2-oxoadipate dehydrogenase (E1a) component of the 2-oxoadipate dehydrogenase complex (OADHC). Participates in the first step, rate limiting for the overall conversion of 2-oxoadipate (alpha-ketoadipate) to glutaryl-CoA and CO(2) catalyzed by the whole OADHC. Catalyzes the irreversible decarboxylation of 2-oxoadipate 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). Can catalyze the decarboxylation of 2-oxoglutarate in vitro, but at a much lower rate than 2-oxoadipate. Responsible for the last step of L-lysine, L-hydroxylysine and L-tryptophan catabolism with the common product being 2-oxoadipate. (R)-N(6)-lipoyl-L-lysyl-[dihydrolipoyllysine-residue succinyltransferase] + 2-oxoadipate + H(+) = (R)-N6-(S8-glutaryldihydrolipoyl)-L-lysyl-[dihydrolipoyllysine-residue succinyltransferase] + CO2 Amino-acid degradation. The 2-oxoadipate dehydrogenase complex is composed of OADH (2-oxoadipate dehydrogenase; E1a), DLST (dihydrolipoamide succinyltransferase; E2) and DLD (dihydrolipoamide dehydrogenase; E3). E1a functional unit is a dimer. 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, respectively). Belongs to the alpha-ketoglutarate dehydrogenase family.