Is atomic rearrangement of type IV PHA synthases responsible for increased PHA production?

Background: Type IV PHA synthase is a key enzyme responsible for catalyzing the formation of non-toxic, biocompatible, and biodegradable short-chain-length polyhydroxyalkanoates (scl-PHA) under the growth-limiting conditions in the members of the genus Bacillus. Results: The comparative in vitro and in silico analysis of the phaC subunit of type IV PHA synthases among Bacillus cereus FA11, B. cereus FC11, and B. cereus FS1 was done in our study to determine its structural and functional properties. Conserved domain analysis demonstrated that phaC subunit belongs to the alpha/beta (α/β) hydrolase fold. The catalytic triad comprising of cysteine (Cys), histidine (His), and aspartate (Asp) was found to be present at the active site. A shorter inter-atomic distance was found between the carboxyl (–COO) group of Asp and amino (NH₂) group of His. Furthermore, slightly long inter-atomic distances between sulfhydryl (SH) group of Cys and NH₂ group of His may be pointing toward the broader substrate specificity of type IV PHA synthases. However, a shorter distance between the SH group of Cys and NH₂ group of His in case of B. cereus FC11 leads to a higher enzymatic activity and maximum PHA yield (49.26%). Conclusion: The in silico study verifies that the close proximity between SH group of Cys and NH₂ group of His in phaC subunit of type IV PHA synthases can be crucial for synthesis of scl-PHA. However, the catalytic activity of type IV PHA synthases declines as the distance between the sulfur (S) atom of the SH group of Cys and the nitrogen (N) atom of NH₂ group of His increases.