Polyketide synthase acyl carrier protein (ACP) as a substrate and a catalyst for malonyl ACP biosynthesis

Background: Using an acyl-acyl carrier protein (ACP) as a starter unit, typ e II polyketide synthases (PKSs) generate a wide range of polyketide produc ts by successive decarboxylative condensations with the two-carbon donor ma lonyl (ACP),ln vitro experiments have demonstrated that polyketide biosynth esis in reconstituted PKS systems requires the fatty acid synthase (FAS) en zyme malonyl COA:ACP acyltransferase (FabD) from streptomycetes, It has als o been shown that holo-ACPs from a type II PKS can catalyze self-malonylati on in the presence of malonyl CoA and negate this FabD requirement. The rel ative roles of FabD and ACP self-malonylation in PKS biosynthesis in vivo a re still not known. Results: We have examined the ACP specificity of the Streptomyces glaucesce ns FabD and shown that it reacts specifically with monomeric forms of ACP, with comparable k(cat)/K-M values for ACPs from both type II PKS and FAS sy stems, Incubations of tetracenomycin ACP (TcmM) with the Escherichia coli F AS ACP (AcpP) unexpectedly revealed that, in addition to the self-malonylat ion process, TcmM can catalyze the malonylation of AcpP. The k(cat)/K-M val ue for the TcmM-catalyzed malonylation of S, glaucescens FAS ACP is two ord ers of magnitude smaller than that observed for the FabD-catalyzed process, Conclusions: The ability of a PKS ACP to catalyze malonylation of a FAS ACP is a surprising finding and demonstrates for the first time that PKS ACPs: and FabD can:catalyze the same reaction. The differences in the catalytic e fficiency of these two proteins rationalizes in vitro observations that Fab D-independent polyketide biosynthesis proceeds only at high concentrations of a PKS ACP.