# Patent

## Title
Glucose dehydrogenase

## Abstract

A modified pyrroloquinoline quinone glucose dehydrogenase that exhibits a high selectivity for glucose is provided. A modified pyrroloquinoline quinone glucose dehydrogenase is disclosed in which the amino acid residue G at Position 99 of a pyrroloquinoline quinone glucose dehydrogenase (PQQGDH) represented by SEQ ID NO: 1, or the amino acid residue G at Position 100 of the pyrroloquinoline quinone glucose dehydrogenase (PQQGDH) represented by SEQ ID NO: 3, is substituted by the amino acid sequence TGZN (where Z is SX, S, or N and X is any amino acid residue). The modified PQQGDH of the present invention may additionally comprise one or more mutations selected from the group consisting of Q192G, Q192A, or Q192S; L193X; E277X; A318X; Y367A, Y367F, or Y367W; G451C; and N452X (where X is any amino acid residue).

## Background
TECHNICAL FIELD
The present application claims priority based on Japanese Patent Application No. 2007-163858 (filed 21 Jun. 2007), the contents of which are hereby incorporated by reference.
The present invention relates to a pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQGDH) and to its preparation and use in the glucose assay.
BACKGROUND ART
The blood glucose level is an important marker for diabetes. The use of PQQGDH has already been commercialized as one method for measuring the glucose concentration.
PQQGDH is a glucose dehydrogenase that employs pyrroloquinoline quinone as a coenzyme, and it catalyzes the oxidation of glucose with the production of gluconolactone. PQQGDH is known to occur as a membrane-bound enzyme and as a water-soluble enzyme. Membrane-bound PQQGDHs are single peptide proteins with molecular weights of approximately 87 kDa and are widely encountered in various Gram-negative bacteria. Water-soluble PQQGDH, on the other hand, has been identified in several strains of Acinetobacter calcoaceticus, and its structural gene was cloned and its amino acid sequence was determined (GenBank accession number X15871; Mol. Gen. Genet. (1989), 217: 430-436). The results of X-ray crystal structural analysis of water-soluble PQQGDH from Acinetobacter calcoaceticus have been reported and the higher order structure of the enzyme, and most importantly the active center, has been elucidated. (A. Oubrie et al., J. Mol. Biol., 289, 319-333 (1999); A. Oubrie et al., The EMBO Journal, 18(19), 5187-5194 (1999); A. Oubrie et al., PNAS, 96(21), 11787-11791 (1999)). Water-soluble PQQGDH from Acinetobacter baumannii has also been identified (GenBank accession number E28183).
PQQGDHs have a high oxidation activity for glucose and do not require oxygen as an electron acceptor because they are coenzyme-linked enzymes. As a result they are expected to find application in glucose assays and particularly as the recognition element of glucose sensors. A problem with PQQGDHs, however, is their low selectivity for glucose. In particular, PQQGDH also has a high activity for maltose, and thus accurate assay is difficult in patients receiving a maltose-containing infusion solution. In this case, the apparent blood sugar level will be higher than the actual blood sugar level, which could lead to a risk of hypoglycemia caused by administering insulin to the patient based on the measured level. Accordingly, a PQQGDH that exhibits a higher selectivity for glucose versus maltose is desired for the enzyme used for measurement of the blood sugar level.
The present inventor has already reported several modified PQQGDHs that exhibit an increased selectivity for glucose (for example, WO 00/66744, Japanese Patent Application Laid-open No. 2001-346587, and WO 2004/005499), but a modified PQQGDH that exhibits an even higher selectivity and/or an even higher enzymatic activity is still required.
The reference documents cited herein are listed below. The contents of these documents are hereby incorporated by reference in its entirety. None of these documents are admitted to constitute a prior art of the present invention.

    Patent Document 1: WO 00/66744
    Patent Document 2: Japanese Patent Application Laid-open No. 2001-346587
    Patent Document 3: WO 2004/005499
    Nonpatent Document 1: Mol. Gen. Genet. (1989), 217:430-436
    Nonpatent Document 2: A. Oubrie et al. (1999) J. Mol. Biol., 289, 319-333
    Nonpatent Document 3: A. Oubrie et al. (1999) The EMBO Journal, 18(19), 5187-5194
    Nonpatent Document 4: A. Oubrie et al. (1999) PNAS, 96(21), 11787-11791


DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a modified pyrroloquinoline quinone glucose dehydrogenase that exhibits a high selectivity for glucose.
The present inventor discovered that the selectivity for glucose is increased by the insertion, in a particular position in water-soluble PQQGDH, of a peptide fragment comprising 4 or 5 amino acids having a predetermined sequence.
The present invention provides a modified pyrroloquinoline quinone glucose dehydrogenase in which the amino acid residue G at Position 99 of a pyrroloquinoline quinone glucose dehydrogenase (PQQGDH) represented by SEQ ID NO: 1 or the amino acid residue G at Position 100 of the PQQGDH represented by SEQ ID NO: 3 is substituted by the amino acid sequence TGZN (where Z is SX, S, or N and X is any amino acid residue), and wherein from 1 to 10 of amino acid residues at Positions 1 to 98 and amino acid residues at Positions 100 to 478 of SEQ ID NO: 1, or amino acid residues at Positions 1 to 99 and amino acid residues at Positions 101 to 480 of SEQ ID NO: 3, may be substituted by any other amino acid residue(s). Z in the modified pyrroloquinoline quinone glucose dehydrogenase of the present invention is preferably SX and is particularly preferably SN.
Preferably, the modified pyrroloquinoline quinone glucose dehydrogenase of the present invention further comprises one or more mutations selected from the group consisting of the following amino acid substitutions:

    Q192G, Q192A, or Q192S;
    L193X (where X is any amino acid residue);
    E277X (where X is any amino acid residue);
    A318X (where X is any amino acid residue);
    Y367A, Y367F, or Y367W;
    G451C; and
    N452X (where X is any amino acid residue).


In another aspect the present invention provides a gene coding for the modified pyrroloquinoline quinone glucose dehydrogenase according to the present invention, a recombinant vector comprising the gene, and a transformant or transfectant that has been transformed with the recombinant vector. The present invention further provides a method of preparing the modified pyrroloquinoline quinone glucose dehydrogenase, comprising culturing a transformant that was transformed by a recombinant vector comprising a gene coding for the modified pyrroloquinoline quinone glucose dehydrogenase according to the present invention; and recovering the modified pyrroloquinoline quinone glucose dehydrogenase from the culture.
In an additional aspect the present invention provides a glucose assay kit comprising the modified pyrroloquinoline quinone glucose dehydrogenase according to the present invention. The present invention additionally provides an enzyme electrode comprising the modified pyrroloquinoline quinone glucose dehydrogenase according to the present invention, as well as a glucose sensor comprising this enzyme electrode as a working electrode.
The modified pyrroloquinoline quinone glucose dehydrogenase of the present invention exhibits a high selectivity for glucose as well as a high glucose oxidation activity, and can therefore be used for the highly selective and highly sensitive assay of glucose.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the enzymatic activity of the modified PQQGDH of the present invention for glucose and maltose.



PREFERRED EMBODIMENT OF THE INVENTION
Structure of the Modified PQQGDH
The modified pyrroloquinoline quinone glucose dehydrogenase of the present invention is characterized in that the amino acid residue G at Position 99 of the water-soluble PQQGDH shown by SEQ ID NO: 1 or the amino acid residue G at Position 100 of the PQQGDH shown by SEQ ID NO: 3 is substituted by the amino acid sequence TGZN (where Z is SX, S, or N and X is any amino acid residue). As used herein, the position of an amino acid in the amino acid sequence of the water-soluble PQQGDHs is numbered by assigning 1 to the initiation methionine.
The PQQGDH represented by SEQ ID NO: 1 is a PQQGDH from Acinetobacter calcoaceticus (GenBank accession number X15871) and the PQQGDH represented by SEQ ID NO: 3 is a PQQGDH from Acinetobacter baumannii (GenBank accession number E28183). These PQQGDHs have an approximately 92% homology at the level of the amino acid sequence. The alignment of the two sequences is shown below.









TABLE 1







_aln.pos
         10        20        30        40        50        60



Calcoace
 MNKHLLAKIALLSAVQLVTL-SAFADVPLTPSQFAKAKSENFDKKVILSNLNKPHALLWGPDNQIWLT
(SEQ ID NO: 1)


Baumannii
 MNKHLLAKITLLGAAQLFTFHTAFADIPLTPAQFAKAKTENFDKKVILSNLNKPHALLWGPDNQIWLT
(SEQ ID NO: 3)


_consrvd
 ************ * ** *   * **** ****** *****************************



 


_aln.p
 70        80        90       100       110       120       130



calocoace
 ERATGKILRVNPESGSVKTVFQVPEIVNDADGQNGLLGFAFHPDFKNNPYIYISGTFKNPKSTD



Baumannii
 ERATGKILRVNPVSGSAKTVFQVPEIVSDADGQNGLLGFAFHPDFKHNPYIYISGTFKNPKSTD



_consrvd
 ************ *** ********** ****************** *****************



 


_aln.pos
  140       150       160       170       180       190       200



calcoace
 KELPNQTIIRRYTYNKSTDTLEKPVDLLAGLPSSKDHQSGRLVIGPDQKIYYTIGDQGRNQLAYLFLP



Baumannii
 KELPNQTIIRRYTYNKTTDTFEKPIDLIAGLPSSKDHQSGRLVIGPDQKIYYTIGDQGRNQLAYLFLS



_consrvd
 **************** *** *** ** ***************************************



 


_aln.pos
    210       220       230       240       250       260       270



calcoace
 NQAQHTPTQQELNGKDYHTYWGKVLRLNLDGSIPKDNPSFNGVVSHIYTLGHRNPQGLAFTPNGKLLQ



Baumannii
 NQAQHTPTQQELNSKDYHTYMGKVLRLNLDGSIPKDNPSFNGVVSHIYTLGHRNPQGLAFAPNGKLLQ



_consrvd
 ************* ********************************************** *******



 


_aln.pos
      280       290       300       310       320       330       340



calcoace
 SEQGPNSDDEINLIVKGGNYGWPNVAGYKDDSGYAYANYSAAANK-SIKDLAQNGVKVAAGVPVTKES



Baumannii
 SEQGPNSDDEINLVLKGGNYGWPNVAGYKDDSGYAYANYSAATNKSQIKDLAQNGIKVATGVPVTKES



_consrvd
 *************  *************************** **  ******** *** ********



 


_aln.pos
        350       360       370       380       390       400



calcoace
 EWTGKNFVPPLKTLYTVQDTYNYNDPTCGEMTYICWPTVAPSSAYVYKGGKKAITGWENTLLVPSLKR



Baumannii
 EWTGKNFVPPLKTLYTVQDTYNYNDPTCGEMAYICWPTVAPSSAYVYTGGKKAIPGWENTLLVPSLKR



_consrvd
 ******************************* *************** ****** *************



 


_aln.p
410       420       430       440       450       460       470



calcoace
 GVIFRIKLDPTYSTTYDDAVPMFKSNNRYRDVIASPDGNVLYVLTDTAGNVQKDDGSVTNTLENPGSL



Baumannii
 GVIFRIKLDPTYSTTLDDAIPMFKSNNRYRDVIASPEGNTLYVLTDTAGNVQKDDGSVTHTLENPGSL



_consrvd
 *************** *** **************** ** ******************* ********



 


_aln.pos
  480



calcoace
 IKFTYKAK



Baumannii
 IKFTYNGK



_consrvd
 *****  *









These two PQQGDHs have similar secondary structures, and it is known that the properties of enzymes, for example, the thermal stability and substrate specificity, are similarly changed by the substitution of corresponding amino acid residues.
Z in the modified pyrroloquinoline quinone glucose dehydrogenase of the present invention is preferably SX and particularly preferably is SN. Thus, the amino acid sequence TGSXN, preferably TGSNN, and particularly preferably TGSKN, TGSRN, or TGSWN is inserted in place of the amino acid residue G at Position 99 in SEQ ID NO: 1 or in place of the amino acid residue G at Position 100 in SEQ ID NO: 3. TGSN or TGNN is also preferably inserted in place of this amino acid residue G.
The modified PQQGDH of the present invention has a higher selectivity for glucose than naturally occurring water-soluble PQQGDH. The modified PQQGDH of the present invention preferably has a lower reactivity for maltose versus reactivity for glucose than does the wild type. Given the reactivity for glucose is 100%, preferably the activity for maltose is not more than 50%, more preferably not more than 30%, even more preferably not more than 20%, and most preferably not more than 10%.
The modified PQQGDH of the present invention may have other mutations in addition to the mutation in the amino acid residue at Position 99 of SEQ ID NO: 1 or at Position 100 of SEQ ID NO: 3. For example, one or more, for example, from 1 to 10, of the amino acid residues at Positions 1 to 98 or at Positions 100 to 478 of SEQ ID NO: 1, or the amino acid residues at Positions 1 to 99 or at Positions 101 to 480 of SEQ ID NO: 3, may be substituted by any other amino acid residue.
In a preferred embodiment of the present invention, the modified PQQGDH of the present invention has one or more mutations, preferably from 1 to 10 mutations, for example, 1, 2, 3, 4, 5, or 6 mutations, selected from the group consisting of

    Q192G, Q192A, or Q192S;
    L193X (where X is any amino acid residue);
    E277X (where X is any amino acid residue);
    A318X (where X is any amino acid residue);
    Y367A, Y367F, or Y367W;
    G451C; and
    N452X (where X is any amino acid residue).

Wherein, the “Q192G” designation, for example, indicates that the glutamine at Position 192 in SEQ ID NO: 1, or the corresponding glutamine at Position 193 in SEQ ID NO: 3, is substituted by glycine. The other substitutional mutations are also indicated in the same manner.



A particularly preferred embodiment of the present invention is a modified pyrroloquinoline quinone glucose dehydrogenase comprising any of the following combinations of amino acid mutations:

    G99 (TGSXN)+Q192G+L193E;
    G99 (TGSXN)+Q192S+N452P;
    G99 (TGSXN)+Q192G+L193E+N452P;
    G99 (TGSXN)+Q192S+N452P;
    G99 (TGSNN)+N452P;
    G99 (TGSNN)+Q192G+L193E+N452P;
    G99 (TGSNN)+Q192S+N452P;
    G99 (TGSNN)+Q192G+N452P;
    G99 (TGSNN)+L193E+N452P;
    G99 (TGSNN)+Q192S+L193M+N452P;
    G99 (TGSNN)+A318Y+N452P;
    G99 (TGSNN)+Q192G;
    G99 (TGSNN)+Q192S;
    G99 (TGSNN)+Q192A;
    G99 (TGSNN)+Q192G+L193E;
    G99 (TGSNN)+Q192S+L193X;
    G99 (TGSNN)+Q192S+L193M;
    G99 (TGSNN)+Q192S+L193T;
    G99 (TGSNN)+Q192S+E277X;
    G99 (TGSNN)+Q192S+N452X;
    G99 (TGSNN)+Q192S+L193X+A318Y+N452P;
    G99 (TGSNN)+Q192S+A318X+N452P;
    G99 (TGSNN)+Q192G+L193E+A318X+N452P;
    G99 (TGSKN)+Q192S+N452P;
    G99 (TGSRN)+Q192S+N452P;
    G99 (TGSWN)+Q192S+N452P;
    G99 (TGSN)+Q192S+N452P;
    G99 (TGSN)+Q192G+L193E;
    G99 (TGSN)+Q192S+L193M;
    G99 (TGNN)+Q192S+N452P;
    G99 (TGNN)+Q192G+L193E; or
    G99 (TGNN)+Q192S+L193M.

Wherein X is any amino acid reside, for example, G99 (TGSXN) indicates that the G at Position 99 in the PQQGDH represented by SEQ ID NO: 1 is substituted by TGSXN.



Another preferred embodiment of the present invention is a modified pyrroloquinoline quinone glucose dehydrogenase comprising any of the following amino acid mutations:

    G99 (TGSXN)+Q192G+L193E;
    G99 (TGSXN)+Q192S+N452P;
    G99 (TGSNN)+Q192G+L193E+N452P;
    G99 (TGSNN)+Q192S+L193M+N452P;
    G99 (TGSNN)+Q192G+L193E+A318K+N452P; or
    G99 (TGSNN)+Q192G+L193E+A318Q+N452P.


WO 04/005499 discloses that the glutamine residue at Position 192, the leucine residue at Position 193, and the asparagine residue at Position 452 are involved in substrate recognition and binding by PQQGDH. In general, however, it is completely unpredictable how the substrate specificity and enzymatic activity will change when mutations are simultaneously introduced in amino acid residues present in different domains. A complete loss of enzymatic activity may even occur in some cases. Accordingly, it was totally unpredictable if the selectivity for glucose would be further enhanced by the simultaneous introduction of the above-indicated substitutions and an insertion mutation at the amino acid residue at Position 99 in SEQ ID NO: 1 or the amino acid residue at Position 100 in SEQ ID NO: 3.
Method of Producing Modified PQQGDH
The sequence of the gene coding for naturally occurring water-soluble PQQGDH from Acinetobacter calcoaceticus is shown in SEQ ID NO: 2, while the sequence of the gene coding for naturally occurring water-soluble PQQGDH from Acinetobacter baumannii is shown in SEQ ID NO:4. A gene coding for the modified PQQGDH of the present invention can be constructed by replacing the nucleotide sequence coding for the amino acid residue targeted for substitution in the gene coding for naturally occurring water-soluble PQQGDH with a nucleotide sequence coding for the desired amino acid residue. Methods for such a site-specific sequence substitution are well known in the art; for example, by PCR using suitably designed primers, as is described in the examples provided below.
The thus obtained mutant gene is inserted in an expression vector (for example, a plasmid), which is then transformed into a suitable host (for example, E. coli). A large number of vector/host systems for the expression of foreign protein are known in the art. A variety of hosts, for example, bacteria, yeast, cultured cells, and so forth are available.
In addition, a portion of other amino acid residues in the modified PQQGDH of the present invention may also be deleted or substituted, and other amino acid residues may be added, insofar as it has a desired glucose dehydrogenase activity. Various methods for site-specific nucleotide sequence substitution are well known in the art.
The modified PQQGDH-expressing transformant obtained as described above is then cultured, and the cells are recovered from the culture fluid by, for example, centrifugation. The recombinant protein present in the periplasmic compartment is subsequently released into culture medium by grinding the cells, for example, with a French press, or by osmotic shock. Ultracentrifugation is thereafter carried out to obtain a water-soluble fraction containing the modified PQQGDH. Alternatively, through the use of a suitable host-vector system, the expressed modified PQQGDH can be secreted into the culture fluid. The modified PQQGDH of the present invention can be isolated by purifying the water-soluble fraction by, for example, ion-exchange chromatography, affinity chromatography, HPLC, and so forth.
Method of Measuring the Enzymatic Activity
The modified PQQGDH of the present invention has the ability to catalyze the oxidation of glucose with PQQ as its coenzyme, with the production of gluconolactone. To measure the enzymatic activity, the quantity of PQQ that is reduced accompanying the PQQGDH-mediated glucose oxidation can be quantitated by the color reaction of a redox dye. For example, phenazine methosulfate (PMS), 2,6-dichlorophenolindophenol (DCIP), potassium ferricyanide, ferrocene, and so forth, can be used as the chromogenic reagent.
Selectivity for Glucose
The selectivity for glucose exhibited by the modified PQQGDH of the present invention can be evaluated by measuring the enzymatic activity in the manner described above using various sugars as substrates, e.g., 2-deoxy-D-glucose, mannose, allose, 3-o-methyl-D-glucose, galactose, xylose, lactose, maltose, and so forth, and determining the relative activity with reference to the activity using glucose as the substrate.
The modified PQQGDH of the present invention provides a higher selectivity for glucose than the wild-type enzyme and in particular has a higher reactivity for glucose than for maltose. Accordingly, an assay kit or enzyme sensor constructed using the modified PQQGDH of the present invention will exhibit a high selectivity with regard to glucose measurement and offers the advantage of high-sensitivity glucose detection even when a maltose-containing sample is used.
Glucose Assay Kit
Another aspect of the present invention is a glucose assay kit comprising a modified PQQGDH according to the present invention. The glucose assay kit of the present invention comprises the modified PQQGDH according to the present invention in a quantity sufficient for conducting at least one assay. In addition to the modified PQQGDH of the present invention, the kit will typically comprise a buffer required for the assay, a mediator, a glucose reference solution for constructing a calibration curve, and instructions for use. The modified PQQGDH according to the present invention can be provided in various forms, for example, as a freeze-dried reagent or as a solution in a suitable storage solution. The modified PQQGDH of the present invention is preferably provided in a form of holoenzyme, but can also be provided as the apoenzyme and then converted into the holoenzyme before use.
Glucose Sensor
Additional aspect of the present invention is an enzyme electrode that carries a modified PQQGDH according to the present invention, and a glucose sensor comprising the enzyme electrode. For example, a carbon electrode, gold electrode, or platinum electrode can be used as the electrode, and the enzyme according to the present invention is immobilized on the electrode. The immobilization method can be exemplified by the use of a crosslinking reagent; enclosure in a polymer matrix; coating with a dialysis film; use of a photocrosslinking polymer, electroconductive polymer, or redox polymer; immobilization in a polymer or adsorptive immobilization on the electrode together with an electron mediator such as ferrocene and its derivatives; and the use of combinations of the preceding. The modified PQQGDH of the present invention is preferably immobilized on the electrode in a holoenzyme form, but can also be immobilized in an apoenzyme form with the PQQ being provided in a separate layer or in a solution. Typically, the modified PQQGDH of the present invention is immobilized on a carbon electrode via glutaraldehyde followed by treatment with an amine group-containing reagent to block the aldehyde groups of glutaraldehyde.
Measurement of the glucose concentration can be carried out as follows. Buffer solution is introduced into a thermostatted cell; PQQ, CaCl2, and a mediator are added; and the cell is held at a constant temperature. Potassium ferricyanide, phenazine methosulfate, and so forth, may be used as the mediator. An electrode bearing the immobilized modified PQQGDH of the present invention is used as the working electrode, in combination with a counterelectrode (for example, a platinum electrode) and a reference electrode (for example, the Ag/AgCl electrode). A constant voltage is applied to the carbon electrode, and after the current becomes constant, a glucose-containing sample is added and the increase in the current is measured. The glucose concentration in the sample can be determined using a calibration curve constructed using glucose solutions of standard concentrations.
The contents of all the patents and reference documents explicitly cited herein are hereby incorporated by reference in its entirety.
The present invention is described in detail based on the following examples, but is not limited to these examples.
EXAMPLE 1
Construction of Modified PQQGDH-Encoding Genes
Mutation was introduced into the structural gene for water-soluble PQQGDH from Acinetobacter calcoaceticus, which is represented by SEQ ID NO: 2. In brief, PCR was carried out using a full length forward primer for the wild-type water-soluble PQQGDH and a mutagenic reverse primer, and another PCR was carried out using a full length reverse primer and a mutagenic forward primer. These PCR products were mixed and PCR was run using a full length forward primer and reverse primer to obtain a gene coding for the mutated full length PQQGDH. The product was sequenced to conform that the desired mutation was correctly introduced.
The sequence of the primers for full length amplification were as follows.










forward:
(SEQ ID NO: 5)









AACAGACCATGGATAAACATTTATTGGC



 










reverse:
(SEQ ID NO: 6)









ACAGCCAAGCTTTTACTTAGCCTTATAGG






The sequences of the mutagenic primers (F: forward, R: reverse) are shown in the following table.










TABLE 2








SEQ





ID


Mutation
Primer Sequence

NO


















TGNN
ACTGGAAATAATCAGAATGGTTTATTAGGTTTT
F
7



CTGATTATTTCCAGTATCAGCATCATTGACAAT
R
8


 


TGQN
ACTGGACAGAATCAGAATGGTTTATTAGGTTTT
F
9



CTGATTCTGTCCAGTATCAGCATCATTGACAAT
R
10


 


TGSN
ACTGGAAGCAATCAGAATGGTTTATTAGGTTTT
F
11



CTGATTGCTTCCAGTATCAGCATCATTGACAAT
R
12


 


TGGN
ACTGGAGGTAATCAGAATGGTTTATTAGGTTTT
F
13



CTGATTACCTCCAGTATCAGCATCATTGACAAT
R
14


 


TGSSN
GATACTGGAAGCAGCAATCAGAATGGTTTATTA
F
15



ATTGCTGCTTCCAGTATCAGCATCATTGACAAT
R
16


 


TGWN
ACTGGATGGAATCAGAATGGTTTATTAGGTTTT
F
17



CTGATTCCATCCAGTATCAGCATCATTGACAAT
R
18


 


TGFN
ACTGGATTTAATCAGAATGGTTTATTAGGTTTT
F
19



CTGATTAAATCCAGTATCAGCATCATTGACAAT
R
20


 


TGDN
ACTGGAGATAATCAGAATGGTTTATTAGGTTTT
F
21



CTGATTATCTCCAGTATCAGCATCATTGACAAT
R
22


 


TGSHN
GATACTGGAAGCCATAATCAGAATGGTTTATTA
F
23



ATTATGGCTTCCAGTATCAGCATCATTGACAAT
R
24


 


TGSLN
GATACTGGAAGCTTAAATCAGAATGGTTTATTA
F
25



ATTTAAGCTTCCAGTATCAGCATCATTGACAAT
R
26


 


TGSVN
GATACTGGAAGCGTCAATCAGAATGGTTTATTA
F
27



ATTGACGCTTCCAGTATCAGCATCATTGACAAT
R
28


 


TGSQN
GATACTGGAAGCCAAAATCAGAATGGTTTATTA
F
29



ATTTTGGCTTCCAGTATCAGCATCATTGACAAT
R
30


 


TGSEN
GATACTGGAAGCGAAAATCAGAATGGTTTATTA
F
31



ATTTTCGCTTCCAGTATCAGCATCATTGACAAT
R
32


 


TGSDN
GATACTGGAAGCGATAATCAGAATGGTTTATTA
F
33



ATTATCGCTTCCAGTATCAGCATCATTGACAAT
R
34


 


TGSPN
GATACTGGAAGCCCTAATCAGAATGGTTTATTA
F
35



ATTAGGGCTTCCAGTATCAGCATCATTGACAAT
R
36


 


TGSTN
GATACTGGAAGCACAAATCAGAATGGTTTATTA
F
37



ATTTGTGCTTCCAGTATCAGCATCATTGACAAT
R
38


 


TGSIN
GATACTGGAAGCATTAATCAGAATGGTTTATTA
F
39



ATTAATGCTTCCAGTATCAGCATCATTGACAAT
R
40


 


TGSAN
GATACTGGAAGCGCTAATCAGAATGGTTTATTA
F
41



ATTAGCGCTTCCAGTATCAGCATCATTGACAAT
R
42


 


TGSWN
GATACTGGAAGCTGGAATCAGAATGGTTTATTA
F
43



ATTCCAGCTTCCAGTATCAGCATCATTGACAAT
R
44


 


TGSGN
GATACTGGAAGCGGTAATCAGAATGGTTTATTA
F
45



ATTACCGCTTCCAGTATCAGCATCATTGACAAT
R
46


 


TGSFN
GATACTGGAAGCTTTAATCAGAATGGTTTATTA
F
47



ATTAAAGCTTCCAGTATCAGCATCATTGACAAT
R
48


 


TGSYN
GATACTGGAAGCTATAATCAGAATGGTTTATTA
F
49



ATTATAGCTTCCAGTATCAGCATCATTGACAAT
R
50


 


TGSCN
GATACTGGAAGCTGCAATCAGAATGGTTTATTA
F
51



ATTGCAGCTTCCAGTATCAGCATCATTGACAAT
R
52


 


TGSMN
GATACTGGAAGCATGAATCAGAATGGTTTATTA
F
53



ATTCATGCTTCCAGTATCAGCATCATTGACAAT
R
54


 


TGSKN
GATACTGGAAGCAAAAATCAGAATGGTTTATTA
F
55



ATTTTTGCTTCCAGTATCAGCATCATTGACAAT
R
56


 


TGSRN
GATACTGGAAGCCGTAATCAGAATGGTTTATTA
F
57



ATTACGGCTTCCAGTATCAGCATCATTGACAAT
R
58


 


TGSNN
GATACTGGAAGCAATAATCAGAATGGTTTATTA
F
59



ATTATTGCTTCCAGTATCAGCATCATTGACAAT
R
60









Other substitutional mutations, e.g., mutations such as Q192G and L193E were introduced using conventional site-specific mutagenesis methods as described in WO 00/66744. In addition, modified PQQGDHs that incorporated a combination of mutations at a plurality of sites were prepared by site-specific mutagenesis using a plurality of primers that corresponded to the individual mutations or were prepared by a recombinant method using restriction enzymes.
EXAMPLE 2
Production of Modified PQQGDH
The gene coding for wild-type PQQGDH or the gene coding for the modified PQQGDH constructed as described above was inserted in the multicloning site of an E. coli expression vector pTrc99A (Pharmacia), and the constructed plasmid was transformed into E. coli. The transformant was cultured with shaking overnight at 37° C. in 450 mL L-broth containing 50 μg/mL ampicillin and 30 μg/mL chloramphenicol. The culture was inoculated into 7 L of L-broth containing 1 mM CaCl2 and 500 μM PQQ. At approximately 3 hours after the start of cultivation, isopropylthiogalactoside was added at a final concentration of 0.3 mM and continued cultivation for 1.5 hours. The cells were recovered from the culture medium by centrifugation (5000×g, 10 minutes, 4° C.) and suspended in 150 μL 10 mM MOPS (pH 7.0). Glass beads (0.105 to 0.125 mm for bacteria) were added in an amount that was 1⁄2 to 2⁄3 of the volume and vortexed for 20 minutes at 4° C. 1 mL 10 mM MOPS (pH 7.0) was added and centrifuged (15,000 rpm, 20 minutes, 4° C.). The supernatant was collected and used as a crude enzyme preparation in the following examples.
EXAMPLE 3
Measurement of the Enzymatic Activity
900 μL of a mixture of 10 mM MOPS (pH 7.0)+1 mM PQQ+1 mM CaCl2 holoenzyme conversion solution was added to 100 μL of the crude enzyme preparation of the wild-type PQQGDH or the modified PQQGDH obtained in Example 2 and left stand (room temperature, in the dark, at least 30 minutes) to allow for conversion to the holoenzyme. After completion of conversion to the holoenzyme, the solution was diluted 10× to prepare an enzyme test solution. To 150 μL of the enzyme test solution was added 50 μL of a given concentration of the substrate (glucose or maltose), 0.6 mM phenazine methosulfate (PMS), and 0.3 mM 2,6-dichlorophenolindophenol (DCIP, final concentrations in each case), and incubated at room temperature in the total volume of 200 μL. Glucose and maltose were used as the substrates at final concentrations of 0, 1, 2, 4, 10, 20, and 40 mM. The change in the DCIP absorbance at 600 nm was monitored with a spectrophotometer. The rate of decline in the absorbance was measured to determine the reaction rate of the enzyme. In the experiments described herein, 1 unit was designated as the enzymatic activity that reduces 1 μmol of DCIP in 1 minute with the molar absorption coefficient of 16.3 mM−1 of DCIP at pH 7.0. The protein concentration was measured using a commercially available protein assay kit (BioRad).
Typical results are given in the following tables indicated by the ratio (%) of the activity for 4 mM maltose with reference to the activity for 4 mM glucose.







TABLE 3





TGSXN


















Wild Type
79



Q192G + L193E
9.0



TGSAN
54



TGSCN
47



TGSDN
60



TGSEN
51



TGSGN
52



TGSHN
46



TGSIN
42



TGSKN
43



TGSMN
43



TGSNN
42



TGSPN
47



TGSQN
42



TGSRN
43



TGSSN
57



TGSTN
47



TGSVN
39



TGSWN
33



TGSFN
58



TGSLN
49



TGSYN
63

















TABLE 4





TGSXN + Q192G + L193E


















Wild Type
79



Q192G + L193E
9.0



TGSGN + Q192G + L193E
6.7



TGSMN + Q192G + L193E
5.7



TGSNN + Q192G + L193E
4.2



TGSPN + Q192G + L193E
6.6



TGSQN + Q192G + L193E
5.2



TGSRN + Q192G + L193E
6.0



TGSSN + Q192G + L193E
9.3



TGSTN + Q192G + L193E
7.4



TGSVN + Q192G + L193E
6.9



TGSAN + Q192G + L193E
15



TGSDN + Q192G + L193E
16



TGSEN + Q192G + L193E
20



TGSHN + Q192G + L193E
9.8



TGSIN + Q192G + L193E
20



TGSKN + Q192G + L193E
5.7



TGSCN + Q192G + L193E
17



TGSWN + Q192G + L193E
7.6

















TABLE 5





TGSNN + Q192S + E277X


















TGSNN + Q192S
12



TGSNN + Q192S + E277A
20



TGSNN + Q192S + E277D
26



TGSNN + Q192S + E277I
15



TGSNN + Q192S + E277T
22



TGSNN + Q192S + E277Q
16



TGSNN + Q192S + E277V
23



TGSNN + Q192S + E277W
38



TGSNN + Q192S + E277Y
27



TGSNN + Q192S + E277C
12



TGSNN + Q192S + E277F
66



TGSNN + Q192S + E277G
34



TGSNN + Q192S + E277H
39



TGSNN + Q192S + E277K
29



TGSNN + Q192S + E277L
76



TGSNN + Q192S + E277M
64



TGSNN + Q192S + E277N
56



TGSNN + Q192S + E277S
13



TGSNN + Q192S + E277R
24



TGSNN + Q192S + E277P
39

















TABLE 6





TGSNN + Q192S + N452X


















TGSNN + Q192S + N452A
8.9



TGSNN + Q192S + N452C
12



TGSNN + Q192S + N452D
20



TGSNN + Q192S + N452E
nd



TGSNN + Q192S + N452F
10



TGSNN + Q192S + N452G
14



TGSNN + Q192S + N452H
13



TGSNN + Q192S + N452I
14



TGSNN + Q192S + N452K
18



TGSNN + Q192S + N452L
7.8



TGSNN + Q192S + N452M
11



TGSNN + Q192S + N452P
1.9



TGSNN + Q192S + N452Q
nd



TGSNN + Q192S + N452R
27



TGSNN + Q192S + N452S
11



TGSNN + Q192S + N452T
14



TGSNN + Q192S + N452V
8.8



TGSNN + Q192S + N452W
79



TGSNN + Q192S + N452Y
9.3

















TABLE 7





TGSNN + Q192G/S/A


















Wild Type
79



Q192G + L193E
9.0



TGSNN + Q192G + L193E
4.2



Q192G
20



TGSNN + Q192G
9.1



Q192A
15



TGSNN + Q192A
7.8



Q192S
28



TGSNN + Q192S
12

















TABLE 8





TGSNN + Q192S + L193X


















TGSNN + Q192S
12



TGSNN + Q192G + L193E
4.2



TGSNN + Q192S + L193G
6.8



TGSNN + Q192S + L193A
7.3



TGSNN + Q192S + L193K
3.7



TGSNN + Q192S + L193R
7.7



TGSNN + Q192S + L193H
8.3



TGSNN + Q192S + L193D
6.8



TGSNN + Q192S + L193E
4.3



TGSNN + Q192S + L193N
8.9



TGSNN + Q192S + L193Q
11



TGSNN + Q192S + L193S
5.7



TGSNN + Q192S + L193T
6.3



TGSNN + Q192S + L193Y
7.4



TGSNN + Q192S + L193C
9.3



TGSNN + Q192S + L193M
4.0



TGSNN + Q192S + L193F
7.8



TGSNN + Q192S + L193W
8.2



TGSNN + Q192S + L193V
7.0



TGSNN + Q192S + L193I
6.9



TGSNN + Q192S + L193P
6.1




















TABLE 9









Wild Type
79



TGSNN
42



TGSNN + Q192S
12



TGSNN + N452P
14



TGSNN + Q192G + L193E
4.2



TGSNN + Q192S + L193M
4.0



TGSNN + Q192S + L193T
5.8



TGSNN + Q192S + N452P
1.9



TGSNN + Q192G + N452P
1.6



TGSNN + Y367A + N452P
50



TGSNN + Y367F + N452P
12



TGSNN + Y367W + N452P
15



TGSNN + Q192S + Y367A + N452P
nd



TGSNN + Q192S + Y367F + N452P
2.7



TGSNN + Q192S + Y367W + N452P
2.0



TGSNN + L193E + N452P
1.0



TGSNN + Q192G + L193E + N452P
0.6




















TABLE 10









Wild Type
79



G451C
74



TCSRN
33



TCSRN + G451C
44



N452P
30



TGSNN + Q192S + L193M
4.0



TGSNN + Q192S + L193M + N452P
0.8



TGSNN + Q192G + L193E + E277K
2.7



TGSNN + Q192G + L193E + N452P
0.6



TGSNN + N452P
14



TGSNN + A318Y + N452P
7.8

















TABLE 11





TGSXN + Q192S + N452P


















TGSAN + Q192S + N452P
7.6



TGSCN + Q192S + N452P
3.9



TGSDN + Q192S + N452P
3.6



TGSFN + Q192S + N452P
26



TGSGN + Q192S + N452P
3.6



TGSLN + Q192S + N452P
2.2



TGSMN + Q192S + N452P
2.5



TGSNN + Q192S + N452P
1.9



TGSPN + Q192S + N452P
19



TGSSN + Q192S + N452P
7.4



TGSTN + Q192S + N452P
3.8



TGSVN + Q192S + N452P
3.7



TGSIN + Q192S + N452P
3.4



TGSWN + Q192S + N452P
1.2



TGSYN + Q192S + N452P
1.4



TGSEN + Q192S + N452P
5.8



TGSHN + Q192S + N452P
4.0



TGSQN + Q192S + N452P
5.1



TGSRN + Q192S + N452P
1.5

















TABLE 12





TGSXN + Q192G + L193E + N452P


















TGSAN + Q192G + L193E + N452P
4.1



TGSFN + Q192G + L193E + N452P
nd



TGSHN + Q192G + L193E + N452P
2.3



TGSIN + Q192G + L193E + N452P
1.6



TGSLN + Q192G + L193E + N452P
1.4



TGSNN + Q192G + L193E + N452P
0.6



TGSQN + Q192G + L193E + N452P
8.4



TGSTN + Q192G + L193E + N452P
nd



TGSVN + Q192G + L193E + N452P
2.3



TGSCN + Q192G + L193E + N452P
44



TGSDN + Q192G + L193E + N452P
7.2



TGSMN + Q192G + L193E + N452P
7.3



TGSKN + Q192G + L193E + N452P
2.4



TGSPN + Q192G + L193E + N452P
5.4



TGSRN + Q192G + L193E + N452P
3.1



TGSSN + Q192G + L193E + N452P
5.2



TGSWN + Q192G + L193E + N452P
2.7



TGSYN + Q192G + L193E + N452P
3.1

















TABLE 13





TGSXN + Q192S + N452P


















TGSNN
42



TGSKN
43



TGSRN
43



TGSWN
33



TGSNN + Q192S
12



TGSKN + Q192S
14



TGSRN + Q192S
8.1



TGSWN + Q192S
7.2



TGSNN + N452P
14



TGSKN + N452P
16



TGSRN + N452P
15



TGSWN + N452P
10



TGSNN + Q192S + N452P
1.9



TGSKN + Q192S + N452P
1.8



TGSRN + Q192S + N452P
1.5



TGSWN + Q192S + N452P
1.2

















TABLE 14





TGSNN + Q192S + L193X + A318Y + N452P


















TGSNN + Q192S + L193M + N452P
0.8



TGSNN + Q192S + A318Y + N452P
1.5



TGSNN + Q192S + L193G + A318Y + N452P
1.0



TGSNN + Q192S + L193A + A318Y + N452P
8.8



TGSNN + Q192S + L193K + A318Y + N452P
0.5



TGSNN + Q192S + L193R + A318Y + N452P
8.8



TGSNN + Q192S + L193H + A318Y + N452P
9.0



TGSNN + Q192S + L193D + A318Y + N452P
7.8



TGSNN + Q192S + L193E + A318Y + N452P
6.6



TGSNN + Q192S + L193N + A318Y + N452P
1.4



TGSNN + Q192S + L193Q + A318Y + N452P
2.2



TGSNN + Q192S + L193S + A318Y + N452P
1.0



TGSNN + Q192S + L193T + A318Y + N452P
8.5



TGSNN + Q192S + L193Y + A318Y + N452P
8.1



TGSNN + Q192S + L193C + A318Y + N452P
8.9



TGSNN + Q192S + L193M + A318Y + N452P
2.4



TGSNN + Q192S + L193F + A318Y + N452P
8.4



TGSNN + Q192S + L193W + A318Y + N452P
9.0



TGSNN + Q192S + L193V + A318Y + N452P
0.9



TGSNN + Q192S + L193I + A318Y + N452P
7.7



TGSNN + Q192S + L193P + A318Y + N452P
8.2

















TABLE 15





TGSNN + Q192S + A318X + N452P


















TGSNN + Q192S + N452P
1.9



TGSNN + Q192S + A318G + N452P
2.6



TGSNN + Q192S + A318K + N452P
0.0



TGSNN + Q192S + A318R + N452P
3.0



TGSNN + Q192S + A318H + N452P
2.0



TGSNN + Q192S + A318D + N452P
2.1



TGSNN + Q192S + A318E + N452P
2.3



TGSNN + Q192S + A318N + N452P
2.4



TGSNN + Q192S + A318Q + N452P
2.8



TGSNN + Q192S + A318S + N452P
nd



TGSNN + Q192S + A318T + N452P
5.1



TGSNN + Q192S + A318Y + N452P
2.3



TGSNN + Q192S + A318C + N452P
nd



TGSNN + Q192S + A318M + N452P
3.0



TGSNN + Q192S + A318F + N452P
3.0



TGSNN + Q192S + A318W + N452P
0.1



TGSNN + Q192S + A318V + N452P
0.4



TGSNN + Q192S + A318L + N452P
3.0



TGSNN + Q192S + A318I + N452P
4.5



TGSNN + Q192S + A318P + N452P
nd

















TABLE 16





TGSNN + Q192G + L193E + A318X + N452P


















TGSNN + Q192G + L193E + N452P
0.6



TGSNN + Q192G + L193E + A318G + N452P
0.7



TGSNN + Q192G + L193E + A318K + N452P
0.7



TGSNN + Q192G + L193E + A318R + N452P
0.7



TGSNN + Q192G + L193E + A318H + N452P
0.8



TGSNN + Q192G + L193E + A318D + N452P
1.0



TGSNN + Q192G + L193E + A318E + N452P
0.9



TGSNN + Q192G + L193E + A318N + N452P
0.6



TGSNN + Q192G + L193E + A318Q + N452P
0.5



TGSNN + Q192G + L193E + A318S + N452P
0.6



TGSNN + Q192G + L193E + A318T + N452P
nd



TGSNN + Q192G + L193E + A318Y + N452P
1.0



TGSNN + Q192G + L193E + A318C + N452P
0.7



TGSNN + Q192G + L193E + A318M + N452P
0.5



TGSNN + Q192G + L193E + A318F + N452P
0.8



TGSNN + Q192G + L193E + A318W + N452P
1.0



TGSNN + Q192G + L193E + A318V + N452P
nd



TGSNN + Q192G + L193E + A318L + N452P
0.6



TGSNN + Q192G + L193E + A318I + N452P
1.9



TGSNN + Q192G + L193E + A318P + N452P
nd

















TABLE 17





TGSI(K, R, W)N


















TGSNN
42



TGSKN
43



TGSRN
43



TGSWN
33



TGSNN + Q192S
12



TGSKN + Q192S
14



TGSRN + Q192S
8.1



TGSWN + Q192S
7.2



TGSNN + N452P
14



TGSKN + N452P
16



TGSRN + N452P
15



TGSWN + N452P
10



TGSNN + Q192S + N452P
1.9



TGSKN + Q192S + N452P
1.8



TGSRN + Q192S + N452P
1.5



TGSWN + Q192S + N452P
1.2

















TABLE 18





TGSN


















TGSN
nd



TGSN + N452P
38



TGSN + Q192S + N452P
9.5



TGSN + Q192G + L193E
11



TGSN + Q192G + L193E + N452P
2.5



TGSN + Q192S + L193M
16



TGSN + Q192S + L193M + N452P
5.5

















TABLE 19





TGNN


















TGNN
38



TGNN + N452P
22



TGNN + Q192S + N452P
5.3



TGNN + Q192G + L193E
11



TGNN + Q192G + L193E + N452P
2.1



TGNN + Q192S + L193M
17



TGNN + Q192S + L193M + N452P
2.3










As is clear from the tables, the modified PQQGDH of the present invention in all cases exhibited a reactivity for glucose that was higher than that for maltose.
EXAMPLE 4
Fabrication and Evaluation of an Enzyme Sensor
20 mg carbon paste was added to 5 units of the modified PQQGDH of the present invention and freeze-dried. After thorough mixing, it was filled onto the surface of a carbon paste electrode that carries approximately 40 mg carbon paste, and polished on filter paper. The electrode was treated for 30 minutes at room temperature in 10 mM MOPS buffer (pH 7.0) containing 1% glutaraldehyde, and then treated for 20 minutes at room temperature in 10 mM MOPS buffer (pH 7.0) containing 20 mM lysine in order to block the glutaraldehyde. The electrode was equilibrated for at least one hour at room temperature in 10 mM MOPS buffer (pH 7.0). The electrode was stored at 4° C.
The glucose concentration was measured using the enzyme sensor prepared above. Glucose was quantitatively measured in the range from 0.1 mM to 5 mM using the enzyme sensor having immobilized the modified PQQGDH of the present invention.
EXAMPLE 5
Measurement of the Enzymatic Activity Using a Purified Enzyme Preparation
A cation-exchange chromatography column packed with TSKgel CM-TOYOPEARL 650M (Tosoh Corporation) was equilibrated with 10 mM phosphate buffer at pH 7.0 and the wild-type crude enzyme or the modified PQQGDH crude enzymes obtained in Example 2 was adsorbed on the column. The column was washed with 750 mL 10 mM phosphate buffer (pH 7.0) and the enzyme was then eluted with 10 mM phosphate buffer (pH 7.0) containing from 0 to 0.2 M NaCl. The flow rate was 5 mL/minute. The fraction exhibiting GDH activity was collected and was dialyzed overnight against 10 mM MOPS-NaOH buffer (pH 7.0) to obtain an electrophoretically-homogeneous modified PQQGDH protein. The enzymatic activity of the purified enzyme preparations for glucose and maltose was measured as in Example 4. Typical results are shown in the following table.










TABLE 20









ENZYMATIC




ACTIVITY
Mal:Glu



(Glu; 4 mM)
(%)










MODIFIED PQQGDH
(U/mg)
4:4
40:4













TGSNN + Q192G + L193E
357
4.1
28*


TGSNN + Q192S + N452P
723
3.5
19*


TGSNN + Q192G + L193E +
213
0.56
  5.7


N452P


TGSNN + Q192S + L193M +
285
2.0
16*


N452P


TGSNN + Q192G + L193E +
381
0.99
  8.92


A318K + N452P


TGSNN + Q192G + L193E +
469
0.55
  6.74


A318Q + N452P





*measured at Mal:Glu = 50:5






In addition, the enzymatic activity for glucose and maltose of the modified PQQGDH having the TGSNN+Q192G+L193E+N452P mutations is shown in FIG. 1.
EXAMPLE 6
Comparison with Prior-Art Modified PQQGDHs
The enzymatic activity for glucose and maltose was measured as in Example 4 using the wild-type crude enzyme obtained in Example 2, the modified PQQGDH crude enzymes obtained in Example 2, and, for comparison, prior-art modified PQQGDHs lacking the mutation at amino acid residue G at Position 99 of PQQGDH but having the 1 or 2 or more amino acid substitutions at other positions.
The following table shows typical results for the enzymatic activity of the modified PQQGDH of the present invention having various mutations and substitution of 99G with TGSNN, a comparative PQQGDH having the same mutation(s) but not substitution at 99G and wild-type PQQGDH. The results are expressed by the activity for 4 mM glucose or 10 mM glucose and for the ratio of the activity for maltose to the activity for glucose (%, 4 mM maltose:4 mM glucose or 10 mM:10 mM).










TABLE 21









ACTIVITY FOR
Mal/Glu



GLUCOSE (U/mg)
(%)












4 mM
10 mM
4:4
10:10















WILD TYPE
47.69
86.96
73.5
75.5


TGSNN + Q192G + L193E
10.63
20.20
4.2
5.1


Q192G + L193E
6.11
11.62
9.0
10.5


TGSNN + Q192G
15.90
26.36
9.1
11.9


Q192G
19.30
31.79
20.2
26.1


TGSNN + Q192A
26.26
48.56
7.8
9.9


Q192A
21.44
41.32
15.2
18.2


TGSNN + Q192S
35.83
60.94
12.9
17.0


Q192S
26.66
44.91
28.0
37.8


TGSNN + N452P
58.71
110.01
16.3
18.7


N452P
56.53
103.43
26.4
33.1


TGSNN + Q192S + N452P
37.98
72.23
1.5
2.2


Q192S + N452P
28.93
56.12
5.0
6.6









As shown in the table, the enzymatic activity of the modified enzymes according to the present invention with the insertion of the TGSNN sequence for glucose is either maintained or increased compared to the corresponding modified enzymes having the same mutations but lacking the insertion of TGSNN. In addition, the maltose-versus-glucose activity ratio is reduced to about one-half to one-fifth, and the selectivity for glucose is thus improved. For example, it has already been reported that the substrate specificity is increased in the modified PQQGDH having a double mutation Q192G/L193E. The activity of the Q192G/L193E modified PQQGDH for 4 mM glucose was 6.1 U/mg and the 4 mM maltose-versus-4 mM glucose activity ratio was 9%. In contrast, the additional insertion of TGSNN provided an approximately 1.7-fold increase in the activity for 4 mM glucose to 10.63 U/mg, and a 2.14-fold increase in substrate specificity, with 4.2% for the 4 mM maltose-versus-4 mM glucose activity ratio.
As demonstrated by these results, the modified PQQGDH of the present invention has a high enzymatic activity for glucose and a high substrate specificity for glucose over maltose.
Industrial Applicability
The present invention is useful for assaying glucose and particularly for measuring blood sugar level.

## Claims

1. A modified pyrroloquinoline quinone glucose dehydrogenase, wherein the amino acid residue G at Position 99 of a pyrroloquinoline quinone glucose dehydrogenase (PQQGDH) represented by SEQ ID NO: 1 is substituted by the amino acid sequence TGZN (where Z is SX, S, or N and X is any amino acid residue), and wherein from 1 to 10 of amino acid residues at Positions 1 to 98 or at Positions 100 to 478 of SEQ ID NO: 1, may be substituted by any other amino acid residue(s).

2. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 1, wherein Z is SX (where X is any amino acid residue).

3. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 1, wherein Z is SN.

4. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 1, further comprising one or more mutations selected from the group consisting of the following amino acid substitutions:
Q192G, Q192A, or Q192S;
L193X (where X is any amino acid residue);
E277X (where X is any amino acid residue);
A318X (where X is any amino acid residue);
Y367A, Y367F, or Y367W;
G451C; and
N452X (where X is any amino acid residue).

5. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 4, comprising any of the following combinations of amino acid mutations:
G99(TGSXN)+Q192G+L193E;
G99(TGSXN)+Q192S+N452P;
G99(TGSXN)+Q192G+L193E+N452P;
G99(TGSXN)+Q192S+N452P;
G99(TGSNN)+N452P;
G99(TGSNN)+Q192G+L193E+N452P;
G99(TGSNN)+Q192S+N452P;
G99(TGSNN)+Q192G+N452P;
G99(TGSNN)+L193E+N452P;
G99(TGSNN)+Q192S+L193M+N452P;
G99(TGSNN)+A318Y+N452P;
G99(TGSNN)+Q192G;
G99(TGSNN)+Q192S;
G99(TGSNN)+Q192A;
G99(TGSNN)+Q192G+L193E;
G99(TGSNN)+Q192S+L193X;
G99(TGSNN)+Q192S+L193M;
G99(TGSNN)+Q192S+L193T;
G99(TGSNN)+Q192S+E277X;
G99(TGSNN)+Q192S+N452X;
G99(TGSNN)+Q192S+L193X+A318Y+N452P;
G99(TGSNN)+Q192S+A318X+N452P;
G99(TGSNN)+Q192G+L193E+A318X+N452P;
G99(TGSKN)+Q192S+N452P;
G99(TGSRN)+Q192S+N452P;
G99(TGSWN)+Q192S+N452P;
G99(TGSN)+Q192S+N452P;
G99(TGSN)+Q192G+L193E;
G99(TGSN)+Q192S+L193M;
G99(TGNN)+Q192S+N452P;
G99(TGNN)+Q192G+L193E; or
G99(TGNN)+Q192S+L193M

where X is any amino acid residue.

6. The modified pyrroloquinoline quinone glucose dehydrogenase according to claim 5, comprising any of the following combinations of amino acid mutations:
G99(TGSXN)+Q192G+L193E;
G99(TGSXN)+Q192S+N452P;
G99(TGSNN)+Q192G+L193E+N452P;
G99(TGSNN)+Q192S+L193M+N452P;
G99(TGSNN)+Q192G+L193E+A318K+N452P; or
G99(TGSNN)+Q192G+L193E+A318Q+N452P.

7. A method for analyzing glucose level using the modified pyrroloquinoline quinone glucose dehydrogenase according to claim 1.

8. A glucose assay kit comprising the modified pyrroloquinoline quinone glucose dehydrogenase according to claim 1.

9. An enzyme electrode comprising the modified pyrroloquinoline quinone glucose dehydrogenase according to claim 1.

10. A glucose sensor comprising the enzyme electrode according to claim 9 as a working electrode.