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J Bacteriol. 2008 Mar;190(6):2050-5. doi: 10.1128/JB.01799-07. Epub 2008 Jan 18.

Nondecarboxylating and decarboxylating isocitrate dehydrogenases: oxalosuccinate reductase as an ancestral form of isocitrate dehydrogenase.

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  • 1Department of Biotechnology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8567, Japan. aomiho@mail.ecc.u-tokyo.ac.jp

Abstract

Isocitrate dehydrogenase (ICDH) from Hydrogenobacter thermophilus catalyzes the reduction of oxalosuccinate, which corresponds to the second step of the reductive carboxylation of 2-oxoglutarate in the reductive tricarboxylic acid cycle. In this study, the oxidation reaction catalyzed by H. thermophilus ICDH was kinetically analyzed. As a result, a rapid equilibrium random-order mechanism was suggested. The affinities of both substrates (isocitrate and NAD+) toward the enzyme were extremely low compared to other known ICDHs. The binding activities of isocitrate and NAD+ were not independent; rather, the binding of one substrate considerably promoted the binding of the other. A product inhibition assay demonstrated that NADH is a potent inhibitor, although 2-oxoglutarate did not exhibit an inhibitory effect. Further chromatographic analysis demonstrated that oxalosuccinate, rather than 2-oxoglutarate, is the reaction product. Thus, it was shown that H. thermophilus ICDH is a nondecarboxylating ICDH that catalyzes the conversion between isocitrate and oxalosuccinate by oxidation and reduction. This nondecarboxylating ICDH is distinct from well-known decarboxylating ICDHs and should be categorized as a new enzyme. Oxalosuccinate-reducing enzyme may be the ancestral form of ICDH, which evolved to the extant isocitrate oxidative decarboxylating enzyme by acquiring higher substrate affinities.

PMID:
18203822
[PubMed - indexed for MEDLINE]
PMCID:
PMC2258884
Free PMC Article
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