Evolutionary engineered polyhydroxyalkanoate (PHA) synthases from Pseudomonas sp. 61-3 enhance PHA accumulation and enable the monomer composition of PHAs to be regulated. We characterized a newly screened Ser477Arg (S477R) mutant of PHA synthase by in vivo analyses of P(3-hydroxybutyrate) [P(3HB)] homopolymer and P(3HB-co-3-hydroxyalkanoate) [P(3HB-co-3HA)] copolymer productions in the recombinants of Escherichia coli. The results indicated that the S477R mutation contributed to a shift in substrate specificity to smaller monomers containing a 3HB unit rather than to an enhancement in catalytic activity. Multiple mutations of S477R with other beneficial mutations, for example, Ser325Cys, exhibited synergistic effects on both an increase in PHA production (from 9 wt % to 21 wt %) and an alteration of substrate specificity. Furthermore, the effects of complete amino acid substitutions at position 477 were characterized in terms of in vivo PHA production and in vitro enzymatic activity. The five mutations, S477Ala(A)/Phe(F)/His(H)/Arg(R)/Tyr(Y), resulted in a shift in substrate specificity to smaller monomer units. The S477Gly(G) mutant greatly enhanced activity toward all different sizes of substrates with carbon numbers ranging from 4 to 12. These results indicated that the residue 477 contributes to both the catalytic activity and substrate specificity of PHA synthase. In recombinant E. coli, the S477A/F/G/H/R/Y mutations consistently led to increases (up to 6 times that of wild-type enzyme) in weight average molecular weights of P(3HB) homopolymers. On the basis of our studies, we created a structural feasibility accounting for the mutational effects on enzymatic activity and substrate specificity of PHA synthase.