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| - Small-Angle X-ray Solution-Scattering Studies on Ligand-Induced SubunitInteractions of the Thiamine Diphosphate Dependent Enzyme PyruvateDecarboxylase from Different Organisms
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| - The quaternary structures of the thiamine diphosphatedependent enzyme pyruvate decarboxylase(EC 4.1.1.1) from the recombinant wild type of Saccharomycescerevisiae and Zymomonas mobilisandfrom germinating Pisum sativum seeds were examined by X-raysolution scattering. The dependence ofthe subunit association equilibrium on the pH and the presence of thecofactors thiamine diphosphate andmagnesium ions were compared, and the differences between the catalyticproperties of the differentenzymes are discussed. The influence of amino acid substitutions at thecofactor binding site of the enzymefrom Saccharomyces cerevisiae (E51 is substitutedby Q or A and G413 by W) on the subunit associationwas examined. Low-resolution models of the P. sativum, Z.mobilis, and S. cerevisiae enzymeswereevaluated ab initio from the scattering data. The enzyme from thebacterium and yeast appear as a dimerof dimers, whereas the plant enzyme is an octamer formed by twotetramers arranged side-by-side. Theshape of the S. cerevisiae enzyme agrees well with theatomic structure in the crystal but suggests thatthe dimers in the latter should be tilted by approximately 10°. Theresulting modification of the atomicstructure also yields a significantly better fit to the experimentalsolution scattering data than that calculatedform the original crystallographic model.
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