Studies on antibiotic biosynthesis

Spavold, Zöe Marie (1987) Studies on antibiotic biosynthesis. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

This thesis describes biosynthetic studies on three antibiotics - narasin, nonactin and tylosin, which belong to the polyether, macrotetrolide and macrolide classes of antibiotics, respectively. In order to determine the origins of the oxygen atoms, and so to investigate the biosynthetic pathway to narasin, feeding experiments with sodium [1-¹³C,¹⁸O₂] labelled acetate and butyrate were carried out, using cultures of S. aureofaciens. A culture was also grown under an atmosphere of ¹⁸O₂. In this manner, the biosynthetic origins of the oxygen atoms at C(20), C(25), C(28) and C(29) were identified as arising from molecular oxygen, and the remaining oxygen atoms from primary precursors. A biosynthetic pathway to narasin, based upon the polyepoxide cyclization model, has been proposed, and is consistent with these results. The biosynthesis of the macrotetrolide antibiotic nonactin has been studied in cultures of S. griseus. The synthesis of both isotopically labelled and unlabelled forms of the methyl ester derivatives (58) and (60), and the N-caprylcysteamine thiol derivative (67), of (2E,6RS)-6-hydroxy-2-methyl-8-oxo-2-nonenoic acid are described. The absolute incorporations of (58), (60) and (67) into nonactin by cultures of S. griseus were 10-25%. Syntheses of the (6R,8R)- and (6S8S-(2E)-6,8-dihydroxy-2-methyl-2-nonenoic acids and their corresponding methyl ester and thiol ester derivatives, in both labelled and unlabelled forms, are also described. The incorporation of the (2E,6R8R) acids (86) and (88), and of their N-caprylcysteamine thiol ester derivatives (87) and (89) into nonactin by cultures of S. griseus were 4% and 20% respectively. For the (2E,6S,8S)-acid (81), and its corresponding methyl ester (73), absolute incorporations of 20% were observed, whilst for the corresponding thiol ester (82), an incorporation of 40% was seen. Importantly, the incorporation of the thiol ester derivatives (87), (89) and (82) into nonactin was shown to occur stereospecifically, thus providing evidence for their intact incorporation by the microorganism. These results provide strong evidence for the involvement of the diols (82) and (89), and possibly the keto-alcohol (67) in the form of thiol esters, as real intermediates on the nonactin biosynthetic pathway. In order to study the late stages of tylactone formation, the first detectable intermediate in tylosin biosynthesis, a synthesis of an isotopically labelled, fully functionalized, open chain derivative of tylactone, activated as its N-caprylcysteamine thiol ester was carried out, based upon synthetic modification of tylactone. This material, in tritiated form, was incorporated into tylactone by S. fradiae, thereby providing the first indication that the last step in tylactone formation involves closure of the macrolide ring.