It has also been shown [129] that synthases possess a
conserved catalytic triad comprising three residues (cysteine,
aspartic acid and histidine) and that the cysteine
bonds are involved in covalent catalysis. It is suggested
that the histidine activates the cysteine for nucleophilic
attack on 3-hydroxybutyryl-CoA, while the aspartic acid
may function as a general base catalyst to activate the
hydroxyl group of HBCoA for ester formation [7].
Two mechanisms have been proposed for chain elongation
to fit with these observations [130] (Fig. 3). Both
involve chain elongation from acylated cysteine. The first
requires only one active cysteine site, and involves both
covalently and noncovalently bound HBn(CoA) intermediates.
The second requires that the active site be at the
interface of two PhaC monomers (with PHA synthase being
present in a dimeric form), with the chain being always
covalently attached to one of the two synthase units,
switching between them with the addition of each new
HA unit. In both mechanisms, the cysteine is activated for
nucleophilic attack by a base residue such as histidine in
the active site. The authors found through labeling studies
using the slower synthase mutant C149S-PhaEC that noncovalently
bound species were present in the early stages
of synthesis, and that these intermediates functioned in
a chemically and kinetically competent fashion, lending
weight to the possibility that the first mechanism is more
likely. In another proposal, Yamanaka et al.[131]