Branched-Chain Keto Acid Decarboxylase fromLactococcus lactis (KdcA), a Valuable Thiamine Diphosphate-Dependent Enzyme for Asymmetric CC Bond Formation

The thiamine diphosphate-dependent, branched-chain 2-keto acid decarboxylase from Lactococcus lactis sup. cremoris B1157 (KdcA) is a new valuable enzyme for the synthesis of chiral 2-hydroxy ketones. The gene was cloned and the enzyme was expressed as an N-terminal hexahistidine fusion protein in Escherichia coli. It has a broad substrate range for the decarboxylation reaction including linear and branched-chain aliphatic and aromatic keto acids as well as phenyl pyruvate and indole-3-pyruvate. The dimeric structure of recombinant KdcA is in contrast to the tetrameric structure of other 2-keto acid decarboxylases. The enzyme is stable between pH 5 and 7 with a pH optimum of pH 6-7 for the decarboxylation reaction. While KdcA is sufficiently stable up to 40 °C it rapidly looses activity at higher temperatures. In this work the carboligase activity of KdcA is demonstrated for the first time. The enzyme shows an exceptionally broad substrate range and, most strikingly, it catalyzes the carboligation of different aromatic aldehydes as well as CH-acidic aldehydes such as phenylacetaldehyde and indole-3-acetaldehyde with aliphatic aldehydes such as acetaldehyde, propanal, and cyclo-propanecarbaldehyde, yielding chiral 2-hydroxy ketones in high enantiomeric excess. Noteworthy, the donor-acceptor selectivity is strongly influenced by the nature of the respective substrate combination.