Differences in the chemical and catalytic characteristics of two crystallographically ‘identical’ enzyme catalytic sites. Characterization of actinidin and papain by a combination of pH-dependent substrate catalysis kinetics and reactivity probe studies targeted on the catalytic-site thiol group and its immediate microenvironment

Abstract

The characteristics of actinidin (EC 3.4.22.14) and papain (EC 3.4.22.2), two cysteine proteinases whose catalytic-site regions appear to superimpose to a degree that approaches atomic co-ordinate accuracy of both crystal structures, were evaluated by determining (a) the pH-dependence in acid media of the acylation process of the catalytic act (k+2/Ks) using N alpha-benzoyl-L-arginine p-nitroanilide (L-Bz-Arg-Nan) as substrate and (b) the sensitivity of the reactivity of the catalytic-site thiol group and its pH-dependence to structural change in small, thiol-specific, two-protonic-state reactivity probes (2,2'-dipyridyl disulphide and methyl 2-pyridyl disulphide) where enzyme-probe contacts should be restricted to areas close to the catalytic site. Distortion of the catalytic sites of the two enzymes at pH less than 4 was evaluated over time-scales appropriate for both stopped-flow reactivity probe kinetics (less than or equal to 1-2 s) and steady-state substrate catalysis kinetics (3-5 min) by using the 2,2'-dipyridyl disulphide monocation as a titrant for non-distorted catalytic sites. This permitted a lower pH limit to be defined for valid kinetic analysis of both types. The behaviour of the enzymes at pH less than 4 requires a kinetic model in which the apparently biomolecular reaction of enzyme with probe reagent is separated from the process leading to loss of conformational integrity by a potentially reversible step. The acylation of actinidin with L-Bz-Arg-Nan in acidic media occurs in two protonic states, one produced by raising the pH across pKa less than 4 which probably characterizes the formation of -S-/-ImH+ ion pair (pKa approx. 3) and the other, of higher reactivity, produced by raising the pH across pKa 5.5, which may characterize rearrangement of catalytic-site geometry. The pH-dependence of the acylation of papain by L-Bz-Arg-Nan is quite different and is not influenced by protonic dissociation with pKa values in the range 5-6. The earlier conclusion that the acylation of papain depends on two protonic dissociations each with pKa approx. 4 was confirmed. This argument is now more firmly based because titration with 2,2'-dipyridyl disulphide permits the loss of conformational integrity to be taken into account in the analysis of the kinetic data at very low pH. Methyl 2-pyridyl disulphide was synthesized by reaction of pyridine-2-thione with methyl methanethiolsulphonate and its pKa at I = 0.1 was determined by spectral analysis at 307 nm to be 2.8.(ABSTRACT TRUNCATED AT 400 WORDS)

Anthelmintic action of plant cysteine proteinases against the rodent stomach nematode,Protospirura muricola,in vitroandin vivo

“…Therefore, papaya latex is efficacious in vivo against P. muricola adult female worms, but only when preceded by an antacid, such as cimetidine, which neutralizes the stomach pH, maintaining it at approximately 7 . 0, close to the optimal pH for cysteine proteinase activity (Salih et al 1987). …”

“…Cysteine proteinases of the papain family are stable between pH 4 and 9, with optimal activity at neutral pH, but their activity decreases greatly below pH 4 and above pH 9, due largely to the pK a s of the active-site thiolate and imidazolium ions (Salih et al 1987). This would explain the lack of effect of papaya latex alone (stomach pH<4) or when preceded by NaHCO 3 (stomach pHy9).…”

“…We hypothesized that this could be due to the low pH of the stomach, conditions in which these enzymes are known to be poorly active (Salih et al 1987 ;Huet et al 2006). When mice were pre-treated with NaHCO 3 or the antacid, cimetidine, to increase the pH of the stomach before papaya latex was administered, a significant reduction in worm burden (61 .…”

“…0 (Salih et al 1987), with the pK a values of the active-site cysteine and histidine residues being around 4 . 0 and 9 .…”

In vitro and in vivo anthelmintic efficacy of plant cysteine proteinases against the rodent gastrointestinal nematode, Trichuris muris

“…It is worth noting that actinidain is very similar in substrate specificity and structure to many other members of the papain family (Varughese et al 1992), making the clear distinction in anthelmintic efficacy truly remarkable. The arrangement of the amino acids in the hydrophobic binding site is not identical for both enzymes however, suggesting that discrimination between possible substrate structures is feasible (Baker, 1980 ;Brocklehurst, Baines and Malthouse, 1981 ;Salih et al 1987 ;Kowlessur et al 1989). The target proteins in the cuticle for the efficacious cysteine proteinases still remain unknown.…”