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Polyhydroxyalkanoate (PHA) synthase is an enzyme that polymerizes the acyl group of hydroxyacyl-coenzyme A (CoA) substrates.  Aeromonas caviae  PHA synthase (PhaC Ac ) is an important biocatalyst for the synthesis of a useful PHA copolymer, poly[( R )-3-hydroxybutyrate- co -( R )-3-hydroxyhexanoate] [P(3HB- co -3HHx)]. Previously, a PhaC Ac  mutant with double mutations in asparagine 149 (replaced by serine [N149S]) and aspartate 171 (replaced by glycine [D171G]) was generated to synthesize a 3HHx-rich P(3HB- co -3HHx) and was named PhaC Ac  NSDG. In this study, to further increase the 3HHx fraction in biosynthesized PHA, PhaC Ac  was engineered based on the three-dimensional structural information of PHA synthases. First, a homology model of PhaC Ac  was built to target the residues for site-directed mutagenesis. Three residues, namely tyrosine 318 (Y318), serine 389 (S389), and leucine 436 (L436), were predicted to be involved in substrate recognition by PhaC Ac . These PhaC Ac  NSDG residues were replaced with other amino acids, and the resulting triple mutants were expressed in the engineered strain of  Ralstonia eutropha  for application in PHA biosynthesis from palm kernel oil. The S389T mutation allowed the synthesis of P(3HB- co -3HHx) with an increased 3HHx fraction without a significant reduction in PHA yield. Thus, a new workhorse enzyme was successfully engineered for the biosynthesis of a higher 3HHx-fraction polymer.

frontiers in bioengineering and biotechnology

Engineering of Aeromonas caviae Polyhydroxyalkanoate Synthase Through Site-Directed Mutagenesis for Enhanced Polymerization of the 3-Hydroxyhexanoate Unit