Cyclosporins, biosynthesis synthesis

It should be noted that while TE domains represent the most common solution in releasing macrocyclic NRPs and PKs, other pathways are known. For instance, in the biosynthesis of cyclosporine, the cyclization is proposed to be catalyzed by the most downstream C-domain [48]. Macrocyclization can also occur under reduction of a carbonyl group mediated by a reduction domain (R-domain) as proposed in the synthesis of the macrocyclic imine nostocyclopeptide [49]. The synthetic utility of these cyclization strategies has not yet been reported. 

In more recent investigations, the assumed multienzyme involved in cyclosporin A biosynthesis could be isolated from T. inflatum. A partially purified enzyme fraction was indeed capable of forming enzyme-substrate complexes by thioester linkage. Although de novo synthesis (in vitro) of cyclosporin A has not yet been achieved, the formation of a partial sequence, namely, the diketopiperazine cyclo(DAla-MeLeu), from D-alanine and L-leucine was observed under consumption of ATP and S-adenosyl-L-methio-nine [25]. 

Table 1.4. IMMUNOSUPPRESSIVE ACTIVITY OF CYCLOSPORINS nat = natural dbs = by directed biosynthesis ps = by partial synthesis syn = by total synthesis + + +, strong immunosuppressive activity + +, moderate activity +, weak activity (+) or -, no significant activity.

Some antibiotics that have been derived from peptides were mentioned in Chapter l. The biosynthesis of penicillins was discussed in Chapter 8. Many peptide antibiotics are known. Some find clinical applications but others such as gramicidin S (9.7), tyrocidine A (9.8) and polymyxins (9.9) are too toxic for use in humans. Cyclosporin A (Figure 1.4), however, has immunosuppressive properties and it has been used in transplant surgery for this reason rather than for its antibiotic properties. Peptide antibiotics have some non-standard structural features and these may explain in part their antibiotic properties. First, cyclic peptides are not found in animal cells. Secondly, peptide antibiotics usually contain some unusual amino acids they may have the d configuration, be A-methylated or have other non-standard structural features. Clearly, these features are not compatible with direct ribo-somal synthesis. 

Even in the case it should be possible to separate ribozyme activity from the ribosome or to isolate an in vitro selected ribozyme that can catalyze the same type of peptide bond formation as a ribosome, however such a biocatalyst seem does not to be suitable for simple practical use rather than using a chemical coupling reagent. In principle, this conclusion is also valid for the nonribosomal poly- or multienzymes which are involved in the biosynthesis of peptide antibiotics[7Z. Up to now, they have only found application in the synthesis field of cyclosporin, gramicidin S, special P-lactam antibiotics and analogs. 

Tlie sequential reactions in elongating acyl transfers in the synthesis of polyketide natural products and non-ribosomal peptide antibiotics such as erythromycin, rapamycin, epotliilone, lovastatin, penicillins, cyclosporin and vancomycin resemble molecular solid-state assembly lines. Such multimodular enzymes may be utilized in combinatorial biosynthesis by way of reprogramming for the manufacture of unnaUiral analogs of natural products. 

Contents L. Jaenicke and F.-J. Marner The Irones and Their Precursors. — M. Lounasmaa and P. Somersalo The Condylocarpine Group of Indole Alkaloids. — U. Sequin The Antibiotics of the Pluramycin Group (4//-Anthra [l,2- ]pyran Antibiotics). — R. M. Wenger Cyclosporine and Analogues — Isolation and Synthesis — Mechanism of Action and Structural Requirements for Pharmacological Activity. — H. Inouye and S. Uesato Biosynthesis of Iridoids and Secoiridoids. 

Dittman J. Wenger RM, Kleirdcauf H, La wen A. Mechanism of cyclosporin A biosynthesis. Evidence for synthesis via a single linear undecapeptide precursor. J Biol Chem 1994 269 2841-2846. 

The in vitro systems established for the synthesis of CyA and the peptolide SDZ 214-103, as welt as the detailed knowledge of the biosynthesis of the amino acid Bmt, will, furthermore, enable access to new cyclosporin and peptolide analogs with an enhanced or altered spectrum of biological effects, for example, exerting anti-HIV activity. 

Offentelier M, Su Z, Samer G, Moser H, Traber R, Memmert K, Schneider-Scherzer E. Biosynthesis of the unusual amino acid (4R) 4-[(E) 3 butenyl] 4 rnethyl L-thrconine of cyclosporin A. Identilkation of 3(K -hydroxv-4(R)-methyl-6(E)-octenoic acid as a key inter-mediate by enzymatic in vitro synthesis and by in vivo labeling techniques. J Biol Chem 1993 268 26127-26134. 

Zocher R, Keller U, Kleinkauf H. Enniatin synthetase, a novel type of multifunctional enzyme catalyzing depsipeptide synthesis in Fusarium oxysporum. Biochem 1982 21 43-48. Zocher R, Nihira T, Paul E, Madry N, Pecters H, Kleinkauf H. Keller U. Biosynthesis of cyclosporin A Partial purification and properties of a mulcifunctional enzyme from Tolypocla-diutn mjkitum. Biochem 1986 25 550-553. 

Zocher, R., Nihira, T., Paul, E., Madry, N., Peeters, H., Kleinkauf, H. and Keller, U. (1986) Biosynthesis of cyclosporin A Partial purification and properties of a multifunctional enzyme from Tolypocladium inflatum. Biochemistry, 25, 550-553. Zocher, R. and Kellei U. (1997) Thiol template peptide synthesis systems in bacteria and fungi. In Poole, R.K. (ed.). Advances in Microbial Physiology. Academic Press, San Diego, pp. 85-131. 

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