Effective microbial production of poly(4-hydroxybutyrate) homopolymer byRalstonia eutropha H16

Abstract

The effective microbial production of copolyesters of 3-hydroxybutyrate (3HB) and 4-hydroxybutyrate (4HB) with high mole fractions of 4HB units by a wild-type strain of Ralstonia eutropha H16 was investigated in culture solutions containing 4-hydroxybutyric acid (4HBA) and various carbon substrates in the presence of a nitrogen source such as ammonium sulfate. The addition of glucose or acetic acid to the culture solution containing 4HBA in the presence of ammonium sulfate resulted in the production of random copolymers of P(3HB-co-4HB) with compositions of up to 82 mol% 4HB, but the yield of copolymers was less than 7 wt% of dried cell weights. In contrast, when n-alkanoic acids such as propionic acid, butyric acid, valeric acid and hexanoic acid, being subject to β-oxidation metabolism in the cell, were used as the co-substrates of 4HBA in the presence of ammonium sulfate, a mixture of copolymers with two different 4HB compositions was produced, and copolyesters with compositions of 93–100 mol% 4HB were isolated from chloroform–n-hexane insoluble fractions in the mixture of copolymers. Especially, when this wild-type Ralstonia eutropha H16 was cultivated in a medium containing 4HBA (15 g litre−1), propionic acid (5 g litre−1) and ammonium sulfate (5 g litre−1), namely C/N (mol/mol) = 10, the P(4HB) homopolymer was produced at maximally 34 wt% of dry cell weight (7.8 g litre−1), and the conversion yield of 4HBA to P(4HB) homopolymer resulted in values as high as 21 mol%. © 1999 Society of Chemical Industry