Picornains 2A are cysteine proteases of picornaviruses, a virus family containing several human and animal pathogens. The pH dependencies of the alkylations of picornain 2A of rhinovirus type 2 with iodoacetamide and iodoacetate show two reactive thiol forms, namely the free thiolate ion at high pH and an imidazole assisted thiol group at low pH. Kinetic deuterium isotope effects do not support general base catalysis by the imidazole group, but rather the existence of a catalytically competent thiolate–imidazolium ion-pair. The nature of the ion-pair differs from that of papain, the paradigm of cysteine proteases. The ion-pair is confined to the same, unusually narrow pH range in which the enzyme exhibits catalytic activity.
