Figure 10

Proposed catalytic mechanism for PhaC1_{P.sp USM 4–55} involving Cys296, Asp451 and His479 with the protein dimer forming the loading and elongation site. In subunit 1 (the loading site), His479 activates Cys296 for nucleophilic attack to the carbonyl carbon of the substrate (3-hydroxyhexanoyl-CoA) yielding a tetrahedral intermediate which then collapses by the release of the CoA group. The elongation process proceeds at the second subunit by the activation of 3-OH group of the bound substrate for nucleophilic attack to the carbonyl carbon of the substrate covalently bound to the Cys296 of the first subunit to form an ester bond. This process repeats by adding more and more monomers to the growing PHA chain.