Biosynthesis of the Monobactams

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. 

The investigation of the role of serine in the biosynthesis of the monobactam (252) was more complex. This amino acid could possibly be incorporated into the D-alanyl residue of the side chain as well as into the P-lactam ring. A mixture of and H-labelled serine was converted to alanine but with loss of 90% of the tritium label. However, when this mixture was fed to growing cells of Acetobacter sp., the tritium label was efficiently retained indicating that the labelled serine was being incorporated into the P-lactam ring rather than into the side chain alanyl residue. 

In a further double-isotope labelling experiment O Sullivan et al. showed that L-serine was incorporated into (253) produced by Agrobacterium radiobacter, again without loss of tritium label. 

In order to determine the origin of the P-lactam 3-methoxyl group the authors fed L-[methyl- C]-methionine. The resulting C-labelled (252) was purified by paper electrophoresis then heated with hydroiodic acid to liberate the methoxyl methyl group as methyl iodide which was trapped and counted. It was found that 87% of the C-label of (252) was recovered in the methyl iodide showing that the methoxyl methyl group was specifically derived from methionine as is the case in cephamycin biosynthesis. 

The bicyclic nucleus of the penicillins (43) is derived from the amino acids cysteine (254) and valine (255) as shown. 

---

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. 

Also included in Volume 3 is a report (Chapter 5) written for the nonmedical scientist describing the clinical uses of the compounds that result from these efforts. The concluding chapters of Volume 3 review two new subjects that are currently at the forefront of p-lactam research. The first of these. Chapter 6, discusses the events that occur in the bacterial cells between cell wall biosynthesis inhibition and cell lysis. It addresses the problem of penicillin-tolerant strains that have the ability to survive in the presence of large amounts of p-lactam antibiotics. The last chapter (7) again turns to a newly discovered class of antibacterial P-lactam natural products. These newly discovered monocyclic compounds, called monobactams, appear to have clinical utility. 

P-Lactams. AH 3-lactams are chemically characterized by having a 3-lactam ring. Substmcture groups are the penicillins, cephalosporias, carbapenems, monobactams, nocardicias, and clavulanic acid. Commercially this family is the most important group of antibiotics used to control bacterial infections. The 3-lactams act by inhibition of bacterial cell wall biosynthesis. 

---