Role of PhaC Type I and Type II Enzymes during PHA Biosynthesis

Abstract

PHA synthases (PhaC) are grouped into four classes based on the kinetics and mechanisms of reaction. The grouping of PhaC enzymes into four classes is dependent on substrate specificity, according to the preference in forming short-chain-length (scl) or medium-chain-length (mcl) polymers: Class I, Class III and Class IV produce scl-PHAs depending on propionate, butyrate, valerate and hexanoate precursors, while Class II PhaC synthesize mcl-PHAs based on the alkane (C6 to C14) precursors. PHA synthases of Class I, in particular PhaCCs from Chromobacterium USM2 and PhaCCn/RePhaC1 from Cupriavidus necator/Ralstonia eutropha, have been analysed and the crystal structures of the C-domains have been determined. PhaCCn/RePhaC1 was also studied by X-ray absorption fine-structure (XAFS) analysis. Models have been proposed for dimerization, catalysis mechanism, substrate recognition and affinity, product formation, and product egress route. The assays based on amino acid substitution by mutagenesis have been useful to validate the hypothesis on the role of amino acids in catalysis and in accommodation of bulky substrates, and for the synthesis of PHB copolymers and medium-chain-length PHA polymers with optimized chemical properties.

Domain-centric dissection and classification of prokaryotic poly(3-hydroxyalkanoate) synthases

“…It has been reported that class I PHA synthase is a homodimer of PhaC [8]. Class II PHA synthase is mainly found in Pseudomonas spp.…”

“…The key enzymes involved in PHA synthesis are PHA synthases, which are a group of heterogeneous enzymes and include PhaC, PhaE, and PhaR that form homodimers, heterodimers, or work concordantly in their active states. These enzymes are currently divided into four categories, Class I, Class II, Class III and Class IV [8]. Class I, III and IV prefer SCL) carbon chain monomers (C3-5) while class II utilizes MCL PHA monomers (C6–14) [8].…”

“…These enzymes are currently divided into four categories, Class I, Class II, Class III and Class IV [8]. Class I, III and IV prefer SCL) carbon chain monomers (C3-5) while class II utilizes MCL PHA monomers (C6–14) [8]. In some exceptions, Class I PHA synthase could produce mixed PHAs incorporated with both SCL and MCL hydroxyalkanoates that show better functional properties compared with homo-polymers [2,7].…”

“…In addition, PHA classes and corresponding descriptions recorded in the database are diverse with no consistent standards. One of the features of protein primary sequences used for identifying PhaCs is a lipase box of G-X-S-X-G [8]. In addition, a characteristic secondary structure containing α/β hydrolase fold was also highlighted [8].…”

“…One of the features of protein primary sequences used for identifying PhaCs is a lipase box of G-X-S-X-G [8]. In addition, a characteristic secondary structure containing α/β hydrolase fold was also highlighted [8]. However, these features are not sufficient to distinguish PhaCs in different classes.…”