X-ray small-angle studies of the pyruvate dehydrogenase core complex from Escherichia coli K-12. II. Subunit structure of the core complex.

The interpretation of X-ray small-angle data of the pyruvate dehydrogenase core complex from E. coli K-12 reveals the fine structure of the complex. Specific inner surface (7.07-10(-2 A-1), inner surface (7.60 - 10(5) A2), MEAN TRANSVERSAL LENGTH (56.6 A), coherence length (123.5 A), structural factor (1.1), and coherence area (3.27 - 10(4) A2) have been determined as further structural parameters characterizing the colloidal distribution of matter. Fouier transformations of scattered intensity and of structural amplitude have been carried out and show the existence of slightly disturbed spherical symmetry of the complex built up from subunits. The mean diameter of the three different subunit components of about 78 A was determined from the correlation function or from the distance distribution. The number of subunits in the complex was ascertained to be 40. The radial excess electron density distribution shows the arrangement of the core complex from a "core" (formed by the transacetylase components) with a small hole inside and a "shell" (formed by the pyruvate dehydrogenase and dihydrolipoamide dehydrogenase components). Although not representing a unique solution, a lot of model calculations indicate how the complex is arranged from subunits. At each edge of a cubic centre, the edge formed by two chains of transacetylase, two chains of pyruvate dehydrogenase and two chains of dihydrolipoamide dehydrogenase components are arranged according to the best fit. Far-reaching conformity between experimental results and model was established.