α-Keto Acid Dehydrogenase Complexes

Abstract

Abstract Dihydrolipoyl transacetylase, one of three enzymes comprising the Escherichia coli pyruvate dehydrogenase complex, dissociates into subunits in acidic solutions and in the presence of sodium dodecyl sulfate at neutral pH. Dissociation of the transacetylase in dilute acetic acid solution (0.83 m, pH 2.6) produces enzymatically inactive subunits with a weight average molecular weight of approximately 70,000. This subunit is apparently a dimer of two identical polypeptide chains. Removal of the dissociating agent by rapid dilution into phosphate or tris(hydroxymethyl)aminomethane buffers (final pH, about 7) results in restoration of enzymatic activity with yields as high as 80 to 90%. The sedimentation characteristics of the reconstituted material and its appearance in the electron microscope are very similar to those of the native transacetylase. The reconstituted transacetylase combines spontaneously with pyruvate decarboxylase and dihydrolipoyl dehydrogenase to produce a large unit that closely resembles the native pyruvate dehydrogenase complex. A model of the transacetylase is proposed, based on biochemical and electron microscopic data, in which multichain morphological subunits are situated at the eight vertices of a cube.