Alteration of substrate chain-length specificity of type II synthase for polyhydroxyalkanoate biosynthesis by in vitro evolution: in vivo and in vitro enzyme assays

In our previous study, in vitro evolution of type 11 polyhydroxyalkanoate (PHA) synthase (PhaC1(Ps))from Pseudomonas sp. 61-3 yielded eleven mutant enzymes capable of synthesizing homopolymer of (R)-3-hydroxybutyrate [P(3HB)] in recombinant Escherichia coli JM109. These recombinant strains were capable of accumulating up to approximately 400-fold more P(3HB) than strains expressing the wild-type enzyme. These mutations enhanced the ability of the enzyme to specifically incorporate the 3HB-coenzyme A (3HB-CoA) substrate or improved catalytic efficiency toward the various monomer substrates Of C-4 to C-12 (R)-3-hydroxyacyl-CoAs which can intrinsically be channeled by PhaC1(Ps), into P(3HB-co-3HA) copolymerization. In this study, beneficial amino acid substitutions of PhaC1(Ps) were analyzed based on the accumulation level and the monomer composition of P(3HB-co-3HA) copolymers generated by E. coli LS5218 [fadR601 atoC(Con)] harboring the monomer supplying enzyme genes. Substitutions of Set by Thr(Cys) at position 325 were found to lead to an increase in the total amount of P(3HB-co-3HA) accmumulated, whereas 3HB fractions in the P(3HB-co-3HA) copolymer were enriched by substitutions of Gln by Lys(Arg, Met) at position 481. This strongly suggests that amino acid substitutions at positions 325 and 481 are responsible for synthase activity and/or substrate chain-length specificity of PhaC1(Ps),. These in vivo results were supported by the in vitro results obtained from synthase activity assays using representative single and double mutants and synthetic substrates, (R,S)-3HB-CoA and (R,S)-3-hydroxydecanoyl-CoA. Notably, the position 481 was found to be a determinant for substrate chain-length specificity of PhaC1(Ps).