A protein engineering study of the role of aspartate 158 in the catalytic mechanism of papain

Abstract

The controversy concerning the various suggested roles for the side chain of Asp158 in the active site of papain has been clarified by using site-directed mutagenesis. Both wild-type papain and an Asp158Asn variant were produced in a baculovirus-insect cell expression system, purified to homogeneity from the culture, and characterized kinetically. With CBZ-Phe-Arg-MCA as substrate, the kcat KM and kcat values obtained for the Asp158Asn papain are 20000 M-1.cntdot.s-1 and 34 s-1, respectively, as compared with values of 120000 M-1.cntdot.s-1 and 51s-1 obtained for the wild-type papain. In addition, the pH-(kcat/KM) profile for the Asp158Asn enzyme is shifted relative to that for the wild-type enzyme to lower values by approximately 0.3 pH unit. This shows clearly that Asp158 is not, as previously postulated, an essential catalytic residue. In addition the pH dependency data are interpreted to indicate that, contrary to earlier suggestions, the negatively charged side chain of Asp158 does not significantly stabilize the active-site thiolate-imidazolium ion pair. However, its presence does influence the pKa''s associated with ion-pair formation in a manner compatible with electrostatic considerations.