (35) 5 62 67 60 54 68 56 63 71 58 65 54 67 Z-D-PheOBn (2 8 ) 31 Z-D-PheGlyNH 2 (36) 4 8 b 0 9 8 12 7 14 4 4 6 8 7 8 Z-D-AlaOBn (29) 31 Z-D-AlaGlyNH 2 (37) 48" 0 61 8 6 80 86 80 79 73 80 77 72 70 Z-D-GluOBn (30) 31 Z-D-GluGlyNH 2 (3S) 4 8 b 0 64 62 60 62 52 74 64 63 62 64 64 25 L-AlaNH 2 HC1(32) Z- -Phe-L-AlaNH 2 (39) 24' 57 50 31 30 33 37 44 36 28 34 31 32 26 32 Z-L-Ala-L-AlaNH 2 (40) 24' 0 10 12 19 21 20 14 11 15 16 22 1 1 27 32 Z-L-Glu- -AlaNH ; (41) 24' 0 64 60 59 61 59 58 60 48 50 51 55 " The reaction was performed in DMF/Water (1/1, v/v) using 0 1 M acyl donor, 0 3 M acyl acceptor, and 0 3 M Et 3 N m the presence of 0 5 mg of active enzyme in 10 mM MES buffer (pH 5 8) containing 1 mM CaCl 2 at rt unless otherwise noted Under same conditions, spontaneous hydrolysis or aminolysis did not occur b After 24 h, 0 5 mg of active enzyme was added and the mixture was stined for another 24 h ' In these cases, 0 2 M of 32 and 0 2 M of Et 3 N were used The reactions were carried out using 0.5 mg of active enzyme with Et 3 N in water solution containing 50% DMF. The activities of enzymes were determined by titration with phenylmethanesulfonyl fluorine ("PMSF"). Hsia et al, Annal. Biochem.. 242:221-227 (1996), which is hereby incoφorated by reference. In all cases, the reactions smoothly proceeded to afford the corresponding dipeptides in good yields. These results indicated that the modification of S166C site by these substituents did not affect the essential ability to accept L-amino acids in peptide coupling. Peptide Ligation ofD-Amino Acids Next, the extension ofthe use ofthe MEs to the coupling reaction of D- amino acids as acyl donor, Z-D-Phe-OBn (28), Z-D-Ala-OBn (29), and Z-D-Glu-OBn (30) with Z-L-Phe-OBn (1) was examined. While WT enzyme did not accept D-amino acids as acyl donors, all ofthe MEs were able to catalyze the coupling of D-amino acids with Z-L-Phe-OBn (1). Although the reactions of Z-D-Phe-OBn (28) in all cases were slow to give Z-D-Phe-Gly-NH 2 (39) in low yield (the best was 14% by using M-n), peptide coupling of Z-D-Ala-OBn (29) or Z-D-Glu-OBn (30) with Gly-NH 2 (31) - 32 - proceeded without remaining substrates. It is noteworthy that using ME-k and -m in case of Z-D-Ala-OBn (29) and ME-o in case of Z-D-Glu-OBn (30) gave Z-D-Ala-Gly-NH 2 (37, 86%) and Z-D-Glu-Gly-NH 2 (38, 74%), respectively, in very high yields. Probably, the CMMs recognized D-amino acids in a different manner from L-amino acids, i. e. , the carbobenzoxy group of α-position seems to bind the Si pocket. On the other hand, repulsion between the phenylmethyl group of Z-D-Phe-OBn (28), which had the biggest substituent among the three kinds of substrate, and the other parts in the pocket of active site ofthe MEs could cause low reactivity of Z-D-Phe-OBn (28). In spite of their small Si' pocket, all selected MEs were also applicable to the coupling of L-amino acids with an α-branched acyl acceptor, L-alaninamide (32). Although the WT enzyme could accept L-alaninamide (32) as acyl acceptor only in the case of Z-L-Phe-OBn (25) as an acyl donor, the MEs also catalyzed the reactions in the cases of not only Z-L-Phe-OBn (25) but also Z-L-Ala-OBn (26) and Z-L-Glu-OMe (27) to afford the corresponding dipeptides Z-L-Phe-L-Ala-NH 2 (39), Z-L-Ala-L-Ala-NH 2 (40), and Z-L-Glu-L-Ala-NH 2 (41), respectively. In the case of Z-L-Ala-OBn (26), mainly competitive hydrolysis ofthe esters was observed (the best was 21% by using ME-m). These results represented a dramatic improvement ofthe specificity of WT. The yields ofthe coupling of Z-L-Glu-OMe (27) with Ala-NH 2 (32) were as good as those of Gly-NH 2 (31) as acyl acceptor, and using ME-j gave the best result (64%). Although not wishing to be bound by theory, it is speculated that the strong interaction between the carboxyl group of Z-L-Glu-OMe (27) and the side chain of S166C site of MEs provides a more stable ES-complex, which could not be easily attacked by water, therefore Z-L-Glu-L-Ala-NH (41) could be obtained in good yield. Dipeptides Produced Z-L-Phe-Gly-NH 2 (33): Η NMR (DMSO-d 6 ) δ 2.74 (dd, J= 11.0, 14.0 Hz, IH, CH 2 Ph), 3.04 (dd, J= 4.0, 14.0 Ηz, 1Η, CH,Ph), 3.59 - 3.72 (m, 2Η, NHCH,CO), 4.21 - 4.35 (m, 1Η, NΗCHCO), 4.93 (d, J= 12.5 Ηz, 1Η, OCH 2 Ph), 4.94 (d, J= 12.5 Ηz, 1Η), OCH 2 Ph), 7.12 (brs, 2Η, NH), 7.16 - 7.38 (m, 5H, Ph), 7.60 (d, J= 8.5 Hz, IH, NH), 8.27 (t, J= 5.5 Hz, IH, NH) ; 13 C NMR (DMSO-d 6 ) δ 37.3, 42.0, 56.3, 65.3, 126.3, 127.5, 127.8, 128.1, 128.4, 129.3, 137.0, 138.2, 156.0, 170.8, 171.8; HRMS (FAB + ) calcd for C 19 H 22 N 3 O 4 (M + H) + 356.1610, found 356.1639; [α] 2, D = -3.94 (c 1.04, MeOH). - 33 - Z-L-Ala-Gly-NH 2 (34): Η NMR (DMSO-d 6 ) δ 1.20 (d, J= 7.0 Hz, 3H, CH 3 ), 3.60 (dd, J = 5.5, 16.0 Hz, IH, CHNH), 3.62 (dd, J= 5.5, 16.0 Hz, IH, CH 2 NH), 4.03 (dq, J= 7.0, 7.0 Hz, IH, CH 3 CHNH), 5.00 (d, J= 12.5 Hz, IH, OCH Ph), 5.03 (d, J= 12.5 Ηz, 1Η, OCH 2 Ph), 7.1 1 (brs, 1Η, NΗ 2 ), 7.18 (brs, IH, NH 2 ), 7.27 - 7.42 (m, 5H, Ph), 7.57 (d, J= 7.0 Hz, IH, NH), 8.11 (t, J= 5.5 Hz, IH, NH); ,3 C NMR (DMSO-d 6 ) δ 17.9, 42.0, 50.3, 65.5, 127.85, 127.89, 128.4, 136.9, 155.9, 170.9, 172.7; HRMS (FAB + ) calcd for C 13 H 18 N 3 O 4 (M + H) + 280.1297, found 280.1307; [α] 25 D = -8.44 (c 0.64, MeOH). Z-L-Glu-GIy-NH 2 (35): Η NMR (DMSO-d 6 ) δ 1.66 - 1.79 (m, IH, CH 2 CH 2 COOH), 1.83 - 1.95 (m, IH, CH 2 CH 2 COOH), 2.26 (t, J= 7.5 Hz, 2H, CH 2 COOH), 3.62 (d, J= 5.5 Hz, 2H, NHCH CO), 3.95 - 4.05 (m, 1Η, NΗCHCO), 5.01 (d, J= 12.5 Ηz, 1Η, OCH 2 Ph), 5.03 (d, J= 12.5 Ηz, 1Η, OCH 2 Ph), 7.07 (brs, 1Η, NΗ), 7.20 (brs, 1Η, NΗ), 7.25 - 7.40 (m, 5Η, Ph), 7.55 (d, J= 7.5 Hz, 1H, NH), 8.11 (t, J= 5.5 Hz, 1H, NH); 12.20 (brs, IH, COOH); 13 C NMR (DMSO d 6 ) δ 27.0, 30.2, 41.9, 54.1, 65.6, 127.8, 127.9, 128.4, 136.9, 156.2, 170.8, 171.7, 174.0; HRMS (FAB + ) calcd for C 15 H 20 N 3 O 6 (M + H) + 338.1352, found 338.1364; [α] 25 D = -9.28 (c 0.69, MeOH). Z-D-Phe-Gly-NH 2 (36): HRMS calcd for C 19 H 22 N 3 O 4 (M + H) + 356.1610, found 356.1592; [α D = +3.42 (c 1.17, MeOH). Z-D-Ala-GIy-NH 2 (37): HRMS calcd for C 13 H 18 N 3 O 4 (M + H) + 280.1297, found 280.1303; [α] 24 D = +8.49 (c 0.86, MeOH) Z-D-Glu-Gly-NH 2 (38): HRMS calcd for C 15 H 20 N 3 O 6 (M + H) + 338.1352, found 338.1353 ; [α] 24 D = +9.07 (c 1.08, MeOH) Z-L-Phe-L-AIa-NH 2 (39): 'H NMR (DMSO-d 6 ) δ 1.22 (d, J= 7.0 Hz, 3H, CH 3 ), 2.71 (dd, J= 13.5, 13.5 Hz, IH, CH 2 Ph), 3.03 (dd, J= 3.5 , 13.5 Ηz, 1Η, CH 2 Ph), 4.18 - 4.31 - 34 - (m, 2H, NHCHCO x 2), 4.93 (s, 2Η, OCH 2 Ph), 7.04 (brs, 1Η, NΗ), 7.14 - 7.22 (m, 1Η, NΗ), 7.55 (d, J= 8.5 Ηz, 1Η, NΗ), 8.08 (d, J= 7.5 Ηz, 1Η, NΗ); ,3 C NMR (DMSO- d 6 ) δ 18.5, 37.4, 48.1, 56.2, 65.2, 126.3, 127.4, 127.7, 128.1 , 128.4, 129.3, 137.1, 138.2, 155.9, 171.1 , 174.1 ; ΗRMS (FAB + ) calcd for C 20 Η 24 N 3 O 4 (M + H) + 370.1767, found 370.1769; [α] 24 D = -8.86 (c 0.44, MeOH). Z-L-Ala-L-Ala-NH 2 (40): Η NMR (DMSO-d 6 ) δ 1.19 (d, J= 7.0 Hz, 3H, CH 3 ), 1.24 (d, J= 7.5 Hz, 3H, CH 3 ), 3.90 - 4.26 (m, 2H, NHCHCO x 2), 5.01 (s, 2Η, CH OPh), 7.02 (brs, 1Η, NΗ), 7.13 (brs, 1Η, NΗ), 7.25 - 7.45 (m, 5Η, Ph), 7.51 (d, J= 6.5 Hz, IH, NH), 7.88 (d, J= 7.5 Hz, IH, NH); 13 C NMR (DMSO- d 6 ) δ 18.1 , 18.5, 47.9, 50.2, 65.4, 127.78, 127.84, 128.4, 137.1, 155.8, 172.0, 174.2; HRMS (FAB + ) calcd for C 14 H 20 N 3 O 4 (M + H) + 294.1454, found 294.1457; [α] 2, D = -20.4 (c 0.77, MeOH). Z-L-Glu-L-Ala-NH 2 (41): Η NMR (DMSO-d 6 ) δ 1.20 (d, J= 8.0 Hz, 3H, CH 3 ), 1.68 - 1.82 (m, IH, CH CH 2 COOH), 1.82 - 2.03 (m, IH, CH CH 2 COOH), 2.21 - 2.40 (m, 2H, CH 2 CH COOH), 3.93 - 4.25 (m, 2H, NHCHCO x 2), 5.02 (s, 2Η, OCH Ph), ' 7.02 (brs, 1Η, NΗ), 7.18 - 7.46 (m, 5Η, Ph), 7.54 (dd, J= 7.5, 24.0 Hz, 2H, NH 2 ), 7.92 (d, J= 7.5 Hz, IH, NH), 12.40 (brs, IH, COOH); 13 C NMR (DMSO-d 6 ) δ 18.4, 26.2, 30.3, 48.0, 53.1, 65.4, 127.0, 127.7, 127.8, 127.9, 128.4, 129.3, 137.0, 156.2, 173.7, 173.8, 174.1 ; HRMS (FAB + ) calcd for C 16 H 22 N 3 O 6 (M + H) + 352.1509, found 352.1502; [α] 25 D = -16.7 (c 0.76, MeOH). Although the invention has been described in detail for the puφose of illustration, it is understood that such detail is solely for that puφose, and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention which is defined by the following claims.

Claims: - 35 - WHAT IS CLAIMED: 1. A modified enzyme with one or more amino acid residues from an enzyme being replaced by cysteine residues, wherein at least some ofthe cysteine residues are modified by replacing thiol hydrogen in the cysteine residue with a thiol side chain to form a modified enzyme, wherein the modified enzyme has high esterase and low amidase activity. 2. A modified enzyme according to claim 1 , wherein the esterase activity is from about 350 s "1 mM "1 to about 11100 s "1 mM '1 . 3. A modified enzyme according to claim 1, wherein the amidase activity is from about 5.6 s "1 mM "1 to about 355 s "1 mM "1 . 4. A modified enzyme according to claim 1 , wherein the enzyme is a protease. 5. A modified enzyme according to claim 4, wherein the protease is a Bacillus lentus subtilisin. 6. A modified enzyme according to claim 1, wherein the amino acid replaced with a cysteine is an amino acid selected from the group consisting of asparagine, leucine, methionine, and serine. 7. A modified enzyme according to claim 1, wherein the amino acid replaced with a cysteine is in a subsite ofthe enzyme. 8. A modified enzyme according to claim 7, wherein the subsite is selected from the group consisting of Si, Si ', and S 2 . 9. A modified enzyme according to claim 1 , wherein the thiol side chain is selected from the group consisting of -SCH 2 (p-COOH-C 6 H 4 ) and -SCH 2 C 6 F 5 . - 36 - 10. A modified enzyme according to claim 9, wherein the thiol side chain is -SCH 2 (p-COOH-C 6 H 4 ). 11. A modified enzyme according to claim 9, wherein the thiol side chain is -SCH 2 C 6 F 5 . 12. A method of producing a modified enzyme comprising: providing an enzyme with one or more amino acids in the enzyme being replaced with cysteine residues and replacing thiol hydrogen in at least some ofthe cysteine residues with a thiol side chain to form a modified enzyme, wherein the modified enzyme has high esterase and low amidase activity. 13. A method according to claim 12, wherein the esterase activity is from about 350 s "1 mM '1 to about 11100 s "1 mM "1 . 14. A method according to claim 12, wherein the amidase activity is from about 5.6 s "1 mM "1 to about 355 s "1 mM "1 . 15. A method according to claim 12, wherein the enzyme is a protease. 16. A method according to claim 15, wherein the protease is a Bacillus lentus subtilisin. 17. A method according to claim 12, wherein the amino acid replaced with a cysteine is an amino acid selected from the group consisting of asparagine, leucine, methionine, and serine. 18. A method according to claim 12, wherein the amino acid replaced with a cysteine is in a subsite ofthe enzyme. 19. A method according to claim 18, wherein the subsite is selected from the group consisting of Si, Si', and S 2 . - 37 - 20. A method according to claim 12, wherein the thiol side chain is selected from the group consisting of -SCH (p-COOH-C H ) and -SCH 2 C 6 F 5 . 21. A method according to claim 20, wherein the thiol side chain is -SCH 2 (p-COOH-C 6 H 4 ). 22. A method according to claim 20, wherein the thiol side chain is -SCH 2 C 6 F 5 . 23. A method of peptide synthesis comprising: providing a modified enzyme with one or more amino acid residues in the enzyme being replaced by cysteine residues, wherein at least some ofthe cysteine residues are modified by replacing thiol hydrogen in the cysteine residue with a thiol side chain, wherein the modified enzyme exhibits high esterase and low amidase activity and combining an acyl donor, an acyl acceptor, and the modified enzyme under conditions effective to form a peptide product. 24. A method according to claim 23, wherein the enzyme is a protease. 25. A method according to claim 24, wherein the protease is a Bacillus lentus subtilisin. 26. A method according to claim 23, wherein the amino acid replaced with a cysteine is an amino acid selected from the group consisting of asparagine, leucine, methionine, and serine. 27. A method according to claim 23, wherein the amino acid replaced with a cysteine is in a subsite ofthe enzyme. 28. A method according to claim 27, wherein the subsite is selected from the group consisting of Si, Si', and S 2 . - 38 - 29. A method according to claim 23, wherein the thiol side chain is selected from the group consisting of -SCH 3 , -SCH 2 CH 3 , -SCH 2 CH(CH 3 ) 2 , -S(CH 2 ) 4 CH 3 , -S(CH 2 ) 5 CH 3 , -S(CH 2 ) 9 CH 3 , -SCH 2 C 6 H 5 , -SCH 2 CH 2 NH 3 + , -SCH 2 CH 2 SO 3 \ -SCH 2 (p- COOH-C 6 H 4 ), and -SCH 2 C 6 F 5 . 30. A method according to claim 29, wherein the thiol side chain is -SCH 2 (p-COOH-C 6 H 4 ). 31. A method according to claim 29, wherein the thiol side chain is -SCH 2 C 6 F 5 . 32. A detergent additive comprising the modified enzyme of claim 1. 33. A feed additive comprising the modified enzyme of claim 1. 34. A method for treating a textile comprising: providing a modified enzyme according to claim 1 and contacting the modified enzyme with a textile under conditions effective to produce a textile resistant to enzyme-sensitive stains.

Inventors: JONES J BRYAN, JONES, J., BRYAN
Applicants: GENENCOR INT, GENENCOR INTERNATIONAL, INC.
Application year: 1999
Main classification: A61K38
IPC codes: C07K1, C07K5, C11D3, C12N9, C12N15, C12P21