The role of electrostatic interactions between the ionizable Asp158 and the active site thiolate-imidazolium ion pair of some cysteine proteinases has been the subject of controversy for some time. This study reports the expression of wild type procaricain and Asp158Glu, Asp158Asn and Asp158Ala mutants from Escherichia coli. Purification of autocatalytically matured enzymes yielded sufficient fully active material for pH (kcat/Km) profiles to be obtained. Use of both uncharged and charged substrates allowed the effects of different reactive enzyme species to be separated from the complications of electrostatic effects between enzyme and substrate. At least three ionizations are detectable in the acid limb of wild type caricain and the Glu and Asn mutants. Only two pKa values, however, are detectable in the acid limb using the Ala mutant. Comparison of pH activity profiles shows that whilst an ionizable residue at position 158 is not essential for the formation of the thiolate-imidazolium ion pair, it does form a substantial part of the electrostatic field responsible for increased catalytic competence. Changing the position of this ionizable group in any way reduces activity. Complete removal of the charged group reduces catalytic competence even further. This work indicates that hydronations distant to the active site are contributing to the electrostatic effects leading to multiple active ionization states of the enzyme.