THE TRYPSIN CATALYZED HYDROLYSIS OF p-nitrophenyl ACETATE1. " Can. J. Chem. Downloaded from by

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1 THE TRYPSIN CATALYZED HYDROLYSIS OF p-nitrophenyl ACETATE1. " The hydrolysis of p-nitrophenyl acetate (NPA) by trypsin has been investigated in the early stage of the reaction using stopped-flow techniques. The influence of ph on the initial rate suggests competitive inhibition of the active site of the enzyme by hydrogen ions. The dissociation constant of the enzyme obtained from the kinetics of this reaction (pk = 6.9) indicates possible catalysis by an amino group or an imidazole group of the enzyme. Lysine methyl ester as an analogue of the enzyme catalyzes the hydrolysis of NPA under similar experimental conditions. The results are described in terms of an assumed mechanism and the nature of the catalytic site is discussed. Hartley and Kilby (1, 2) first reported that the enzyme chymotrypsin was capable of catalyzing the hydrolysis of p-nitrophenyl acetate (NPA). From their results they concluded that the mechanism consisted of three distinct stages. The first stage is extremely rapid and concerns the combination of enzyme with substrate, the second stage is moderately rapid and involves the liberation of phenol and the acetylation of the enzyme, while the last stage, which is slow, liberates acetate with the consequent regeneration of enzyme. Gutfreund and Sturtevant (3), employing stopped-flow techniques, showed that the rate constant for acetylation (second stage) of chymotrypsin with NPA was ph independent. This is contrary to their results for dinitrophenyl acetate (4) and the results of other workers (5) who report ph dependence. The purpose of our investigation into the mechanism of trypsin catalyzed hydrolysis of NPA is mainly to clarify the problem as to whether or not the rate constant related to acetylation is ph inclependent, and compare the suggested mechanism with a mechanism recently proposed by Cunningham (6). His scheme, which emphasizes the hydroxyl group of a serine residue as primary site rather than the imidazolyl group of histidine, satisfies the available experimental evidence (7). However, when trypsin (8) or chymotrypsin (9) are treated with DFP3"diisopropyl fluorophosphate) no histidine residue occurs in the vicinity of the phosphorylated serine, so that we would like to present some evidence that an amino group other than the imidazolyl might function as part of the catalytic site. EXPERIMENTAIL Twice crystallized trypsin (salt-free) was obtained from the Mann Research Laboratories Inc., New York. The substrate p-nitrophenyl acetate was synthesized as described by Chattaway (10). All reactions were carried out in 20% (v/v) isopropyl alcohol at 25" C with M phosphate as buffer. The ph values quoted refer to the buffer and were measured in the absence of alcohol, but in the presence of enzyme, using a Beckman Model G ph meter. The rapid acetylation reaction was studied employing the stopped-flow apparatus 'Manz~script received Az~gz~st 22, Contribution from tlze Department of Chemistry, University of Ottawa, Otlawa, and tlze Laboratories of the Food and Drug Directorale, Deparlw~enl of Nalional Heallh and Welfare, Ottawa, Canada.?Presenled al the Chemical Inslilz~le of Canada Conference, Toronlo, May Present address: Laboratories of the Food and Drug Directorale, Deparl~nenl of Nalional H1,alll~ and Welfare, Ollawa, Canada. LPresenl address: Deparlnlent of Cl~ertzislry, Lava1 Universily, Quebec, Que. Can. J. Chem. Vol. 37 (1959)

2 752 CANADIAN JOURNAL OF CHEMISTRY. VOL described previously (11), while the slow deacetylation reaction was followed with a Cary Model 11 spectrophotometer using the differential technique. The data were treated accordiilg to the suggested mechanism by the theoretical method outlined in a preceding paper (12). RESULTS AND DISCUSSION Efect of ph 012 Acetylation The initial rapid production of p-nitrophenol, corresponding to the forination of acetyl-trypsin was recorded at several substrate and hydrogen-ion concentrations. The constant F (12) was the11 calculated from the reaction trace using the Guggenheim technique (13). A typical Lineweaver-Burk plot (14) of some of the results is given in Fig. 1, where the extrapolatioil of l/f at infinite substrate concentratioil yields the same value for l/kz at two widely separated ph values. The fact that seven such plots between ph 6.2 and 7.8 produced the same result for l/kz strongly suggests competitive inhibition by hydrogen-ions. The same result, though not elucidated as competitive inhibition, was obtained with chymotrypsin (3). Similar to the usual Michaelis-Menten treatment, the slopes obtained from the Lineweaver-Burk plot in Fig. 1 involve K,/k2. According to Laidler (Is), the variation of K,JkZ with ph provides the dissociation constants of the groups which comprise the active site of the enzyme. It should also be pointed out, as has been done previously (3), that should the three-step mechanism be general for other enzyme systems, then the standard Michaelis constant may consist of a rather large number of constants.!/so FIG Lineweaver-Burk plot of the hydrolysis of NPA by trypsin (1 mg/ml) at ph 6.4 and 7.8. The results given in Fig. 1 can be explained if we assume that hydrogen-ion competitively inhibits the initial combination of enzyme and substrate. With this in mind, the scheme for acetylation becomes, (M-I)

3 STEWART AND OUELLET: 9-NITROPHENYL ACETATE where E is the active form and EH+ the inactive form of the enzyme, S is the substrate, ES1 is the R4ichaelis complex, ES2 is the acetylated enzyme, P1 is the phenol, and kl, kfl, kz, kl, and klr are the appropriate rate constants. When this scheme is treated theoretically (12) and the effect of hydrogen-ion taken into account (15, 16, 17) - F, the quantity to be determined, formulates to Equation [2] may be also written as, Figure 2 was obtained by plotting the slopes of equation [6] against [HI. The resultant dissociatio~l constant Ka = 1.4X10L7, which is related to the active site of the enzyme, was calculated from the intercept K,/k2 and the slope K,/k2 Ka. Equation [2] may be rearranged in still another fashion, r 0" M aec FIG. 2. The determination of K,,/kp and K,,,/Ka k? for the trypsin-npa system, from a plot of slopes against [H+].

4 754 CAN.4DIAN JOURNAL OF CHEMISTRY. VOL The constants kz and K, were evaluated by plotting the intercepts of equation [7] against l/so, as illustrated in Fig. 3. The rate constant of acetylation kn = 1.5 sec-i was estimated from the intercept l/kz, which allowed a Michaelis constant K, = 2.1 )(lo-? ilf to be determined from K,/kz, the slope of the line. FIG. 3. The estimation of 1/kn and K,,,/k? by means of a plot of intercepts versus so, for trypsin catalyzed hydrolysis of NPA. The results for k2, Ka, and K, are included in Table I, where the constants for trypsin and:chymotrypsin are compared. TABLE I Kinetic constants for the enzymatic hydrolysis of p-nitrophenyl acetate Constant Trypsin Chymotrypsin* *Calculated from the results of Gutfreund. PI. and Sturtevant. J. M. Biochem. J (1966). t6.8 for ester hydrolysis (ref. 5). Eflect of ph on Deacetylation The reactions which represeilt deacetylation are: where ES2 is the unstable forin and ESZH+ the stable form of the acetylated enzyme, P2 is acetate, ancl kl, ks, ancl k-5 the appropriate rate constants. When t is large the rate is

5 STE\trART.AND OUELLET: p-nitrophenyl ACETATE [8] PI = ~2 = kt3 eo (refs. 3, lo), where and [lo1 Ka' = k_6/k~,. The slow deacetylation reaction was studied spectrophotometrically by following the phenol produced after the rapid acetylation reaction had subsided. The results were conveniently corrected for the spontaneous hydrolysis of NPA by placing the reaction solution in the sample cell and the same solution less enzyme in the reference cell of a double beam spectrophotorneter, and recording the difference in absorbance. The reaction was started by introducing equal portioils of NPA simultaneously into both cells with hypodermic syringes. The necessary corrections were then applied to the records to take into account that the fraction of colored species of p-nitrophenol, i.e. pk, = 7.15 (18), varies with ph. In the above manner the rates at different hydrogen-ion concentrations, which are plotted as the reciprocal rate against [HI in Fig. 4, were obtained. This plot gives a dissociation constant Ka' = 1X10-7 for acetylated trypsin and compares favorably with the constant Ka, concerned with the active site. The rate constant for deacetylation, k3 = 1.3X10-? sec-l, was determined from the intercept of Fig. 4; the extinction coefficient of p-nitrophenol was taken as 1.8X lo1 (19), and the molecular weight of trypsi11 as 21,000 (20). I 8 r I - 25' C So = 4~ M Eo= Irng /Irnl! / [HI x 10 ~ K = O 7.03 kr =.Ol26 rsc-1 FIG. 4. A plot of the reciprocal rate of liberation of p-~~itrophe~~ol against [HC], for the determination of kjeo and Ka' in the catalyzed hydrolysis of SPA by trypsin. Mechanism of Hydrolysis When the effect of protor1 inhibition is introduced, the result suggests a mechanism similar to that proposed by Cunningham (6). Quite different from former mechanisms his scheme advocates the hydroxyl group as primary site, rather than the imidazolyl which is now thought to be secondary. To facilitate a comparison of the two mechanisms those stages which correspond to reactions (a) to (e) are represented in the scheme outlined in Fig. 5. As our present experimental method does not detect the acyl-transfer

6 CANADIAN JOURNAL OF CHEMISTRY. VOL K o (inoctive) (inoctive) 7 (inhibition) (octive) FIG. 5. The mechanism of enzymatic ester hydrolysis. reaction, the absence of this transfer is the main difference in the two schemes. Thus, the rate constant k3 for deacetylation is a composite constant involving two stages: the transfer of acetyl to nitrogen and its subsequent release. In the scheme as shown in Fig. 5 the enzyme is inactive to complex formation unless a proton is released from the active site, and explains why inhibition is competitive rather than non-competitive. On examination of equation [I] it is seen that under certain conditions competitive inhibition may not be realized. This is especially true when the rate constants of acetylation and deacetylation are of the same order of magnitude. A Lineweaver-Burk plot of the data will not produce a straight line with constant intercept, and competitive inhibition may appear to be non-competitive. The initial point of attack of substrate is the hydroxyl group of serine where the complex undergoes partial decomposition, releasing phenol. The hydroxyl group becomes acetylated, but before deacetylation can proceed there must be a transfer of the acetyl group to the nitrogen. The stability of certain acylated enzymes at low ph has been reported (5, 21, 22). This can be explained 011 the basis that transfer from the hydroxyl only takes place at higher ph values where the nitrogen does not have the inhibiting hydrogen intact. Therefore, proton inhibition at the deacylation stage accounts for this inactivity and stability of acetyltrypsin and chymotrypsin below a ph of 5 to 6. A similar approach may be used to explain inhibition by certain organophosphorus compounds. For instance, in the case of DFP the hydroxyl group of a serine residue in the enzyme is phosphorylated, but transfer of this phosphoryl group to the nitrogen is difficult, resulting in an inactive enzyme. ikture of the Catalytic Site Tracer experiments show that DIP3"diisopropyl phosphate) is attached to the hydroxyl group of a serine residue and no histidine (imidazole) occurs in the vicinity (8, 9). However, Westheimer (23) presents in detail the necessary orientation of the active site of chymotrypsin that enables the imidazolyl and hydroxyl groups to come together by folding. Also, spectrophotometric evidence has been published to show that acetyl-imidazolyl is an intermediate when acetyl-chymotrypsin decomposes (7). The lack of histidine in the neighborhood of DIP3"serine1 and the differences in the pk values and the degree of activity of trypsin and chymotrypsin (see Table I) has led us to postulate that other groups might also function in partnership with the hydroxyl group to constitute an active site. As for model hydrolysis (24) the facts present the possibility that the basicity of the nitrogen containing group of the active site may

7 STEWART AND OUELLET: 0-NITROPHENYL ACETATE I MOLES LY ME X lo4 '088 % ISOPROPYL ALCOHOL FIG. 6. The effect of LYME on the rate of hydrolysis of NPA in the presence of 20% (v/v) isopropyl alcohol. FIG. 7. The effect of isopropyl alcohol on the rate of hydrolysis of NPA in the presence of 4x10-" M LYME. be the important factor. Since the pk of a free a-amino group in an ester is in the neighborhood of the pi< for the active site of trypsin, lysine methyl ester (LYME) was tested and found to influence the rate of hydrolysis of NPA under conditions comparable to enzymatic hydrolysis. This effect is demonstrated in Fig. G, where the rate of hydrolysis of NPA is proportional to the amino acid concentration. The same result was obtained by varying the isopropyl alcohol concentration and is given in Fig. 7. These results show that the aliphatic hyclroxyl and anliilo groups could play a significant part in the constitution of an enzymatic site. Further evidence to support this hypothesis can be secured from the literature. Hartley and Icilby (?) have reported that the phenolic hydroxyl and the aliphatic amino groups may act as acceptors of the acetyl group. The work of Balls and co-worlters reveals that chymotrypsin mediates the transfer of acetyl to ethanol (22) or trimethylacetyl to n-butanol (25), illustrating the importance of hydroxyl; while the significance of an amino group is shown by the transfer of acetyl from chymotrypsin to hydroxylamine (22) and the fact that certain monoacylated chymotrypsin derivatives are reactivated by tyrosine ethyl ester (25). Also, Wagner- Jauregg and Haclcley (26) discovered during an investigation of the model hydrolysis

8 758 CANADIAN JOURNAL OF CHEMISTRY. VOL of organophosphorus inhibitors that certain amino and hydroxyl substituted pyridines, and such compounds as catechol, increased the rate of hydrolysis as compared to pyridine. Finally, some results were obtained by infrared spectroscopy using the KBr-pellet technique which indicate the a-amino group is acetylated when NPA is allowed to hydrolyze in the presence of tyrosine ethyl ester (TYEE). From the infrared spectra in Fig. 8 it is seen that both acetyl-tyee and the system of NPA hydrolyzed in the presence of TYEE possess appreciable absorbance at 1660 cm-i, while TYEE and hydrolyzed NPA do not; thereby disclosing that tyrosine ethyl ester is acetylated during the hydrolysis of NPA. HYDROLYZED NPA FREOUENCY (CM-I) ACETYL TYEE FIG. 8. '1'he infrared spectra of freeze-dricd solutions of hydrolyzed NPA, TYEE, acetyl-'1'yee, allti NPX hydrolyzed in the presence of 'TYEE; each containing 20% (v/v) isopropyl alcohol a11d 0.05.\f phosphate ph 7.8. The rate constant related to the acetylation stage during the trypsin or chyn~otrypsin catalyzed hydrolysis of NPA appears to be ph independent. The mechanism implied by the result is in agreement with the postulated mechanism of Cunningham. There is some degree of suggestion that this three-step scheme might be general, and in the instance of other enzyme systems the 3/lichaelis constant may be a con~posite of a larger number of constants than formerly anticipated. Even though it has been sho\v11 that hydroxyl groups and amino groups influence the rate of hydrolysis of NPX, further investigations will be required before clefinitely establishing the nat~~re of their relationship to enzymatic hydrolysis. This is particularly true in the case of the free a-amino group. Although the pk of this group in an ester and possibly a peptide mould be in agreement with the pk reported for the active site of trypsin, free a-amino groups are believed to exist only in the terminal residues of proteins. We wish to thank the National Research Council of Canacla for their financial support of a portion of this research.

9 STEWART AND OUELLET: p-nitrophenyl ACETATE REFERENCES 1. HARTLEY, B. S. and I<ILBY, B. A. Biochem. J. 50, 672 (1952). 2. HARTLEY, B. S, and KILBY, B. A. Biochem. J. 56, 288 (1954). 3. GUTFREUND, H. and STURTEVANT, J. M. Biochem. J. 63, 656 (1956). 4. GUTFREUND. H. and STURTEVANT. T. M. Proc. Natl. Acad. Sci (1956). \, 5. DIXON, G. H: ~~~~NEURATH, H.' j. Biol. Chem. 225, 1049 (1957).' 6. CUNNINGHAM, L. W. Science, 125, 1145 (1957). 7. DIXON, G. H. and NEURATH, H. J. Am. Chem. Soc. 79, 4558 (1957). 8. DIXON, G. H., KAUFFMAN, D. L., and NEURATH, H. J. Am. Chem. Soc. 80, 1260 (1958). 9. SCHAFFER. N. I<.. SIMET., L.., HARSHMAN.. S... ENGLE. R. R.. and DRISKO. R. W. T. Biol. Chem. 225, lj7 (195ij. 10. CHATTAWAY, F. D. J. Chen~. Soc. 134, 2495 (1931). 11. STEWART, J. A. and OUELLET, L. Can. J. Chem. 37, 744 (1959). 12. OUELLET, L. and STEWART, J. A. Can. J. Chem. 37, 737 (1959). 13. GUGGENHEIM, E. A. Phil. Mag. 2, 538 (1926). 14. LINEWEAVER, H. and BURK, D. J. Am. Chem. Soc. 56, 658 (1934). 15. LAIDLER, K. J. Trans. Faraday Soc. 51, 550 (1955). 16. LAIDLER, I<. J. Trans. Faraday Soc. 51, 528 (1955). 17. LAIDLER, K. J. Trans. Faraday Soc. 51, 540 (1955). 18. ROBINSON, R. A. and BIGGS, A. I. Trans. Faraday Soc. 51, 901 (1955). 19. BIGGS, A. I. Trans. Faraday Soc. 50, 800 (1954). 20. SEDLACEK, B. and BARTL, P. Chem. Listy, 49, 996 (1955). 21. BALLS, A. K. and ALDRICH, F. L. Proc. Natl. Acad. Sci. 41, 190 (1955). 22. BALLS. A. I<. and WOOD. H. N. T. Biol. Chem (1956) IVEST~EIMER F. H. roc. Natl. Acad. Sci ?l957).' 24. BRUICE, T- C. and SCHMIR, G. L. J. Am. ~him. SO~. 79; 1663 (1957). 25. MCDONALD, C. E. and BALLS, A. K. J. Biol. Chem. 227, 727 (1957). 26. ~VAGPI'ER-JAUREGG, P. and HACKLEY, B. E. J. Am. Chem. Soc. 75, 2125 (1953).