FUNCTIONAL CONSEQUENCES OF SUBSTITUTION OF THE ACTIVE-SITE (PHOSPHO) HISTIDINE RESIDUE OF ESCHERICHIA-COLI SUCCINYL-COA SYNTHETASE

Succinyl-CoA synthetase (EC 6.2.1.5, succinate:CoA ligase (ADP-forming)) of Escherichia coli is an alpha-2-beta-2 tetramer, with the active site believed to be located at the point of contact between the two subunit types. It has been previously established that the reaction involves the intermediate participation of a phosphorylated enzyme form in the process of catalysis. The site of phosphorylation (His-246) and the binding sites for the substrates ADP and ATP are located in the alpha-subunit, and the succinate and CoA binding sites are in beta. A mutant form of this enzyme, with the active site histidine residue replaced by aspartate, has been produced in large quantities and purified to homogeneity. This form appears to be indistinguishable from the native enzyme with respect to its subunit assembly, but has no ability to catalyze the overall reaction. As expected, the His-246-alpha --> Asp mutant is incapable of undergoing phosphorylation. We have developed an assay based upon the arsenolysis of succinyl-CoA that effectively isolates the partial reaction that occurs in the portion of the active site contributed by the beta-subunit; this reaction does not involve covalent participation of His-246-alpha. We have found that the His-246-alpha --> Asp mutant is also devoid of activity in this arsenolysis reaction, indicating that an intact His-246-alpha is required for the establishment of the microenvironment in this portion of the active site that is required for the corresponding step of the overall reaction.