Many of the functional differences between acetohydroxyacid synthase (AHAS) isozyme I and other AHASs are a result of the rapid formation and breakdown of the covalent acetolactate-thiamin diphosphate adduct in AHAS I.

Acetohydroxy acid synthase (AHAS; EC 2.2.1.6) is a thiamin diphosphate (ThDP)-dependent decarboxylase-ligase that catalyzes the first common step in the biosynthesis of branched-chain amino acids. In the first stage of the reaction, pyruvate is decarboxylated and the reactive intermediate hydroxyethyl-ThDP carbanion/enamine is formed. In the second stage, the intermediate is ligated to another 2-ketoacid to form either acetolactate or acetohydroxybutyrate. AHAS isozyme I from Escherichia coli is unique among the AHAS isozymes in that it is not specific for 2-ketobutyrate (2-KB) over pyruvate as an acceptor substrate. It also appears to have a different mechanism for inhibition by valine than does AHAS III from E. coli. An investigation of this enzyme by directed mutagenesis and knowledge of detailed kinetics using the rapid mixing-quench NMR method or stopped-flow spectroscopy, as well as the use of alternative substrates, suggests that two residues determine most of the unique properties of AHAS I. Gln480 and Met476 in AHAS I replace the Trp and Leu residues conserved in other AHASs and lead to accelerated ligation and product release steps. This difference in kinetics accounts for the unique specificity, reversibility and allosteric response of AHAS I. The rate of decarboxylation of the initially formed 2-lactyl-ThDP intermediate is, in some AHAS I mutants, different for the alternative acceptors pyruvate and 2-KB, putting into question whether AHAS operates via a pure ping-pong mechanism. This finding might be compatible with a concerted mechanism (i.e. the formation of a ternary donor-acceptor:enzyme complex followed by covalent, ThDP-promoted catalysis with concerted decarboxylation-carboligation). It might alternatively be explained by an allosteric interaction between the multiple catalytic sites in AHAS.