Nocardicins biosynthesis

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. 

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). 

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). 

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. 

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). 

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. 

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). 

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. 

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