Mechanism of asymmetric decarboxylation of α-aryl-α- methylmalonate catalyzed by arylmalonate decarboxylase originated from Alcaligenes bronchisepticus

Arylmalonate decarboxylase (EC. 4.1.1.76, originated from Alcaligenes bronchisepticus KU 1201) is an enzyme which catalyzes asymmetric decarboxylation of arylmalonate. We have once proposed the intermediary formation of a thiol ester between the substrate and the enzyme based on the inhibition studies with α-bromophenylacetate. We misinterpreted the binding mode of this acid as formation of thiol ester and estimated that the substrate also bound to the enzyme in the same manner. However, reinvestigation indicated that the mode of inhibition by this acid is irreversible, different from the previous conclusion. Accordingly the above mechanism became very unlikely. Instead, we would like to propose that Cys 188 is working as a proton donor on the basis of following evidence. The pH-rate of reaction profiles of the native and C188S mutant enzyme greatly differed in alkaline region. This is estimated to come from the difference in pK_a values of Cys and Ser, and suggested that Cys 188 is a proton donor. Homology alignment showed that this enzyme has some homology with glutamete racemase and some other isomerases. The presence of Cys 188 is conserved to all these enzymes as well as to AMDase. The role of this amino acid residue in glutamate racemase has been established to interchange a proton between the substrate. This fact also supports that Cys 188 of AMDase is working as a proton donor to form the asymmetric center of the product.