2-oxoglutarate dehydrogenase (E1o) component of the 2-oxoglutarate dehydrogenase complex (OGDHC). Participates in the first step, rate limiting for the overall conversion of 2-oxoglutarate to succinyl-CoA and CO(2) catalyzed by the whole OGDHC. Catalyzes the irreversible decarboxylation of 2-oxoglutarate (alpha-ketoglutarate) via the thiamine diphosphate (ThDP) cofactor and subsequent transfer of the decarboxylated acyl intermediate on an oxidized dihydrolipoyl group that is covalently amidated to the E2 enzyme (dihydrolipoyllysine-residue succinyltransferase or DLST). Plays a key role in the Krebs (citric acid) cycle, which is a common pathway for oxidation of fuel molecules, including carbohydrates, fatty acids, and amino acids. Can catalyze the decarboxylation of 2-oxoadipate in vitro, but at a much lower rate than 2-oxoglutarate. Mainly active in the mitochondrion. A fraction of the 2-oxoglutarate dehydrogenase complex also localizes in the nucleus and is required for lysine succinylation of histones: associates with KAT2A on chromatin and provides succinyl-CoA to histone succinyltransferase KAT2A. (R)-N(6)-lipoyl-L-lysyl-[dihydrolipoyllysine-residue succinyltransferase] + 2-oxoglutarate + H(+) = (R)-N(6)-(S(8)-succinyldihydrolipoyl)-L-lysyl-[dihydrolipoyllysine-residue succinyltransferase] + CO2 Calcium ions and ADP stimulate, whereas ATP and NADH reduce catalytic activity. The 2-oxoglutarate dehydrogenase complex is composed of OGDH (2-oxoglutarate dehydrogenase; E1), DLST (dihydrolipoamide succinyltransferase; E2) and DLD (dihydrolipoamide dehydrogenase; E3). It contains multiple copies of the three enzymatic components (E1, E2 and E3). In the nucleus, the 2-oxoglutarate dehydrogenase complex associates with kat2a. Mainly localizes in the mitochondrion. A small fraction localizes to the nucleus, where the 2-oxoglutarate dehydrogenase complex is required for histone succinylation. Expressed in the brain. The mitochondrial 2-oxoglutarate and 2-oxoadipate dehydrogenase complexes (OGDHC and OADHC, respectively) share their E2 (DLST) and E3 (dihydrolipoyl dehydrogenase or DLD) components, but the E1 component is specific to each complex (E1o and E1a (DHTK1), respectively). Belongs to the alpha-ketoglutarate dehydrogenase family.