Principles of quasi-equivalence and Euclidean geometry govern the assemblyof cubic and dodecahedral cores of pyruvate dehydrogenase complexes

The pyruvate dehydrogenase multienzyme complex (M-r of 5-10 million) is ass embled around a structural core formed of multiple copies of dihydrolipoyl acetyltransferase (E2p), which exhibits the shape of either a cube or a dod ecahedron, depending on the source. The crystal structures of the 60-meric dihydrolipoyl acyltransferase cores of Bacillus stearothermophilus and Ente rococcus faecalis pyruvate dehydrogenase complexes were determined and reve aled a remarkably hollow dodecahedron with an outer diameter of approximate to 237 Angstrom, 12 large openings of approximate to 52 Angstrom diameter across the fivefold axes, and an inner cavity with a diameter of approximat e to 118 Angstrom. Comparison of cubic and dodecahedral E2p assemblies show s that combining the principles of quasi-equivalence formulated by Caspar a nd Klug [Caspar, D. L. & Klug, A. (1962) Cold Spring Harbor Symp. errant. B iol. 27, 1-4] with strict Euclidean geometric considerations results in pre dictions of the major features of the E2p dodecahedron matching the observe d features almost exactly.