[Small angle X-ray scattering studies on the structure and symmetry of yeast pyruvate decarboxylase in solution]

The holo-enzyme of the pyruvate decarboxylase EC 4.1.1.1 isolated from brewer's yeast was studied by small-angle X-ray scattering. The point-group symmetry 222 deduced from electron microscopic investigations was confirmed for molecules in solution. The best approximation of the overall shape of the enzyme was reached by an homogeneous triaxial ellipsoid, with the half-axes of a = 2.47 nm, b = 5.08 nm and c = 8.0 nm. The ellipsoid is halved vertical to the a-axis, and both halves are turned from each other by about 30 degrees. As an explanation of the existence of four binding sites for Mg2+ and thiamine pyrophosphate on the dimeric molecule the hypothesis of a gene duplication is discussed.