Chain termination in polyhydroxyalkanoate synthesis: involvement of exogenous hydroxy-compounds as chain transfer agents.

We have identified a range of compounds which, when present during poly(3-hydroxybutyrate) [P(3HB)] accumulation by Ralstonia eutropha (reclassified from Alcaligenes eutrophus), can act as chain transfer agents in the chain termination step of polymerization. End-group analysis by 31P NMR of polymer derivatized with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane revealed that all these compounds were covalently linked to P(3HB) at the carboxyl terminus. All chain transfer agents possessed one or more hydroxyl groups, and glycerol was selected for further investigation. The number-average molecular mass (Mn) of P(3HB) produced by R. eutropha from glycerol was substantially lower than for polymer produced from glucose, and we identified two new end-group structures. These were attributed to a glycerol molecule bound to the P(3HB) chain via the primary or secondary hydroxyl groups. When a primary hydroxyl group of glycerol is involved in chain transfer, the end-group structure is in both [R] and [S] configurations, implying that chain transfer to glycerol is a random transesterification and that PHA synthase does not catalyse chain transfer. 3-Hydroxybutyric acid is the most probable chain transfer agent in vivo, with propagation and termination reactions involving transfer of the P(3HB) chain to enzyme-bound and free 3-hydroxybutyrate, respectively. Only carboxyl end-groups were detected in P(3HB) extracted from exponentially growing bacteria. It is proposed that a compound other than 3-hydroxybutyryl-CoA acts as a primer in the initiation of polymer synthesis.