Biosynthesis of the copolymer of 3-hydroxybutyrde and 3-hydroxyvalerate inMethylobacterium extorquens: Metabolism of propanol, propionate, pentanol,and valerate

Methylobacterium extorquens 15 was found to accumulate a considerable amoun t of poly-3-hydroxybutyrate (PHB) during growth on methanol and the copolym er of 3-hydroxybutyrate and 3-hydroxyvalerate (PHBV) during growth in the p resence of propanol, pentanol, propionate, and valerate. In the initial sta ges of PHBV synthesis, propanol and pentanol were oxidized to propionyl-CoA and valeryl-CoA by the phenazine methosulfate-dependent alcohol and aldehy de dehydrogenases and acyl-CoA synthetase. Propionyl-CoA was further metabo lized with the involvement of propionyl-CoA carboxylase and methylmalonyl-C oA mutase. Valeryl-CoA was cleaved into propionyl-CoA and acetyl-CoA via a pathway involving acyl-CoA dehydrogenase, L-crotonase, NAD(+)-dependent ace toacetyl-Coa reductase, and 3-ketothiolase. Methanol was found to induce de carboxylation of the IC-atom of propionyl-CoA and valeryl-CoA and incorpora tion of these compounds Into PHBV, although the enzymes responsible for thi s decarboxylation were not revealed. PHB was synthesized with the involveme nt of 3-ketothiolase, NADPH-dependent acetoacetyl-CoA reductase, and PHB sy nthase. Analysis of the rates of incorporation of radioactive carbon from C -14-methanol, 2-C-14-acetate + methanol, 2-C-14-acetate, and 2-C-14-butyrat e into CO2, PHB, and various cellular constituents showed that, during meth ylotrophic growth, the tricarboxylic acid cycle and NADP(+)-dependent isoci trate dehydrogenase weakly contributed to the production of NADPH necessary for PHB synthesis.