Teicoplanin

# Overview
Teicoplanin is an antibiotic used in the prophylaxis and treatment of serious infections caused by Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus and Enterococcus faecalis. It is a semisynthetic glycopeptide antibiotic with a spectrum of activity similar to vancomycin. Its mechanism of action is to inhibit bacterial cell wall synthesis.
Teicoplanin is marketed by Sanofi-Aventis under the trade name Targocid. Other trade names include Teconin marketed by Biocon ltd (India).
Oral teicoplanin has been demonstrated to be effective in the treatment of pseudomembranous colitis and Clostridium difficile-associated diarrhoea, with comparable efficacy with vancomycin.
Its strength is considered to be due to the length of the hydrocarbon chain.

# Susceptibility data
Teicoplanin targets peptidoglycan synthesis making it an effective antimicrobial against Gram-positive bacteria including Staphylococci and Clostridium spp. The following represents MIC susceptibility data for a few medically significant pathogens:
- Clostridium difficile: 0.06 g/ml - 0.5 g/ml
- Staphylococcus aureus: 0.06 g/ml - 128 g/ml
- Staphylococcus epidermidis: 0.06 g/ml - 32 g/ml

# Chemistry
Teicoplanin (TARGOCID, marketed by Sanofi Aventis Ltd) is actually a mixture of several compounds, five major (named teicoplanin A2-1 through A2-5) and four minor (named teicoplanin RS-1 through RS-4).
All teicoplanins share a same glycopeptide core, termed teicoplanin A3-1  a fused ring structure to which two carbohydrates (mannose and N-acetylglucosamine) are attached. The major and minor components also contain a third carbohydrate moiety  -D-glucosamine  and differ only by the length and conformation of a side-chain attached to it.
The structures of the teicoplanin core and the side-chains that characterize the five major teicoplanin compounds are shown below.

# Biosynthesis
Teicoplanin refers to a complex of related natural products isolated from the fermentation broth of a strain of Actinoplanes teichomyceticus, consisting of a group of five structures.  These structures possess a common aglycone, or core, consisting of seven amino acids bound by peptide and ether bonds to form a four-ring system.  These five structures differ by the identity of the fatty acyl side-chain attached to the sugar.  The origin of these seven amino acids in the biosynthesis of teicoplanin was studied by 1H and 13C nuclear magnetic resonance.  The studies indicate amino acids AA1, AA2, AA4, AA5, and AA6 are derived from tyrosine, and amino acids AA3 and AA7 are derived from acetate. To be specific, teicoplanin contains 4-hydroxyphenylglycine and 3,5-dihydroxyphenylglycine residues, a chlorine atom attached on each of the tyrosine residues, and three sugar moieties, N-fatty acyl--D-glucosamine, N-acetyl--D-glucosamine, and D-mannose.

## Gene cluster
The study of the genetic cluster encoding the biosynthesis of teicoplanin identified 49 putative open reading frames (ORFs) involved in the compound's biosynthesis, export, resistance, and regulation.  Thirty-five of these ORFs are similar to those found in other glycopeptide gene clusters. The function of each of these genes is described by  Li and co-workers.  A summary of the gene layout and purpose is shown below.
Gene layout.  The genes are numbered.  The letters L and R designate transcriptional direction.  The presence of the - symbol means a gene is found after NRPs, which are represented by A, B, C, and D.  Based on the figure from: Li, T-L.; Huang, F.; Haydock, S. F.; Mironenko, T.; Leadlay, P. F.; Spencer, J. B. Chemistry & Biology. 2004, 11, p. 109.

## Heptapeptide backbone synthesis
Analysis indicated tyrosine and three types of nonproteinogenic amino acids, (S)-4-hydroxyphenylglycine, 3,5-dihydroxyphenylglycine, and -hydroxytyrosine as the building blocks of the teicoplanin group of glycopeptides.  In all, six of the seven total amino acids of the teicoplanin backbone are composed of nonproteinogenic or modified amino acids.  Eleven enzymes are coordinatively induced to produce these six required residues.  Teicoplanin contains two chlorinated positions, 2 (3-Cl-Tyr) and 6 (3-Cl--Hty). The putative halogenase Orf8- has been proposed to catalyze the halogenation on both amino acids.  Chlorination is thought to occur at a very early point in the biosynthesis prior to phenolic oxidative coupling, with the possibility of tyrosine or -hydroxytyrosine being the substrate of chlorination.
The biosynthesis of the heptapeptide backbone is carried out by four nonribosomal peptide synthetases designated TeiA, TeiB, TeiC, and TeiD.  Each of the modules has a domain for amino acid selection and activation as the aminoacyl-AMP.  The catalytic domains in modules one and three of the nonribosomal peptide synthetase assembly line select and activate (S)-4-hydroxyphenylglycine and 3,5-dihydroxyphenylglycine.  In addition to these modules for amino acid selection and activation, each module has a thiolation domain modified with phosphopantetheine to provide a thiol for covalent aminoacyl-S-enzyme formation.

## Modification after heptapeptide backbone formation
Once the heptapeptide backbone has been formed, the cyclization of the linear structure is begun.  Gene disruption studies indicate a cytochrome P450 oxygenase as the enzyme that performs the coupling reactions.  OxyB has been suggested to form the first ring by coupling residues 4 and 6.  OxyA then couples residues 2 and 4, followed by the formation of a C-C bond between residues 5 and 7 by OxyC.  A fourth enzyme catalyzes the coupling of residues 1 and 3, although where this coupling fits into the OxyB/OxyA/OxyC order is not known.  The regioselectivity and atropisomer selectivity of these probable one-electron coupling reactions has been suggested to be due to the folding and orientation requirements of the partially crossed-linked substrates in the enzyme active site.  The coupling reactions are shown below.
Specific glycosylation has been shown to occur after the formation of the heptpeptide aglycone.  Data suggest three separate glycosyl transferases are required for the glycosylation of the teicoplanin aglycone.  Two of these glycosyl transferases are involved in the addition of the N-fatty acyl--D-glucosamine and N-acetyl--D-glucosamine units.  A third mannosyl transferase is responsible for the addition of the D-mannose unit onto residue 7.  The fatty acyl chain is connected by amide bond to the glucosamine moiety by the action of an acyl transferase.  In addition to glycosylation, some genes have been suggested to code for deacetylases.  In addition to the ability to salvage portions of the molecular structure, it provides a way to protect/deprotect the molecule.