Biosynthesis of salinosporamides from alpha,beta-unsaturated fatty acids: implications for extending polyketide synthase diversity

Abstract

A new series of coenzyme A-tethered polyketide synthase extender units were discovered in relation to the biosynthesis of the salinosporamide family of anticancer agents from the marine bacterium Salinispora tropica. In vivo and in vitro experiments revealed that the crotonyl-CoA reductase/carboxylase SalG has broad substrate tolerance toward 2-alkenyl-CoAs that give rise to the salinosporamide C-2 substitution pattern.

Scheme 1

Biosynthesis of salinosporamide A (3), its analogs and their substituted malonyl-CoA PKS building blocks (boxed). Abbreviations: ACP, acyl carrier protein; KS, ketosynthase; AT, acyltransferase; C, condensing domain; A adenylation domain; PCP, peptidyl carrier protein.

Figure 1

HPLC analysis at 210 nm of organic fractions of (A) wild-type S. tropica, (B) S. tropicasalL^- mutant, (C) S. tropica Strop_3612^- mutant (D) S. tropicasalG^- mutant, (E) S. tropicasalL^- + 0.8 mM pentanoic acid, (F) S. tropicasalL^- + 0.8 mM trans-2-pentenoic acid, (G) S. tropicasalL^- + 0.12 mM 4-bromocrotonoic acid, and (H) S. tropicasalL^- + 0.15 mM 4-fluorocrotonoic acid. Salinosporamide analogs 1-4 and 6-7 are noted, while derivatives of 3 are marked with an asterisk.