Small-angle X-ray solution-scattering studies on ligand-induced subunit interactions of the thiamine diphosphate...

The quaternary structures of the thiamine diphosphate dependent enzyme pyruvate decarboxylase (EC 4.1.1.1) from the recombinant wild type of Saccharomycescerevisiae and Zymomonas mobilis and from germinating Pisum sativum seeds were examined by X-ray solution scattering. The dependence of the subunit association equilibrium on the pH and the presence of the cofactors thiamine diphosphate and magnesium ions were compared, and the differences between the catalytic properties of the different enzymes are discussed. The influence of amino acid substitutions at the cofactor binding site of the enzyme from Saccharomyces cerevisiae (E51 is substituted by Q or A and G413 by W) on the subunit association was examined. Low-resolution models of the P. sativum, Z.mobilis, and S. cerevisiae enzymes were evaluated ab initio from the scattering data. The enzyme from the bacterium and yeast appear as a dimer of dimers, whereas the plant enzyme is an octamer formed by two tetramers arranged side-by-side. The shape of the S. cerevisiae enzyme agrees well with the atomic structure in the crystal but suggests that the dimers in the latter should be tilted by approximately 10 degrees. The resulting modification of the atomic structure also yields a significantly better fit to the experimental solution scattering data than that calculated form the original crystallographic model.