The influence of effectors of yeast pyruvate decarboxylase, phosphate, pyruvamide, thiamin diphosphate and Mg*, on the pH-dependent equilibrium between dimers and tetramers was studied by synchrotron radiation X-ray solution scattering. Thiamin diphosphate and phosphate shift the equilibrium to higher values without altering the structure of the oligomers. Pyruvamide, a substrate analogue activator, induces a sigruficant change in the structure of the tetramer. By eliminating radiation damage

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... addition of dithioerythrol to the buffers, the scattering curves could be measured accurately over a large angular range. They were expanded in terms of spherical harmonics to obtain the shapes of the dimers and tetramers with higher resolution than was hitherto possible. This also allowed us to position the dimers, which are centrosymrnetric at low resolution, in the tetramers which have 222 symmetry. The results indicate that addition of pyruvamide results in a less compact tetramer owing to structural changes in the dimers and to their displacements. Results The extended scattering curves of the solutions containing essentially dimers (pH 9.2) or tetramers (pH 6.5) are illustrated in Fig. 1 . The orthogonal expansion method indicates that the scattering data from the dimer can be described by ten independent parameters and those of the tetramer and of the tetramer + PA Article published online by EDP Sciences and available at http://dx.