Synchrotron radiation solution x-ray scattering study of the pH-dependence of the quarternary structure of yeast pyruvate decarboxylase

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Abstract
The pH dependence of the quaternary structure of pyruvate decarboxylase from yeast wasstudied in therange 6.2 < pH < 8.4. There is an equilibrium with a midpoint around pH 7.5 betweentetramers and dimers, and the catalytic activity of the enzyme depends on the volume fraction of tetramer.This equilibrium may provide an additional regulating mechanism besides substrate activation sinceaccumulation of pyruvate would lead to a reduction in pH and hence an increase of the concentration ofthe catalytically active tetramer. Radiation damage during the X-ray scattering experiments results in ashift of this equilibrium and in the formation of octamers. These effects could be circumvented and analyzedusing experimental and data processing methods which can be readily applied to other radiation-sensitivesystems. The low-resolution shapes of the dimers and tetramers were determined from the scattering curvesusing spherical harmonics. The results indicate that a conformational change must occur in the dimersupon formation of the tetramers, in agreement with earlier circular dichroism measurements.

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