2-oxoadipate dehydrogenase (E1a) component of the 2- oxoadipate dehydrogenase complex (OADHC) (PubMed:29191460, PubMed:29752936, PubMed:32303640, PubMed:32633484, PubMed:32695416). 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 (PubMed:29191460, PubMed:32695416). 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) (Probable) (PubMed:29752936, PubMed:32303640, PubMed:32633484). Can catalyze the decarboxylation of 2-oxoglutarate in vitro, but at a much lower rate than 2-oxoadipate (PubMed:29191460, PubMed:29752936, PubMed:32633484, PubMed:32695416). Responsible for the last step of L-lysine, L-hydroxylysine and L- tryptophan catabolism with the common product being 2-oxoadipate (Probable). {ECO:0000269|PubMed:29191460, ECO:0000269|PubMed:29752936, ECO:0000269|PubMed:32303640, ECO:0000269|PubMed:32633484, ECO:0000269|PubMed:32695416, ECO:0000305|PubMed:29191460, ECO:0000305|PubMed:29752936, ECO:0000305|PubMed:32303640, ECO:0000305|PubMed:32633484, ECO:0000305|PubMed:32695416}. This is the function of DHTKD1 (dehydrogenase E1 and transketolase domain containing 1, Ensembl gene identifier ENSG00000181192).