Intrinsic fluorescence of succinyl-CoA synthetase and four tryptophan mutants. Tryptophan 76 and tryptophan 248 of the .beta.-subunit are responsive to CoA binding

Abstract

In the present study, single mutations in which each of the three tryptophans (.beta.-Trp43, .beta.-Trp76, and .beta.-Trp248) has been changed to phenylalanine (designated W34F, W76F, and W248F) have been accomplished by the technique of site-directed mutagenesis and the mutant proteins isolated. In addition, a double mutant in which .beta.-Trp43 and .beta.-Trp248 were changed to phenylalanines (W43,248F) has also been isolated. Each of the mutant enzymes was practically as active as wild type. Since the emission spectrum of .beta.-Trp76 reflected a low fluorescence intensity for this residue, it was possible to obtain the emission spectrum of each tryptophan residue by using W43F, W248F, and W43,248F. From the positions of the emission maxima and the results of iodide quenching of fluorescence, it was deduced that .beta.-Trp248 is a surface residue, .beta.-Trp43 is buried, and .beta.-Trp76 is intermediate in location. Coenzyme A, but no other substrate, protected the fluorescence of .beta.-Trp76 and .beta.-Trp248, but not of .beta.-Trp43, against quenching by acrylamide. These results are consistent with an interaction between .beta.-Trp76 and .beta.-Trp248 and the binding site for CoA.