Erythromycin Biosynthesis: Kinetic Studies on a Fully Active Modular Polyketide Synthase Using Natural and Unnatural Substrates

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Abstract
6-Deoxyerythronolide B synthase (DEBS) is a modular polyketide synthase (PKS) that catalyzes the biosynthesis of the parent macrolide of erythromycin. On the basis of a recently developed cell-free assay (Pieper et al., 1995a) we report the results of steady-state kinetic studies on a modular PKS. A truncated form of DEBS (DEBS 1+TE), in which DEBS 1 is fused to the thioesterase domain from the C-terminal end of DEBS 3, was used for most of these studies. The overall kcat for (2S,3S,4S,5R)-2,4-dimethyl-3,5-dihydroxy-n-heptanoic acid δ-lactone (C9-lactone) synthesis is 3.4 min-1, indicating that the enzyme is at least as active in vitro as in vivo. The apparent Km for (2S)-methylmalonyl-CoA consumption by DEBS 1+TE is 24 μM. The catalytic activity of DEBS 1+TE is strongly dependent on the phosphate concentration in the reaction buffer in the range 0−250 mM, suggesting that hydrophobic interactions may be crucial to the assembly of DEBS monomers into a functional complex. Although DEBS 1+TE can convert acetyl-, propionyl-, or butyryl-CoA into the corresponding C8-, C9-, and C10-lactones (Pieper et al., 1995b), it has a 32-fold preference for a propionate primer over an acetate primer and a 7.5-fold preference for a propionate primer over a butyrate primer. In the absence of any added primer unit, synthesis can be primed via decarboxylation of methylmalonyl-CoA; under these conditions the overall kcat for polyketide synthesis remains unchanged. Decarboxylation of methylmalonyl-CoA is negligible in the presence of saturating concentrations of propionyl-CoA but competes with the priming of the enzyme by acetyl-CoA or butyryl-CoA. The kcat for 6-deoxyerythronolide B synthesis by the complete DEBS is 0.5 min-1. Under these assay conditions, the C9-lactone is also produced as an abortive chain elongation product with a kcat of 0.23 min-1, presumably due to inefficient assembly of the multimeric protein complex involving DEBS 1, 2, and 3. Together, these results provide the first comprehensive kinetic insights into a fully active modular PKS.