Studies on the regulation of the human E1 subunit of the 2-oxoglutarate dehydrogenase complex, including the identification of a novel calcium-binding site

The regulation of the 2-oxoglutarate dehydrogenase complex is central to intramitochondrial energy metabolism. In the present study, the active full-length E1 subunit of the human complex has been expressed and shown to be regulated by Ca2+, adenine nucleotides and NADH, with NADH exerting a major influence on the K-0.5 value for Ca2+. We investigated two potential Ca2+-binding sites on E1, which we term site 1 (D(114)ADLD) and site 2 ((ESDLD)-S-139). Comparison of sequences from vertebrates with those from Ca2+-insensitive non-vertebrate complexes suggest that site 1 may be the more important. Consistent with this view, a mutated form of E1, D114A, shows a 6-fold decrease in sensitivity for Ca2+, whereas variant Delta site1 (in which the sequence of site 1 is replaced by A(114)AALA) exhibits an almost complete loss of Ca2+ activation. Variant Delta site2 (in which the sequence is replaced with A(139)SALA) shows no measurable change in Ca2+ sensitivity. We conclude that site 1, but not site 2, forms part of a regulatory Ca2+-binding site, which is distinct from other previously described Ca2+-binding sites.