||This is the key enzyme in the biosynthesis of polyhydroxyalkanoates (PHA), linear polyesters produced by bacteria as a means of carbon and energy storage . The enzyme catalyses the stereoselective, covalent linkage of (3R)-3-hydroxyacyl-CoA thioesters in a transesterification reaction with concomitant release of coenzyme A. The growing polymer is attached to a conserved active site L-cysteine residue. Three types of PHA synthases have been proposed based on their substrate specificity and enzyme structure. Type I and type III synthases preferentially polymerize short chain hydroxyalkanoate monomers containing 3-5 carbon atoms [1,2]. The difference between these two types is that type I synthases are composed of only a single subunit (PhaC), whereas type III synthases are composed of two different subunits, PhaC and PhaE [3,5]. Type II synthases are also composed of a single subunit (PhaC), but preferentially polymerize monomers containing more than 5 carbon atoms .
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