A Streptomyces collinus Thiolase with Novel Acetyl-CoA:Acyl Carrier Protein Transacylase Activity,

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Abstract
Acetyl-CoA:acyl carrier protein (ACP) transacylase (ACT) activity has been demonstrated for the 3-ketoacyl-ACP synthase III (KASIII) which initiates fatty acid biosynthesis in the type II dissociable fatty acid synthases of plants and bacteria. Several lines of evidence have indicated the possibility of ACT activity being associated with proteins other than KASIII. Using a crude extract of Streptomyces collinus, we have resolved from KASIII an additional protein with ACT activity and subsequently purified it 85-fold in five chromatographic steps. The 45 kDa protein was shown by gel filtration to have a molecular mass of 185 +/- 35 kDa, consistent with a homotetrameric structure for the native enzyme. The corresponding gene (fadA) was cloned and sequenced and shown to encode a protein with amino acid sequence homology to type II thiolases. The fadA was expressed in Escherichia coli, and the resulting recombinant FadA enzyme purified by metal chelate chromatography was shown to have both ACT and thiolase activities. Kinetic studies revealed that in an ACT assay FadA had a substrate specificity for a two-carbon acetyl-CoA substrate (K(m) 8.7 +/- 1.4 microM) but was able to use ACPs from both type II fatty acid and polyketide synthases (Streptomyces glaucescens FabC ACP, K(m) 10.7 +/- 1.4 microM; E. coli FabC ACP, K(m) 8.8 +/- 2 microM; FrenN ACP, K(m) 44 +/- 12 microM). In the thiolase assay kinetic analyses revealed similar K(m) values for binding of substrates acetoacetyl-CoA (K(m) 9.8 +/- 0.8 microM) and CoA (K(m) 10.9 +/- 1.8 microM). A Cys92Ser mutant of FadA possessed virtually unchanged K(m) values for acetoacetyl-CoA and CoA but had a greater than 99% decrease in k(cat) for the thiolase activity. No detectable ACT activity was observed for the Cys92Ser mutant, demonstrating that both activities are associated with FadA and likely involve formation of the same covalent acetyl-S-Cys enzyme intermediate. An ACT activity with ACP has not previously been observed for thiolases and in the case of the S. collinus FadA is significantly lower (k(cat) 3 min(-1)) than the thiolase activity of FadA (k(cat) 2170 min(-1)). The ACT activity of FadA is comparable to the KAS activity and significantly higher than the ACT activity, reported for a streptomycete KASIII.