Identification of Interactions Involved in the Generation of Nucleophilic Reactivity and of Catalytic Competence in the Catalytic Site Cys/His Ion Pair of Papain

Understanding the roles of noncovalent interactions within the enzyme molecule and between enzyme and substrate or inhibitor is an essential goal of the investigation of active center chemistry and catalytic mechanism. Studies on members of the papain family of cysteine proteinases, particularly papain (EC 3.4.22.2) itself, continue to contribute to this goal. The historic role of the catalytic site Cys/His ion pair now needs to be understood within the context of multiple dynamic phenomena. Movement of Trp177 may be necessary to expose His159 to solvent with consequent decrease in its degree of electrostatic solvation of (Cys25)-S–. Here we report an investigation of this possibility using computer modeling of quasi-transition states and pH-dependent kinetics using 3,3′-dipyridazinyl disulfide, its n-propyl and phenyl derivatives, and 4,4′-dipyrimidyl disulfide as reactivity probes that differ in the location of potential hydrogen-bonding acceptor atoms. Those interactions that influence ion pair geometr...