Biosynthesis of the copolymer of 3-hydroxybutyrate and 3-hydroxyvalerate in methylobacterium extorquens: metabolism of propanol, propionate, pentanol, and valerate

Methylobacterium extorquens 15 was found to accumulate a considerable amount of poly-3-hydroxybutyrate (PHB) during growth on methanol and the copolymer of 3-hydroxybutyrate and 3-hydroxyvalerate (PHBV) during growth in the presence of propanol, pentanol, propionate, and valerate. In the initial stages of PHB V synthesis, propanol and pentanol were oxidized to propionyl-CoA and valeryl-CoA by the phenazine methosulfate-dependent alcohol and aldehyde dehydrogenases and acyl-CoA synthetase. Propionyl-CoA was further metabolized with the involvement of propionyl-CoA carboxylase and methylmalonyl-CoA mutase. Valeryl-CoA was cleaved into propionyl-CoA and acetyl-CoA via a pathway involving acyl-CoA dehydrogenase, L-crotonase, NAD⁺-dependent acetoacetyl-CoA reductase, and 3-ketothiolase. Methanol was found to induce decarboxylation of the IC-atom of propionyl-CoA and valeryl-CoA and incorporation of these compounds into PHBV, although the enzymes responsible for this decarboxylation were not revealed. PHB was synthesized with the involvement of 3-ketothiolase, NADPH-dependent acetoacetyl-CoA reductase, and PHB synthase. Analysis of the rates of incorporation of radioactive carbon from ¹⁴C-methanol, 2-¹⁴C-acetate + + methanol, 2-¹⁴C-acetate, and 2-¹⁴C-butyrate into CO₂, PHB, and various cellular constituents showed that, during methylotrophic growth, the tricarboxylic acid cycle and NADP⁺-dependent isocitrate dehydrogenase weakly contributed to the production of NADPH necessary for PHB synthesis.