Extended substrate range of thiamine diphosphate-dependent MenD enzyme by coupling of two C-C-bonding reactions.

ABSTRACT

Carboligations catalyzed by aldolases or thiamine diphosphate (ThDP)-dependent enzymes are well-known in biocatalysis to deliver enantioselective chain elongation reactions. A pyruvate-dependent aldolase (2-oxo-3-deoxy-6-phosphogluconate aldolase [EDA]) introduces a chiral center when reacting with the electrophile, glyoxylic acid, delivering the (S)-enantiomer of (4S)-4-hydroxy-2-oxoglutarate [(S)-HOG]. The ThDP-dependent enzyme MenD (2-succinyl-5-enol-pyruvyl-6-hydroxy-3-cyclohexene-1-carboxylate synthase (SEPHCHC synthase)) enables access to highly functionalized substances by forming intermolecular C-C bonds with Michael acceptor compounds by a Stetter-like 1,4- or a benzoin-condensation 1,2-addition of activated succinyl semialdehyde (ThDP adduct formed by decarboxylation of 2-oxoglutarate). MenD-catalyzed reactions are characterized by high chemo- and regioselectivity. Here, we report (S)-HOG, in situ formed by EDA, to serve as new donor substrate for MenD in 1,4-addition reactions with 2,3-trans-CHD (2,3-trans-dihydroxy-cyclohexadiene carboxylate) and acrylic acid. Likewise, (S)-HOG serves as donor in 1,2-additions with aromatic (benzaldehyde) and aliphatic (hexanal) aldehydes. This enzyme cascade of two subsequent C-C bond formations (EDA aldolase and a ThDP-dependent carboligase, MenD) generates two new stereocenters.