Protonated form of histidine 238 quenches the fluorescence of tryptophan 241 in subtilisin Novo.

The pH dependence of the fluorescence of phenylmethanesulfonyl-subtilisin Novo after excitation at 300 nm is studied. pK values of 5.9 and 10.3 were determined from the titration curve. The major source of indole fluorescence is tryptophan 106. Evidence is presented that the protonated form of histidine 238 quenches the fluorescence of tryptophan 241 in this enzyme. The quenching is explained by a charge-transfer complex formation between the excited indole and imidazole rings which are parallel and quite close to each other in the X-ray model.