Make it or break it: a review on PHA synthase and depolymerase proteins

Petroleum-based plastics are recalcitrant world-wide used materials that severely pollute the environment, thus biodegradable bioplastics are emerging as a viable alternative. From this group, the study of polyhydroxyalkanoates (PHAs) has stood out for their potential in diverse applications including medicine, packaging and agriculture. The enzyme responsible for PHAs synthesis inside the microbial cell is the PHA synthase (PhaC). PhaCs are present in a wide variety of microorganisms and are classified according to their substrate specificity and subunit composition into 4 classes. Class I, class III and class IV use the acyl-CoA as a precursor to synthesize short-chain-length PHAs while Class II enzymes use an intermediate of the β-oxidation pathways to synthesize medium-chain-length PHAs. Enzymes from this pathway that have been upregulated and downregulated to optimize PHAs production are described in this review. Another important enzyme is the PHA depolymerase (PhaZ) which is responsible for all PHA degradation inside and outside the cell. This review describes both enzymes in detail, including classification, structure, substrate specificity and proven protein engineering techniques for enzymatic rate enhancement and modified substrate specificity of the proteins. It also includes a mutation map for the class II PhaC sequence of Pseudomonas putida that suggest point mutations for future protein engineering work.