X-RAY STRUCTURE OF EUKARYOTIC E3, LIPOAMIDE DEHYDROGENASE, FROM YEAST. T. Toyoda and A. Takenaka, Department of Life Science, Tokyo Institute of Technology, Yokohama, Japan; and T. Sekiguchi, Department of Fundamental Science, Iwaki Meisei University, Fukushima, Japan.

[[alpha]]-Keto acid dehydrogenase complex is a family of well-organized multienzyme systems which are composed of three kinds of enzymes, E1, E2, and E3. Depending on the substrates, E1 and E2 are varied to construct the different architectures with 532 symmetry for pyruvate dehydrogenase complex and 432 symmetry for [[alpha]]-ketoglutarate dehydrogenase complex in eukaryotes. The third component E3 (lipoamide dehydrogenase) is, however, commonly used among them. To reveal the structure of E3 which binds to both complexes and to elucidate the reaction mechanism, the crystal structure of E3 from yeast was solved by the molecular replacement method with diffraction data collected up to 2.98(max. 2.49)Å resolution using synchrotron radiation. The initial phases were improved by non-crystallographic symmetry averaging and solvent flattening of electron density. The atomic coordinates of the molecular model constructed by computer graphics were refined with molecular dynamics. The final R-factor is 19.0% at 2.49Å resolution. Compared with prokaryotic Gram negative bacteria E3 (A. vinelandii ) which binds only to complexes with 432 symmetry, large differences occur in the loop regions with insertion or deletion of amino acids. There are no differences in topology of the secondary structures. One [[beta]]-sheet (246-276 residues) is changed the normal on the molecular surface. It is expected that E3s have different molecular surfaces between those incorporated into the cores with only 432 and with both 432 and 532 symmetries. A characteristic feature, found on an electrostatic molecular surface, may be concerned with such binding properties.