Biosynthesis of Nocardicin

Examination of the structure of nocardicin A (1) reveals that the compound contains three a-amino acid units and one a-oximino acid, residue and suggests that the compound is probably derived from a peptide. 

The first reported study on the biosynthesis of nocardicin A was by Hosoda etal, who administered a variety of C-labelled compounds to shaken fermentations of Nocardia miformis subsp. tsuyamonensis 206). Examination of the resulting nocardicin A showed that L-[U- C]-tyrosine, [G- CJ-shikimic acid, and L-[U- C]-serine were all incorporated into the antibiotic. [U- C]-Glycine and L-[U- C]-homoserine were also incorporated but to a lesser degree, whereas the labels from L-[l- C]-tyrosine, L-[U- C]-phenylalanine, L-[U- C]-alanine and DL-[U- C]-a-amino-butyric acid were not incorporated to any significant extent. 

By use of the above degradations the authors showed that L-[U- C]-tyrosine was incorporated specifically into the two aromatic residues, L-[U- C]-homoserine was incorporated specifically into the D-homo-serinyl residue and L-[U- C]-serine was incorporated specifically into the p-lactam ring of nocardicin A. 

Townsend et al. extended their studies on serine incorporation by showing that L-[2- H, l- C]-serine was incorporated into nocardicin A with a 19% retention of tritium label which they concluded was adequate to rule out the possibility of a dehydroalanyl intermediate 208). Neither the carbon label nor the tritium label of D-[2- H, l- C]-serine was incorporated. The authors also synthesised serines labelled with a deuterium atom on C-3 in the form of the enantiomeric pairs (247) and (248), and (249) and (250). These pairs were not resolved as only the L-isomer would be 

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Tritium at C-2 of either (110) or its D-isomer was lost on formation of (113), and the L-isomer (110) was the preferred precursor of nocardicin. These results parallel those for the utilization of valine in the biosynthesis of penicillins, and it has been suggested that the configurational inversion of L-(/ -hydroxyphenyl)glycine (110) which necessarily occurs in the course of the biosynthesis of nocardicin may also parallel the inversion of L-valine which occurs in the biosynthesis of penicillins (cf. above). 

An example of 0-amino-carboxy-propylation is provided by the biosynthesis of nocardicin A (p-lactam antibiotic produced by the actinomyceteNocarafia uniformis) (Section 4.1.5) (Figure 1.8). 

Neither L-alanine nor L-cysteine was incorporated into (113), but both L-serine (112) and glycine were. Good evidence was obtained that utilization of glycine is by way of L-serine. L-Serine was utilized without loss of the tritium that was sited at C-3, so the construction of the /Mactam ring occurs without change in the oxidation state at this carbon atom,97 in contrast to the parallel situation in the biosynthesis of penicillins.2 For nocardicin A (113), direct nucleophilic displacement of a (presumably activated) hydroxy-group of a seryl unit by amide nitrogen apparently occurs. 

The monocyclic lactams include the nocardicins formed by actinomycetes and monobactams formed mostly by bacteria and possessing antibiotic activities with differing sensitivities to /8-lactamases. For biosynthesis of clavulanic acid see Lit. 

In order to confirm the role of serine the same workers administered a mixture of L-[U- C]-serine and L-[3- H]-serine to a C. violaceum fermentation. The resulting " C-labelled (34) showed 101% retention of the tritium label. Similarly, feeding a mixture of L-[3- C]-serine and L-[3- H]-serine resulted in 83% tritium retention in the product. However in two experiments where mixtures of L-[U- C]-cystine and L-[3,3 - H]-cysteine were fed only 14% and 20% of the tritium label was retained in the products. These experiments clearly indicated that serine is a closer precursor of (34) than cysteine. Again, the retention of tritium label from L-[3- H]-serine is reminiscent of nocardicin biosynthesis and suggests that the 3-lactam ring is closed by an Sjq2 displacement of the serine hydroxyl. 

P-Lactams. AH p-lactams are chemically characterized by having a p-lactam ring. Substmcture groups are the penicillins, cephalosporins, carbapenems, monobactams, nocardicins, and clavulanic acid. Commercially this family is the most important group of antibiotics used to control bacterial infections. The P-lactams act by inhibition of bacterial cell wall biosynthesis. 

Nocardicins.—Nocardicin A has the unusual structure (167), part of which is a /3-lactam ring. Its biosynthesis has been studied and it has been observed that two molecules of L-tyrosine (with proven loss of the carboxy-group), L-homoserine, and L-serine account for the carbon skeleton of (167). It has been suggested that the tyrosine is utilized by way of L-p-hydroxy-phenylglycine (168). 

Since the spectrum of a compound is relatively simple compared with its analogue, chemical shifts may be measured directly. This can be extremely useful in stereochemical problems. One example. Nocardicin biosynthesis, has already been discussed. The NMR of selectively substituted 2-benza-mido-4,5-d2"" orborneols allows easy distinction between the exo and endo deuterons (Figure 9) (1). Such stereochemical information was crucial to mechanistic studies of reactions involving protonated cyclopropanes (4,7). 

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