alpha-Ketoglutarate dehydrogenase complex may be heterogeneous in quaternary structure.

The quaternary structure of the alpha-ketoglutarate dehydrogenase complex (KGDC) from Escherichia coli has been investigated by electron microscopy. KGDC consists of an octahedral cube-shaped structural core, lipoyl transsuccinylase (E2), to which 12 polypeptide chains each of alpha-ketoglutarate dehydrogenase (E1) and dihydrolipoyl dehydrogenase (E3) are non-covalently bound. The analysis was greatly simplified by analyzing subcomplexes of KGDC prepared by assembly of the purified component enzymes in vitro; the subcomplexes consisted of the E2 component to which only a few E1 or E3 subunits were attached. We find that both the E1 and E3 bind on the surface of the E2 molecule approximately midway between the 4-fold and 2-fold symmetry axes of E2. There are 24 such positions per E2 molecule but, based upon the observed stoichiometries of the component enzymes, it is clear that at least half of these sites are unoccupied in KGDC. If KGDC possesses symmetry, then a mechanism must exist for selecting a symmetrically distributed subset of the potential binding sites for the E1 and E3. However, analysis of images of subcomplexes in which two E1 or E3 subunits are present suggests that binding to the E2 occurs through random selection of the potential binding sites. If native KGDC is assembled by such a mechanism, then KGDC would not have a unique quaternary structure, but instead would consist of a family of structural isomers having up to approximately 125,000 members. Consideration of independent structural and biochemical data regarding the mechanism of action of the E2 indicates that the kind of structural heterogeneity being proposed is consistent with a functional KGDC.