Expression of six genes from two
glutamate fermenting clostridia converted Escherichia coli into a producer of
glutaconate from
2-oxoglutarate of the general metabolism (Djurdjevic, I. et al. 2010, Appl. Environ. Microbiol.77, 320-322). The present work examines whether this pathway can also be used to reduce 2-oxoadipate to (R)-2-hydroxyadipic
acid and dehydrate its
CoA thioester to 2-hexenedioic
acid, an unsaturated precursor of the biotechnologically valuable
adipic acid (
hexanedioic acid).
2-Hydroxyglutaryl-CoA dehydratase from Clostridium symbiosum, the key
enzyme of this pathway and a potential radical
enzyme, catalyzes the reversible
dehydration of (R)-2-hydroxyglutaryl-CoA to (E)-
glutaconyl-CoA. Using a spectrophotometric assay and mass spectrometry, it was found that (R)-2-hydroxyadipoyl-CoA, oxalocrotonyl-
CoA, muconyl-
CoA, and butynedioyl-
CoA, but not 3-methylglutaconyl-CoA, served as alternative substrates. Hydration of butynedioyl-
CoA most likely led to 2-oxosuccinyl-CoA, which spontaneously hydrolyzed to
oxaloacetate and
CoASH. The
dehydratase is not specific for the
CoA-moiety because (R)-2-hydroxyglutaryl-thioesters of
N-acetylcysteamine and
pantetheine served as almost equal substrates. Whereas the related 2-hydroxyisocaproyl-CoA
dehydratase generated the stable and inhibitory 2,4-pentadienoyl-CoA radical, the analogous allylic ketyl radical could not be detected with muconyl-
CoA and 2-hydroxyglutaryl-CoA dehydratase. With the exception of (R)-2-hydroxyglutaryl-CoA, all mono-
CoA-thioesters of dicarboxylates used in this study were synthesized with
glutaconate CoA-transferase from Acidaminococcus fermentans. The now possible conversion of (R)-2-hydroxyadipate via (R)-2-hydroxyadipoyl-CoA and 2-hexenedioyl-CoA to 2-hexenedioate paves the road for a bio-based production of
adipic acid.