Escherichia coli succinyl-CoA synthetase (EC 6.2.1.5) was irreversibly inactivated on incubation with the adenine nucleotide analogue 5′-p-fluorosulphonylbenzoyladenosine (5′-FSBA). Optimal inactivation by 5′-FSBA took place in 40% (v/v) dimethylformamide. ATP and ADP protected the enzyme against inactivation by 5′-FSBA, whereas desulpho-CoA, an analogue of CoA, did not. Inactivation of succinyl-CoA synthetase by 5′-FSBA resulted in total loss of almost four thiol groups per alpha beta-dimer, of which two groups appeared to be essential for catalytic activity. 5′-FSBA at the first instance appeared to interact non-specifically with non-essential thiol groups, followed by a more specific reaction with essential thiol groups in the ATP(ADP)-binding region. Plots of the data according to the method of Tsou [(1962) Sci. Sin. 11, 1535-1558] revealed that, of the two slower-reacting thiol groups, only one was essential for catalytic activity. When succinyl-CoA synthetase that had been totally inactivated by 5′-FSBA was unfolded in acidic urea and then refolded in the presence of 100 mM-dithiothreitol, 85% of the activity, in comparison with the appropriate control, was restored. These data are interpreted to indicate that inactivation of succinyl-CoA synthetase by 5′-FSBA involves the formation of a disulphide bond between two cysteine residues. Disulphide bond formation likely proceeds via a thiosulphonate intermediate between 5′-p-sulphonylbenzoyladenosine and one of the reactive thiol groups of the enzyme.
Adenosine 5'-O-(3-thio)triphosphate, a substrate and potent inhibitor of Escherichia coli succinyl-CoA synthetase. Additional evidence for a cooperative alternating-sites mechanism.
