A Study of the Quenching of the Intrinsic Fluorescence of Succinyl-CoA Synthetase from Escherichia coli by Acrylamide, Iodide, and Coenzyme A

Escherichia coli succinyl-CoA synthetase (SCS) contains three tryptophan residues per mole of αβ dimer, and all of them are on the β subunit. SCS shows an emission maximum at 335 nm which is shifted to 350 nm upon denaturation by urea or guanidine hydrochloride. Acrylamide is able to quench the tryptophan fluorescence in SCS by static and dynamic mechanisms. Substrates give protection against quenching by acrylamide. Binding of ATP to the α subunit which has no tryptophans gives as large an effect on the quenching by acrylamide as the binding of coenzyme A (CoA) to the β subunit. Addition of CoA eliminates the curvature observed in Stern-Volmer plots for acrylamide quenching obtained by lifetime measurements. Potassium iodide does not quench the SCS fluorescence in the presence of CoA. These results suggest that there are heterogeneously emitting tryptophan residues in SCS that are located at the αβ subunit contact region close to the CoA binding site. Hence, the tryptophan residues can act as intrinsic reporters of events taking place at the active site of this enzyme. Further, the present results support models for SCS that put the active site at the αβ subunit contact region.