Butyrate metabolism in Streptomyces

Abstract

Streptomyces produce a mixture of branched-chain and straight-chain fatty acids. A series of in vivo and in vitro experiments in Streptomyces collinus demonstrated that branched-chain fatty acids are produced using the amino acid catabolites isobutyryl CoA and methylbutyryl CoA as starter units. Similarly, butyryl CoA was shown to be an efficient starter unit for even-numbered straight-chain fatty acids. In vitro analysis of fatty acid biosynthesis demonstrated that acetyl CoA functioned less efficiently than butyryl CoA as a starter unit for straight-chain fatty acid biosynthesis. In vivo analysis demonstrated that isomerization of the valine-catabolite isobutyryl CoA contributed to the butyryl CoA pool for straight-chain fatty acid biosynthesis. Butyryl CoA was also hypothesized to be derived from the reduction of crotonyl CoA. An NADPH specific alpha-beta-enoyl CoA reductase with a high substrate specificity for crotonyl CoA was purified and characterized. Effective inhibition of this enzyme by the major straight-chain fatty acids produced by S. collinus was consistent with the proposal that this enzyme provides butyryl CoA for straight-chain fatty acid biosynthesis. Thiolactomycin, a known Type II fatty acid synthase inhibitor was shown, in vitro, to inhibit palmitate biosynthesis from butyryl CoA. Thiolactomycin in vivo produced a dramatic increase in the ratio of straight-chain to branched-chain fatty acids. Stable isotope incorporation studies suggested that thiolactomycin suppressed the utilization of both isobutyryl CoA and butyryl CoA as starter units for fatty acid biosynthesis. These results suggest an additional pathway for the initiation of straight-chain fatty acid biosynthesis, possibly utilizing acetyl CoA.