Crystallographic structure of E. coli isocitrate dehydrogenase.[1] There are three active sites. Three isocitrates, one isocitrate in the binding site for NADP+.
Isocitrate dehydrogenase (IDH) (EC1.1.1.42) and (EC1.1.1.41) is an enzyme that catalyzes the oxidative decarboxylation of isocitrate, producing alpha-ketoglutarate (α-ketoglutarate) and CO2. This is a two-step process, which involves oxidation of isocitrate (a secondary alcohol) to oxalosuccinate (a ketone), followed by the decarboxylation of the carboxyl group beta to the ketone, forming alpha-ketoglutarate. In humans, IDH exists in three isoforms: IDH3 catalyzes the third step of the citric acid cycle while converting NAD+ to NADH in the mitochondria. The isoforms IDH1 and IDH2 catalyze the same reaction outside the context of the citric acid cycle and use NADP+ as a cofactor instead of NAD+. They localize to the cytosol as well as the mitochondrion and peroxisome.[2]
^PDB: 1CW7; Cherbavaz DB, Lee ME, Stroud RM, Koshland DE (January 2000). "Active site water molecules revealed in the 2.1 A resolution structure of a site-directed mutant of isocitrate dehydrogenase". Journal of Molecular Biology. 295 (3): 377–385. doi:10.1006/jmbi.1999.3195. PMID10623532.