A Novel Δ3,Δ2-Enoyl-CoA Isomerase Involved in the Biosynthesis of the Cyclohexanecarboxylic Acid-Derived Moiety of the Polyketide Ansatrienin A,

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Abstract
The side chain of the antifungal polyketide ansatrienin A produced by Streptomyces collinus contains a cyclohexanecarboxylic acid (CHC) derived moiety. This CHC in the coenzyme A activated form (CHC-CoA) is derived from shikimic acid via a pathway in which the penultimate step is the isomerization of 2-cyclohexenylcarbonyl-CoA to 1-cyclohexenylcarbonyl-CoA. We have purified a 28 kDa 2-cyclohexenylcarbonyl-CoA isomerase (ChcB) from S. collinus and cloned and sequenced the corresponding chcB gene. The predicted amino acid sequence of ChcB showed moderate sequence identity to members of the hydratase/isomerase superfamily of enzymes. The recombinant ChcB was overexpressed in Escherichia coli and purified to homogeneity using metal chelate chromatography. Kinetic analysis demonstrated that recombinant ChcB had wide substrate specificity and could catalyze a double bond isomerization using 2-cyclohexenylcarbonyl-CoA (Km 116 ± 68 μM, kcat3.7 ± 1.0 min-1), trans-3-hexenyl-CoA (Km 39 ± 10 μM, kcat12.8 ± 1 min-1), and vinylacetyl-CoA (Km 156 ± 34 μM, kcat29 ± 3 min-1) as substrates. ChcB activity in cell extracts of S. collinus SP1, an insertionally disrupted chcB mutant, was shown to decrease by more than 99% (as compared to the wild-type strain) using all three of these substrates. The S. collinus SP1 strain, unlike the wild-type strain, could not produce ω-cyclohexyl fatty acids but was still able to grow efficiently on methyl oleate as a sole carbon source. These observations demonstrate that the S. collinus ChcB is required for catalyzing the isomerization of 2-cyclohexenylcarbonyl-CoA to 1-cyclohexenylcarbonyl-CoA during CHC-CoA biosynthesis but not for degradation of unsaturated fatty acids. The chcB gene does not appear to be associated with the ansatrienin biosynthetic gene cluster, which has previously been shown to contain at least one gene known to be essential for CHC-CoA biosynthesis. This finding represents a notable exception to the general rule regarding the clustering of polyketide biosynthetic pathway genes.