Effect of Boric Acid on the Catalytic Activity of Streptomyces griseus Protease 3

Transcription

1 Eur. J. Biochem. 45, (1974) Effect of Boric Acid on the Catalytic Activity of Streptomyces griseus Protease 3 CarlAxel BAUER and Gosta PETTERSSON Avdelningen for Biokemi, Kemicentrum, 1,unds Universitet (Received February 28, 1974) 1. Boric acid acts as a competitive inhibitor of the Streptomyces griseus protease3catalyzed hydrolysis of pnitrophenylacetate, indicating that there is no significant interaction between inhibitor and the acylenzyme formed during the reaction. 2. Boric acid affects the rate of the transient burst of pnitrophenol formation from pnitrophenylacetate in a way suggesting that the inhibitor binds equally firmly to the free enzyme and the Michaelis complex formed between enzyme and substrate. The same conclusion can be drawn from the observation that boric acid acts as a noncompetitive inhibitor of the enzymatic hydrolysis of glutaryllphenylalaninepnitroanilide. 3. Apparent equilibrium constants for the binding of boric acid to the free enzyme and the Michaelis complex have been determined kinetically at different ph between 5 and 10. The data obtained provide evidence that the stability of the enzyme * inhibitor complex is dependent upon an ionizing group in the protein with apparent pk, of 6.6, and further show that there is no interaction between enzyme and the ionized form of the inhibitor. 4. The above results suggest that boric acid has no effect on the process of Michaeliscomplex formation, but inhibits enzyme activity by binding covalently to the activesite seryl residue under formation of a tetrahedral adduct, which might represent a transitionstate analogue for the enzymatic conversion of bound substrate into bound product. It is concluded that the nucleophilic reactivity of the activesite seryl residue is unaffected by the binding of pnitrophenylacetate to the protein, and that Streptomyces griseus protease 3 may be considered as homologous with chymotry5sin as concerns the structure and function of the catalytic site. The hydrolytic activity of the serine enzyme Streptomyces griseus protease 3 exhibits in the absence of boric acid a phdependence closely similar to that of pancreatic serine proteases [I], which suggests that the anomalous phdependence observed for the bacterial enzyme in boratecontaining buffer systems [2] can be attributed to effects of boric acid on the catalytic activity of the protein. Boric acid and boronic acid derivatives have been reported to act as transitionstate analogues for serine hydrolases such as chymotrypsin [371 and subtilisin [7,8], inhibiting enzyme activity by binding to the active site serine under formation of a tetrahedral adduct, and a similar mechanism of enzyme inhibition might be operative in the interaction between boric acid and Streptomyces griseus protease 3. Enzyme, (CBN Recommendations 1972) Streptomyces grisew. protease 3 or peptide peptidohydrolase (EC ). In order to test this idea, and hence to obtain further information about the detailed mechanistical similarities or dissimilarities between the bacterial enzyme and pancreatic serine proteases, the inhibitory effect of boric acid on the steadystate and presteadystate ratebehaviour of Xtreptomyces griseus protease 3 has been investigated and analyzed. EXPERIMENTAL PROCEDURE Materials and methods used in the present investigation were the same as those described in the precceding paper [l], with the exception that the enzymatic hydrolysis of pnitrophenylacetate was followed at 410 nm [9] instead of 348 nm. Orthoboric acid of analytical degree was obtained from BDH Chemicals (Poole, England). Eur. J. Riochem. 45 (1974)

2 474 Inhibition of Streptornyces griseus Protease 3 RESULTS Effect of Boric Acid on the SteadyState RateBehaviour of Streptomyces griseus Protease 3 The inhibitory effect of boric acid on the catalytic activity of Streptomyces griseus protease 3 will be interpreted in terms of the kinetic scheme [1,9] S pi Hz0 Pz E ES A EA E (1) 1 1 ESI EAT where K4, K,, and K, denote equilibrium constants for the binding of inhibitor (I) to the free enzyme (E), the Michaelis complex (ES), and the acylenzyme (EA), respectively. MichaelisMenten parameters for such an inhibition mechanism are given by Eqn (3) predicts that the interaction between boric acid and the free enzyme can be quantitatively evaluated from the effect of the inhibitor on the quotient Kmlkcat (the slope of LineweaverBurk graphs). According to Eqn (2), further, interactions between boric acid and the Michaelis complex or the acylenzyme are kinetically manifested as an inhibition of the acylation (k,) or deacylation (k,) steps, and may or may not be quantitatively evaluated from the effect of the inhibitor on kcat (the intercept of LineweaverBurk graphs) depending on the relative magnitudes of k, and k,. Fig. 1 shows that boric acid acts as a competitive inhibitor of the enzymatic hydrolysis of pnitrophenylacetate at ph 7.5. Since deacylation is ratelimiting with this ester substrate (k, < k, [1,9]) the k,term in Eqn (2) may be complet,ely neglected, and the observation that kcat remains unaffected by boric acid thus provides evidence that binding of the inhibitor to the acylenzyme is kinetically insignificant (K, = 0). The estimate of K4 obtained on fitting Eqn (3) to the data in Fig. 1 was 9 (& 1) Ml. The same estimate of the equilibrium constant for the binding of inhibitor to the free enzyme was obtained by evaluation of the effect of boric acid on slopes of LineweaverBurk graphs for the enzymatic hydrolysis of glutaryllphenylalaninepnitroanilide at ph 7.5, shown in Fig.2. With this peptidesubstrate acylation of the enzyme is ratelimiting (k, < k3 [l]), and the k,term in Eqn (2) may be neglected. The observation (Fig. 2) that boric acid inhibits the hydrolysis of the peptide substrate competitively, therefore, provides evidence that K, Q K4, which means that the affinity of Strepto (3) c 0 m?. " c $ c GI L m E 5 15 e a B I/ [Sl (rnm') Pig.i. Effect of boric acid on LineweaverBurk graphs for the Streptomyces griseus protease3catalyzed hydrolysis of pnitrophenylacetate. 0.9 pm enzyme and varied concentrations of substrate (0.21 mm) and inhibitor (0200 mm) in a buffer system containing 5O//, (v/v) methanol, 30 mm CaCI,, 50 mm maleic acid. and 50 mm Tris at ph 7.5, 25 "C I /[51 (mm') Fig.2. Effect of boric acid on LineweaverBurk graphs for the Streptomyces griseus protease3catalyzed hydrolysis of glutaryllphenylalaninepnitroanilide. Conditions as in Fig. 1

3 C.A. Bauer and G. Pettersson w 2 1.o u *. + v 0.5 Fig.3. Effect of boric acid on the transient burst of pnitrophenol formation during the Streptomyces griseus protease3 catalyzed hydrolysis of pnitrophenylucetate. Buffer system as in Fig.1 (A) 13 pm enzyme mixed with 2 mm substrate. (B) 13 pm enzyme mixed with 2 mm substrate preequilibrated with 400 mm boric acid. (C) 13 pm enzyme preequilibrated with 400mM boric acid mixed with 2mM substrate myces griseus protease 3 for boric acid decreases considerably on the binding of the peptide substrate. Effect of Boric Acid on the TransientState Rate Behaviour of Streptomyces griseus Protease 3 The typical oscilloscope traces shown in Fig.3 illustrate that boric acid affects the rate of the transient burst of pnitrophenol formation obseived on mixing Streptomyces griseus protease 3 with pnitrophenylacetate at ph 7.5 [9], and that identical traces ale obtained in the presence of boric acid irrespectively of the choice of premixing conditions (substrate and inhibitor mixed with enzyme, or substrate mixed with enzyme preequilibrated with inhibitor). The latter observation strongly indicates that boric acid equilibrates rapidly with the enzyme, under which condition scheme (i) predicts that the apparent firstorder rateconstant ktr for the transient burst is determined by [3] Fig.4 shows the effect of boric acid on plots of l/(lctr kcat) ws i/[s] for the enzymatic hydrolysis of pnitrophenylacetate at ph 7.5. In accordance with Eqn (4), linear graphs are obtained in this plot, and the observation that the straight lines intersect on the i/[s] axis provides evidence that the affinity I I 5 l/[s] (mm ) Fig.4. Plot according to Eqn (4) for the transient burst of pnitrophenol formation during the Streptomyces griseus protease3catalyzed hydrolysis of pnitrophenylacetate. Conditions as in Fig. 3 (B), except that the substrate concentration was varied of the enzyme for boric acid is unaffected by the binding of pnitrophenylacetate ( K4 = K5). The estimate of the binding constant obtained on fitting Eqn (4) to the data in Fig.4 was K4 = K, = 10 (&2) Ml, which agrees well with the estimate calculated from the steadystate kinetic data. Effect of ph on the Inhibition of Streptomyces griseus Protease 3 by Boric Acid The effect of boric acid on the steadystate rate of enzymatic hydrolysis of glutaryllphenylalaninepnitroanilide was investigated at different ph between 5 and i0. No significant deviations from a competitive inhibition pattern were observed over this range of phvalues, and Fig.5 shows the effect of ph on the kinetically estimated equilibrium constant (K4) for the binding of inhibitor to the free enzyme. Experimental points fit well to the bellshaped titration curve indicated in Fig. 5, which was calculated under the assumption that the affinity of the enzyme for boric acid is dependent upon ionization of a group with apparent pka of6.6 and protonation of a group with apparent pk, of 9.0. DISCUSSION The above results show that the inhibitory effect of boric acid on the catalytic activity of Xtreptomyces griseus protease 3 can be explained in terms of Eur. J. Kiochem. 45 (1974)

4 476 Inhibition of Streptomyces griseus Protease 3 0 I I I I PH Fig. 5. Effect of ph on the kinetically estimated equilibrium constant for the binding of boric acid to Streptomyces griseus protease 3 Eqns (2) (4), corresponding to the minimal kinetic Scheme (1). The fact that MichaelisMenten kinetics are obeyed in the presence of inhibitor excludes the possibility that inhibitorcontaining enzyme species participate in reaction steps leading to product formation, and the lack of effect of boric acid on the maximum velocity of the reaction for which deacylation of the enzyme is ratelimiting provides direct evidence that there is no significant interaction between inhibitor and the acylenzyme. The observation that boric acid binds equally firmly to the free enzyme and the Michaelis complex formed with pnitrophenylacetate, however, indicates that the latter substrate and the inhibitor bind independently of each others, for which reason the random scheme (5) appears to give a more adequate de '11 S EI '11 ESI scription of the actual inhibition mechanism. This formal extension of the reaction mechanism does not call for any reinterpretation of the kinetic results, since the additional step of substratebinding to the complex EI will have no influence on the ratebehaviour of the system when substrate and inhibitor preequilibrate rapidly with the enzyme. The present results, therefore, provide strong evidence that boric acid does not interfere with the process of Michaelis complex formatino between enzyme and pnitrophenylacetate, but prevents the enzyme from being acylated by the substrate. The other way round, substrate binding per se does not preclude the interaction between enzyme and boric acid, whereas acylation of the reactive seryl residue in the protein h does so. In view of the tendency of trigonal boron to form tetrahedral adducts with nucleophiles [lo], these observations strongly suggest that boric acid inhibits enzyme activity through binding covalently to the reactive seryl residue under formation of a tetrahedral complex enzymeser0b( OH),, which might represent a transitionstate analogue for the enzymatic conversion of bound substrate into bound product. Confirmatory evidence for such a mechanism of inhibition is given by the observed phdependence of the kinetically estimated apparent equilibrium constant for the binding of boric acid to the enzyme (Fig.5). The decreased affinity of the protein for boric acid below ph 7 can thus be attributed to the enzymatic group with apparent pk, of 6.6 previously shown to be involved in the activation of the active site serine [l]. Furthermore, boric acid ionizes as a Lewis acid with a pk, approximating 9 [lo] B(OH), + H20 Ft B(OH), + H+ (6) forming a tetrahedral anion that would not be expected to combine to t,he reactive seryl residue, and a corresponding decrease in the apparent stability of the enzyme * inhibitor complex is evident from the data in Fig.5. The present data for the inhibition of Streptomyces griseus protease 3 by boric acid agree in all essential features with those reported for the boricacidinhibition of the pancreatic serine enzyme chymotrypsin [3,4], although the latter data were taken to indicate an interactioin between inhibitor and the histidinyl residue involved in the chargerelay system for activation of the reactive seryl residue. Arguments in support of a direct interaction between inhibitor and the serine hydroxyl group have, however, been presented in later studies on the effect of boronic acid derivatives on the catalytic activity of chymotrypsin and the closely related 1serine protease subtilisin [6,8]. The present results, therefore, lead to the mechanistically interesting conclusion that the nucleophilic reactivity of the activesite seryl residue in Streptomyces griseus protease 3, and in chymotrypsin, by analogy, is unaffected,by the binding pnitrophenylacetate to the protein. Previous investigations have revealed several striking sequential [ 1111 and mechanistical [ 1,9] similarities between Streptomyces griseus protease 3 and chymotrypsin. The data reported here, in fact, suggest that the two proteins, despite their wide phylogenetic and physiological differences, may be considered as homologous, at least as concerns the structure and function of the catalytic site around the reactive seryl residue Further studies of Streptomyces griseus protease 3 will now be directed mainly towards the substratespecificity of the protein, which appears to be considerably broader than that of chymotrypsin [2].

5 C.A. Bauer and G. Pettersson 477 This investigation was supported by grants from the Swedish Natural Science Research Council and Kungliga Fysiografisku Sallskupet i Lund. REFERENCES 1. Bauer, C.A. & Pettersson, G. (1974) Eur. J. Biochem. 45, Bauer, C.A. & Lofqvist, B. (1973) Acta Chem. Scand. 27, , 3. Berezin. 1. V.. Kolomiitseva. G. Ya.. Levashov. ' A. V. & Martinek, K. (1967) Mol. Biol. 1,' Berezin, I. V., Kolomiitseva, G. Ya., Levashov, A. V. & Martinek, K. (1967) Mol. Biol. 1, Antonov, V. K., Ivanina, T. V., Berezin, I. V. & Martinek, K. (1970) FEBS Lett. 7, Koehler, K. A. & Lienhard, G. E.' (1971) Biochemistry, 10, Philitm. M. & Bender. M. L. (1971) Proc. Natl. Acad. \ I SC;.~U. S. A. 68, Lindauist. R. N. & Terrv. *, C. (1974) Arch. Biochem. Bio \ I phis. 160, Bauer, C.A., Lofqvist, B. & Pettersson, G. (1974) Eur. J. Biochem. 41, Nelson, P. N. & CampbeI1, G. W. (1964) in Boron, MetalloBoron Compounds and Borunes (Adams, R. M., ed.) pp , Interscience, London. 11. Johnson,P. & Smillie,L. (1972) Can. J. Biochem. 50, C.A. Bauer and G. Pettersson, Avdelningen for Biokemi, Kemicentrum, Lunds Universitet, Box 740, S Liind 7, Sweden