Crystal versus solution structures of thiamine diphosphate-dependent enzymes

The quaternary structures of the thiamine diphosphate-dependent enzymes tra nsketolase (EC 2.2.1.1; from Saccharomyces cerevisiae), pyruvate oxidase (E C 1.2.3.3; from Lactobacillus plantarum), and pyruvate decarboxylase (EC 4. 1.1.1; from Zymomonas mobilis and brewers' yeast, the latter in the native and pyruvamide-activated forms) were examined by synchrotron x-ray solution scattering. The experimental scattering data were compared with the curves calculated from the crystallographic models of these multisubunit enzymes. For all enzymes noted above, except the very compact pyruvate decarboxylas e from Z. mobilis, there were significant differences between the experimen tal and calculated profiles. The changes in relative positions of the subun its in solution were determined by rigid body refinement, For pyruvate oxid ase and transketolase, which have tight intersubunit contacts in the crysta l, relatively small modifications of the quaternary structure (root mean sq uare displacements of 0.23 and 0.27 nm, respectively) sufficed to fit the e xperimental data, For the enzymes with looser contacts (the native and acti vated forms of yeast pyruvate decarboxylase), large modifications of the cr ystallographic models (root mean square displacements of 0.58 and 1.53 nm, respectively) were required. A clear correlation was observed between the m agnitude of the distortions induced by the crystal environment and the inte rfacial area between subunits.