Penicillins antibiotics, tripeptide precursor

A simple example is the tripeptide precursor of the penicillin antibiotics, called ACV, an abbreviation for S-(L-a-aminoadipyl)-L-cysteinyl-D-valine. The amino acid precursors for ACV are L-a-aminoadipic acid (an unusual amino acid derived by modification of L-lysine), L-cysteine, and L-valine (not o-valine). 

Penicillin and cephalosporin antibiotics are usually classed as P-lactam antibiotics, since their common feature is a lactam function in a four-membered ring, typically fused to another ring system. This second ring takes in the P-lactam nitrogen atom and also contains sulfur. In the case of penicillins, e.g. benzylpenicillin, the second ring is a thiazolidine, and in the cephalosporins, e.g. cephalosporin C, this ring is a dihydrothiazine. What is not readily apparent from these structures is that they are both modified tripeptides and their biosyntheses share a common tripeptide precursor. 

P-Lactam Antibiotics.—The tripeptide [as (159)] isolated from Penicillium chry-sogenum151 has been accepted as a likely key intermediate in the biosynthesis of penicillins with penicillin N (158), the first of the fi -lactams to be formed.152 It has been shown recently that this tripeptide is formed from radio-active L-a-amino-adipic acid, L-valine, and L-cysteine in P. chrysogenum and has the absolute configuration (159).153 Moreover, it has been shown to be a precursor for penicillin... 

The penicillins, from the fungus Penicillium chryso-genum, are the oldest and most widely used antibiotics. They are formed through stepwise build-up from a tripeptide (ACV) derived from a-Amino adipic acid, cysteine, and valine. Successive oxidation steps form the p-lactam and close the thiazolidine ring to form isopenicillin N. Action of an acyltransferase then yields penicillin G (Fig. 47). Alternatively, hydrolysis of isopenicillin N (or penicillin G) yields 6-aminopenicillanic acid, a key precursor for the wide range of semisynthetic pencillins used therapeutically. 

Although cephalosporin C is divisable into a-aminoadipic acid, cysteine, and valine, the actual mechanism whereby Cephalosporium sp. incorporates the three amino acids into cephalosporin C has not been established, Arnstein and Morris isolated 8 (a-aminoadipyl) cysteinyl valine from mycelia of Penicillium chrysogenum and suggested that the tripeptide is a precursor in all penicillin biosynthesis.. This same tripeptide also appears to be found in the intracellular pool of Cephalosporium sp.- The final postulated step in the biosynthesis of penicillin is an acyl transfer reaction, or the production of 6-aminopeni-cillanic acid if precursor is not added. Cephalosporium sp. apparently do not produce sidechain amidases or acyl transferases, and no 7-ACA has been reported found in the fermentation. Thus, to obtain clinically useful antibiotics, chemical manipulation of cephalosporin C is necessary. Synthesis of many 7-acyl derivatives was possible once a practical cleavage reaction made available large amounts of 7-ACA from cephalosporin C. of these derivatives, sodium cephalothin was the first... 

The thiol-template mechanism is utilized in other enzymes involved in production of peptide-based antibiotics. Actinomycin synthetase II (ACMSII) and b-L-(a-aminoadipolyl)-L-cysteinyl-D-valine (ACV) synthetase catalyze the stereoinversion of valine residues vithin peptide-based antibiotics, and employ ATP and the PAN cofactor in a mechanism similar to that depicted in Fig. 7.14 [58, 59]. ACMSII catalyzes the stereoinversion of a valine within the tripeptide 4-MHA-L-Thr-D-Val (MHA, 4-methyl-3-hydroxyanthranilic acid), which is a precursor for the antibiotic actinomycin D. ACV synthetase catalyzes the stereoinversion of the valine within ACV, which is a precursor for penicillin and cephalosporin [60-63]. ACV synthetase has been shown to have much broader substrate specificity, also accepting non-natural substrates [64, 65]. 

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