In vitro analysis of the chain termination reaction in the synthesis of poly-(R)-β-hydroxybutyrate by the class III synthase from Allochromatium vinosum

Allochromatium vinosum polyhydroxyalkanoate synthase catalyzes formation of poly-(R)-3-hydroxybutyrate (PHB) from (R)-3-hydroxybutyryl-coenzyme A (HB-CoA). (R)-3-Hydroxybutyryl-N-acetylcysteamine (HB-NAC) is an alternative substrate for this synthase in vitro, with a turnover 1% that of HB-CoA. With HB-NAC, the molecular weight (M-w) of PHB produced at substrate-to-enzyme ratios of 1500-15000 is similar to 75 kDa. H-1 NMR shows that PHB produced has hydroxybutyrate at the alcohol end and N-acetylcysteamine (NAC) at the carboxylate end of the polymer. Exogenous NAC has no effect on the Mw of PHB produced with HB-CoA or HB-NAC in vitro, whereas PHB from a polymerization reaction with both HB-CoA and HB -NAC has intermediate M-w. These results can be accommodated by two models. In the first, NAC liberated at the active site during polymerization acts as a chain transfer agent. In the second, there is a noncovalent polymer intermediate covalently linked to NAC, which can dissociate from the active site.