Structural insights into the mechanism of acetolactate decarboxylase

Abstract

Acetolactate decarboxylase (ALDC) is a bacterial enzyme of the butanediol fermentation pathway that decarboxylates S‐acetolactate into R‐acetoin. The remarkable feature of this enzyme is that it also catalyses the decarboxylation of the opposite enantiomer R‐acetolactate to give the same product R‐acetoin. The enzyme is widely used commercially by the brewing industry. It shortens the maturation step by preventing the formation of diacetyl, which gives an off taste to beer. The structure of Bacillus brevis ALDC has been solved using X‐ray crystallography, in the presence of a series of chiral substrate analogues. In the resting state the active site zinc ion is coordinated to three conserved histidines and a glutamate. Upon binding of the substrate analogues the C‐terminal loop moves away losing the glutamate coordination bond. The structural information combined with experimental results from active site mutants allows us to present a mechanism for the stereospecific decarboxylation of acetolactate. The structural and biochemical data we have obtained will be used to develop further modifications and applications of the enzyme in future work.The authors thank EPSRC for funding through the Molecular Organisation and Assembly in Cells (MOAC) Doctoral Training Centre.