The kinetics of papain- and ficin-catalysed hydrolyses in the presence of alcohols

Papain kinetics in the presence of a water-miscible organic solvent

The effects of various concentrations of a water-miscible organic solvent [a 7:3 (v/v) mixture of N, N dimethylformamide and dimethylsulfoxide] on the kinetics of papain have been investigated. The parameters k(cat) and K(m) for the amidase and esterase activity of papain using N-alpha-benzoyl-DL-arginine-p-nitroanilide (BAPNA) and N-alpha-benzoyl-L-arginine ethyl ester (BAEE) as substrates were determined. For both types of activity, k(cat) initially increased (up to about 15% solvent), and then decreased with increasing concentrations of organic solvent. In contrast, K(m) increased sharply with the organic solvent concentration. Active site titration at 0 and 50% solvent indicated no change in the amount of active enzyme. Fluorometric measurements of the emission spectrum of papain did not indicate any major conformational changes with increasing concentrations of organic solvent.

Evidence that binding to the s2-subsite of papain may be coupled with catalytically relevant structural change involving the cysteine-25-histidine-159 diad. Kinetics of the reaction of papain with a two-protonic-state reactivity probe containing a hydrophobic side chain

A method is proposed by which site-specific reactivity probes that exhibit different reactivities in two ionization states can be used to detect association-activation phenomena that involve repositioning of acid/base groups in enzyme active centres. The pH-dependences of the apparent second-order rate constants (k) for the reactions of the thiol group of papain (EC 3.4.22.2) with a series of two-protonic-state reactivity probes are compared. The short-chain probes, 2,2'-dipyridyl disulphide and n-propyl 2-pyridyl disulphide, react at pH6 in adsorptive complexes and/or transition states with geometries that do not permit hydrogen-bonding of the pyridyl nitrogen atom with the active-centre imidazolium ion, as evidenced by the rate minima at pH6 and the rate maxima at pH4 provided by reagent protonation. Only when the probe molecule, e.g. 4-(N-aminoethyl 2'-pyridyl disulphide)-7-nitrobenzo-2-oxa-1,3-diazole [compound(III)], contains a long hydrophobic side chain is the reaction characterized by maximal rates at about pH6, as in the acylation step of the catalytic act (at pH6, k(compound III)/k(2,2'-dipyridyl disulphide) approximately 100). It is proposed that this striking difference in profile shape may result from binding of the hydrophobic side chain of compound (III) possibly in the S(2)-subsite of papain, which promotes a change in catalytic-site geometry involving repositioning of the imidazolium ion of histidine-159 and hydrogen-bonding with the N atom of the leaving group, as has been postulated to occur in the acylation step of substate hydrolysis.

Rate constants of individual steps in papain-catalysed reactions

Rate constants for acylation of papain (EC 3.4.22.2) by specific substrates and its subsequent deacylation are derived from kinetic analysis of the reactions in the presence of aminoacetonitrile and methanol. Methyl and ethyl hippurate and methyl N-benzyloxycarbonylglycinate have marginally higher values of rate constants for acylation than for deacylation, while the reverse is true for ethyl N-benzoyl-L-arginate. Both acylation and deacylation are rate-determining for these substrates, while only deacylation irate-determining for methyl-N-acetyl-L-phenylalanylglycinate. Deacylation is the only rate-determining step for p-nitrophenyl esters of hippuric acid, N-benzyloxycarbonylglycine and N-acetyl-L-phenylalanylglycine. These results are discussed in relation to those from inactivation of the enzyme by alkylating agent in the presence of substrate.

Papain-catalyzed reactions at subzero temperatures

As a first step in the investigation of papain catalysis using subzero temperatures to detect, accumulate, and characterize enzyme-substrate intermediates, we have studied some potential cryosolvents and carried out preliminary intermediate trapping experiments. The effects of subzero temperatures and aqueous dimethyl sulfoxide solutions on the papain-catalyzed hydrolysis of Nalpha-carbobenzoxy-L-lysine p-nitrophenyl ester have been investigated in detail. At 0 degrees C, the value of kcat decreases with increasing dimethyl sulfoxide concentration, decreasing in proportion to the decreased water concentration; however, the value of Km increases exponentially. The effect on Km can be accounted for by a combination of both dielectric and competitive inhibition effects. The Arrhenius plot for the deacylation reaction in 7.65 M (60% v/v) dimethyl sulfoxide is linear over the temperature range 0 to -45 degrees C and extrapolates to a calculated value of kcat at 25 degrees C in excellent agreement with that obtained in the absence of organic solvent. The pH-rate profile is not substantially perturbed by the presence of 7.65 M dimethyl sulfoxide. At -45 degrees C and below, turnover occurs extremely slowly, and is essentially negligible, although acylation is still quite rapid. Consequently, the acyl enzyme, Na-carbobenzoxy-L-lysyl-papain, can be readily accumulated and trapped at temperatures below -50 degrees C. At these low temperatures, under conditions of excess substrate, the amount of p-nitrophenol liberated in the acylation reaction is equivalent to the active-site normality of the enzyme, indicating a 1:1 stoichiometry in formation of the acyl enzyme. The effect of dimethyl sulfoxide up to 7.65 M, on the intrinsic ultraviolet, fluorescence, and circular dichroic properties of the enzyme shows no evidence of any solvent-induced structural changes. All experimental observations are consistent with the conclusion that 7.65 M dimethyl sulfoxide and subzero temperatures have no deleterious effects on papain-catalyzed reactions. A related series of experiments indicate that aqueous ethanol cryosolvents up to 13.7 M (80% v/v) are also suitable. Preliminary experiments at subzero temperatures using Na-carbobenzoxy-L-lysine methyl ester suggest the existence of three enzyme-substrate intermediates which can be detected and accumulated.

Behavior of soluble and immobilized acid phosphatase in hydro-organic media

The hydrolysis of p-nitrophenyl phosphate by wheat germ acid phosphatase (orthophosphoric monoester phosphohydrolase, EC 3.1.3.2) has been investigated in mixtures of aqueous buffers with acetone, dioxane and acetonitrile. The enzyme was either in free solution or immobilized on a pellicular support which consisted of a porous carbonaceous layer on solid glass beads. The highest enzyme activity was obtained in acetone and acetonitrile mixed with citrate buffer over a wide range of organic solvent concentration. In 50% (v/v) acetone both V and Km of the immobilized enzyme were about half of the values in the neat aqueous buffer, but the Ki for inorganic phosphate was unchanged. In 50% (v/v) mixtures of various solvents and citrate buffers of different pH, the enzymic activity was found to depend on the pH of the aqueous buffer component rather than the pH of the hydro-organic mixture as measured with the glass-calomel electrode. The relatively high rates of p-nitrophenol liberation in the presence of glucose even at high organic solvent concentrations suggest that transphosphorylation is facilitated at low water activity.