Structure and catalytic mechanistic insight into Enterobacter aerogenes acetolactate decarboxylase

Abstract

α-Acetolactate decarboxylase (ALDC) catalyses α-acetolactate into acetoin (3-hydroxy-2-butanone, AC) and is considered to be the rate-limiting enzyme in the synthesis of 2,3-butanediol. In this work, the enzymatic activity of ALDC from Enterobacter aerogenes ALDC (E.a.-ALDC) was fully characterized with enzyme kinetics, indicating a K_m of 14.83 ± 0.87 mM and a k_cat of 0.81 ± 0.09 s^-1. However, compared with the activities of ALDCs reported from other bacteria, the activity of E.a.-ALDC was determined to present a relatively lower value of 849.08 ± 35.21 U/mg. The enzyme showed maximum activity at pH 5.5. In addition, the activity of E.a.-ALDC was promoted by Mg²⁺. The crystal structure of E.a.-ALDC firstly solved by X-ray crystallography at resolution of 2.4 Å revealed a chelated zinc ion with conserved His199, His201, His212, Glu70 and Glu259. In the active center, the conservative Arg150 was particularly proven to deviate from the zinc ion of the active centre, by adopting a flexible conformational change, resulting in a weak interaction network of the enzyme and the substrate. Further in silico docking of E.a.-ALDC with two enantiomers, (R)-acetolactate and (S)-acetolactate, unaltered the interaction network of E.a.-ALDC from the apo structure, which confirmed the weakened role of Arg150 in the catalytic properties of E.a.-ALDC. Our results reveal a unique structure-function relationship of acetolactate decarboxylase and provide a fundamental basis for the enzymatic synthesis of acetoin.

Keywords: Enterobacter aerogenes; X-ray crystal structure; α-Acetolactate decarboxylase.