Editing Pyruvate dehydrogenase complex (section)

===Gram-positive bacteria and eukaryotes===
In contrast, in [[Gram-positive]] bacteria (e.g. ''[[Bacillus stearothermophilus]]'') and eukaryotes the central PDC core contains 60 E2 molecules arranged into an icosahedron. This E2 subunit “core” coordinates to 30 subunits of E1 and 12 copies of E3.

Eukaryotes also contain 12 copies of an additional core protein, [[E3 binding protein]] (E3BP) which bind the E3 subunits to the E2 core.<ref>{{cite journal |last1=Brautigam |first1=C. A. |last2=Wynn |first2=R. M. |last3=Chuang |first3=J. L. |last4=Machius |first4=M. |last5=Tomchick |first5=D. R. |last6=Chuang |first6=D. T. |title=Structural insight into interactions between Dihydrolipoamide Dehydrogenase (E3) and E3 binding protein of Human pyruvate dehydrogenase complex |journal=Structure |date=2006 |volume=14 |issue=3 |pages=611–621 |doi=10.1016/j.str.2006.01.001|pmid=16442803 |pmc=2879633 }}</ref>  The exact location of E3BP is not completely clear. Cryo-electron microscopy has established that E3BP binds to each of the icosahedral faces in yeast.<ref>Stoops, J.K., Cheng, R.H., Yazdi, M.A., Maeng, C.Y., Schroeter, J.P., Klueppelberg, U., Kolodziej, S.J., Baker, T.S., Reed, L.J. (1997) On the unique structural organization of the Saccharomyces cerevisiae pyruvate dehydrogenase complex. J. Biol. Chem. 272, 5757-5764.</ref> However, it has been suggested that it replaces an equivalent number of E2 molecules in the bovine PDC core.

Up to 60 E1 or E3 molecules can associate with the E2 core from Gram-positive bacteria - binding is mutually exclusive. In eukaryotes E1 is specifically bound by E2, while E3 associates with E3BP. It is thought that up to 30 E1 and 6 E3 enzymes are present, although the exact number of molecules can vary ''in vivo'' and often reflects the metabolic requirements of the tissue in question.