The oxoglutarate dehydrogenase complex (OGDC) or α-ketoglutarate dehydrogenase complex is an enzyme complex, most commonly known for its role in the citric acid cycle. Units Much like pyruvate dehydrogenase complex (PDC), this enzyme forms a complex composed of three components: Three classes of these multienzyme complexes have been characterized: one specific for pyruvate, a second specific for 2-oxoglutarate, and a third specific for branched-chain α-keto acids. The oxoglutarate dehydrogenase complex has the same subunit structure and thus uses the same coenzymes as the pyruvate dehydrogenase complex and the branched-chain alpha-keto acid dehydrogenase complex (TTP, CoA, lipoate, FAD and NAD). Only the E3 subunit is shared in common between the three enzymes. Properties Metabolic pathways This enzyme participates in three different pathways: Citric acid cycle (KEGG link: MAP00020) Lysine degradation (KEGG link: MAP00310) Tryptophan metabolism (KEGG link: MAP00380) Kinetic properties The following values are from Azotobacter vinelandii (1): KM: 0.14 ± 0.04 mM Vmax : 9 ± 3 μmol.min−1.mg−1 Citric acid cycle Reaction The reaction catalyzed by this enzyme in the citric acid cycle is: α-ketoglutarate + NAD+ + CoA → Succinyl CoA + CO2 + NADH This reaction proceeds in three steps: decarboxylation of α-ketoglutarate, reduction of NAD+ to NADH, and subsequent transfer to CoA, which forms the end product, succinyl CoA. ΔG°' for this reaction is -7.2 kcal mol−1. The energy needed for this oxidation is conserved in the formation of a thioester bond of succinyl CoA. Regulation Oxoglutarate dehydrogenase is a key control point in the citric acid cycle. It is inhibited by its products, succinyl CoA and NADH. A high energy charge in the cell will also be inhibitive. ADP and calcium ions are allosteric activators of the enzyme. By controlling the amount of available reducing equivalents generated by the Krebs cycle, Oxoglutarate dehydrogenase has a downstream regulatory effect on oxidative phosphorylation and ATP production. Reducing equivalents (such as NAD+/NADH) supply the electrons that run through the electron transport chain of oxidative phosphorylation. Increased Oxoglutarate dehydrogenase activation levels serve to increase the concentrations of NADH relative to NAD+. High NADH concentrations stimulate an increase in flux through oxidative phosphorylation. While an increase in flux through this pathway generates ATP for the cell, the pathway also generates free radical species as a side product, which can cause oxidative stress to the cells if left to accumulate. Oxoglutarate dehydrogenase is considered to be a redox sensor in the mitochondria,