Effect of cysteine-25 on the ionization of histidine-159 in papain as determined by proton nuclear magnetic resonance spectroscopy. Evidence for a his-159--Cys-25 ion pair and its possible role in catalysis.

Papain was succinylated in order to increase its solubility above pH 8 so that proton NMR spectroscopy could be used to study the ionization of His-159 at the active site of the enzyme. The pH dependence of NMR spectra of catalytically active succinyl-papain and the methylthio derivative of the active-site cysteinyl residue of succinyl-papain (succinyl-papain-S-SCH3) were determined between pH 6 and 10. The pH dependence of the C epsilon 1 H resonance of His-159 in catalytically active succinyl-papain indicates that His-159 has a pK of about 8.6 in the catalytically active form of the enzyme. The position of this resonance in succinyl-papain-S-SCH3 indicates that when the active-site cysteinyl residue is methylthiolated, His-159 is completely deprotonated between pH 6 and 10. This result is taken as evidence for an imidazolium--thiolate ion-pair interaction between His-159 and Cys-25 wherein neutralization of the charge on the thiolate anion by methylthiolation would be expected to cause a marked decrease in the pK of His-159. A possible catalytic role for the ion pair in the acylation step in papain-catalyzed reactions is proposed wherein attack of a substrate by the imidazolium--thiolate ion pair is accompanied by an increase in the acidity of the imidazolium group that facilitates expulsion of the leaving group of the substrate.