Chemistry with an Artificial Primer of Polyhydroxybutyrate Synthase Suggests a Mechanism for Chain Termination

[Image: see text] Polyhydroxybutyrate (PHB) synthases (PhaCs) catalyze the conversion of 3-(R)-hydroxybutyryl CoA (HBCoA) to PHB, which is deposited as granules in the cytoplasm of microorganisms. The class I PhaC from Caulobacter crescentus (PhaC(Cc)) is a highly soluble protein with a turnover number of 75 s(–1) and no lag phase in coenzyme A (CoA) release. Studies with [1-(14)C]HBCoA and PhaC(Cc) monitored by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE) and autoradiography reveal that the rate of elongation is much faster than the rate of initiation. Priming with the artificial primer [(3)H]sTCoA and monitoring for CoA release reveal a single CoA/PhaC, suggesting that the protein is uniformly loaded and that the elongation process could be studied. Reaction of sT-PhaC(Cc) with [1-(14)C]HBCoA revealed that priming with sTCoA increased the uniformity of elongation, allowing distinct polymerization species to be observed by SDS–PAGE and autoradiography. However, in the absence of HBCoA, [(3)H]sT-PhaC unexpectedly generates [(3)H]sDCoA with a rate constant of 0.017 s(–1). We propose that the [(3)H]sDCoA forms via attack of CoA on the oxoester of the [(3)H]sT-PhaC chain, leaving the synthase attached to a single HB unit. Comparison of the relative rate constants of thiolysis by CoA and elongation by PhaC(Cc), and the size of the PHB polymer generated in vivo, suggests a mechanism for chain termination and reinitiation.