SMALL-ANGLE X-RAY SOLUTION-SCATTERING STUDIES ON LIGAND-INDUCED SUBUNIT INTERACTIONS OF THE THIAMINE DIPHOSPHATE DEPENDENT ENZYME PYRUVATE DECARBOXYLASE FROM DIFFERENT ORGANISMS

The quaternary structures of the thiamine diphosphate dependent enzyme pyruvate decarboxylase (EC 4.1.1.1) from the recombinant wild type of Saccharomyces cerevisiae and Zymomonas mobilis and from germinating P isum sativum seeds were examined by X-ray solution scattering. The dep endence of the subunit association equilibrium on the pH and the prese nce of the cofactors thiamine diphosphate and magnesium ions were comp ared, and the differences between the catalytic properties of the diff erent enzymes are discussed. The influence of amino acid substitutions at the cofactor binding site of the enzyme from Saccharomyces cerevis iae (E51 is substituted by Q or A and G413 by W) on the subunit associ ation was examined. Low-resolution models of the P. sativum, Z. mobili s, and S. cerevisiae enzymes were evaluated ab initio from the scatter ing data. The enzyme from the bacterium and yeast appear as a dimer of dimers, whereas the plant enzyme is an octamer formed by two tetramer s arranged side-by-side. The shape of the S. cerevisiae enzyme agrees well with the atomic structure in the crystal but suggests that the di mers in the latter should be tilted by approximately 10 degrees. The r esulting modification of the atomic structure also yields a significan tly better fit to the experimental solution scattering data than that calculated form the original crystallographic model.