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Cohn, M., and Townsend, J. (1954), Nature (Lundun) 173, 1090. Cooperman, B. S., and Buc, H. (1972), Eur. J . Biochem. 27. 503. Cooperman, B. S., and Chiu, N. Y. (1973), Biochemrstrj, 12, 1676. Cooperman, B. S., Chiu, N . Y . ,Bruckmann, R . H., Bunick, G. J., and McKenna, G. P. (1973), Biochemistrj 12,1665. Cooperman, B. S.,and Mark, D. H. (1971), Biochim. Biophj‘s. Acta 252,221. Cottam, G. L.,and Ward, R . L. (1969), Arch. Biochem. Biophys. 132,308. Dixon, M., and Webb, E. C. (1964), Enzymes, 2nd ed, New York, N . Y., Academic Press. Eisinger, J., Shulman, R . G., and Szymanski, B. M. (1962), J . Chem. P1zj.s. 36,1721. Kabachnik, M. I., Lastovskii, R . P., Medved, T. Ya., Medyntsev, V. V., Kolpakova, I. D., and Dyatiova, B. M. (1967), Dukl. Chem. 177. 1060. Kunitz, M . (1952), J . Gen. Phj,siol. 35, 423. Larsen, M., Willet, R . , and Yount, R . G. (1969), Science 166, 1510. Leigh, J. S. (1970), J . Chem. P1ij.s. 50,2605. Malmstrom, B. G. (1953), Arch. Biochenr. Biophj.s. 46, 345. Mildvan, A. S.(1970), Enzymes 2,466. Mildvan, A. S.,and Cohn, M. (1965), J . Biol. Chenr. 240,238. Mildvan, A. S.. and Cohn, M . (1966), J . B i d . Chem. 241, 1178. Mildvan, A . S., and Cohn, M. (1970), Adcan. Enzj.niol. 3 3 , l . Moe, 0 . A,, and Butler, L. G . (1972a), J . Biol. Chern. 247, 7308.
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Yeast Inorganic Pyrophosphatase. 111. Active-Site Mapping by Electrophilic Reagents and Binding Measurements? Barry S . Cooperman* and Ning Yu Chiu
ABSTRACT: The effects of electrophilic reagents on the enzymatic activity of inorganic pyrophosphatase are studied. Phenylglyoxal incubation results in complete inactivation, while incubation with l-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride leads to a retention of 7 % activity. Trinitrobenzenesulfonate, 0-methylisourea, and iodoacetic acid are ineffective as inactivating agents. The rates of both inactivation processes are virtually unaffected by added Mg2+, but Mg2*-hydroxymethanebisphosphonate complex, a n inhibitor of enzymatic activity, slows the rate considerably. This protective effect is used to measure inhibitor binding and
I
n the first two papers of this series we presented methods for measuring three functions of yeast inorganic pyrophosphatase: enzymatic activity, divalent metal ion binding, and
t From the Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104. Received Augitsf 28, 1972. This work was supported by Research Grant AM-13202 from the National Institutes ofHealth.
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BIOCHEMISTRY,
\OL.
1 2 , N O . 9, 1 9 7 3
the dissociation constant so obtained is found comparable to a K, value found previously from steady-state kinetic measurements (Cooperman, B. S., and Chiu, N. Y. (1973), Biochemistry 12, 1670). Magnetic resonance measurements show that incubation of enzyme with both inactivating reagents has only a minor effect of Mn2+ binding, but that binding of hydroxymethanebisphosphonate to the phenylglyoxal-inactivated enzyme has been abolished. These results and those of related studies are used to construct a plausible enzymatic mechanism.
pyrophosphate analog binding. In this paper we begin using these methods to construct a preliminary structure-function map of the active site. Our strategy has been to (1) determine the sensitivity of the enzyme to a wide range of “group specific’’ electrophilic reagents, and for those reagents which inactivate the enzyme; (2) test whether inactivation is inhibited by Mg*+ or pyrophosphate analog added either separately or together; and (3) determine whether inactivation can be
ACTIVE-SITE MAPPING OF PYROPHOSPHATASE IO0
IO0
60
-+> I= 0
k
f0
30
a d
t
50
10 0
10
20
30
40
U
z
TIME (MINUTES)
FIGURE 2: Mg2+-PCHOHP protection of enzyme inactivation by
0.08 M PhGx in 0.2 M N-ethylmorpholine, pH 7.4 (1 .O N in KCl). No protection (0);1.2 mM Mgz+, 7 X M PCHOHP (cI); 1.2 mM Mg*+,2 X M PCHOHP (A).
x 14.0
0
60
120
180
240
TI ME ( MINUTES)
1: Enzyme stability at various pH values. Enzyme (2-5 unitsiml) was incubated at a series of pH values, and aliquots were assayed at various times in the standard assay medium. Buffers were: pH 4.0, 0.05 M acetate; pH 5.0, 0.05 M N,N'-dimethylpiperazine ; pH 6.0, 0.003 M 2-(N-morpholinoethanesulfonate); pH's 7.0 and 8.0, 0.2 M N-ethylmorpholine; pH's 9.0, 10.0, and 11.0, 0.05 M borate. FIGURE
accounted for by a blockage of either the metal ion or pyrophosphate binding site. Our results offer evidence for the presence of at least one essential arginine at the active site which is necessary for pyrophosphate binding. By contrast, there appear to be no essential aspartates, glutamates, lysines, or cysteines at the active site.
in enzyme activity on standing in buffer. As can be seen in Figure 1, such corrections were minor in the pH range 6.08.0. Runs measuring PhGx inactivation of enzyme were begun by addition of solid PhGx. Solution was complete within ten seconds. These studies could not be done in Tris buffer because of a side reaction between PhGx and Tris. Runs measuring R(NCN)Et inactivation of enzyme were begun by addition of an aliquot of a freshly prepared (daily) R(NCN)Et solution. TI (measurement). The proton relaxation rate of water was measured at 24.3 MHz at 30" by a pulsed nuclear magnetic resonance (nmr) technique described previously (Cohn and Leigh, 1962). Values of the observed enhancement (Eisinger et al., 1962), Eobsd, were calculated from
Experimental Section Materials Phenylglyoxal monohydrate (PhGx)' (Aldrich), was twice recrystallized from water before use. N-Ethylmorpholine and N,N'-dimethylpiperazine (Aldrich) were distilled before use. 1Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (R(NCN)Et) was obtained from Sigma. Yeast inorganic pyrophosphatase (35-45 Kunitz units/mg) was prepared as described elsewhere (Cooperman et al., 1973; Cooperman and Chiu, 1973).
where TI and TI,oare the observed relaxation times in the presence and absence of Mn2+, respectively. The terms with asterisks represent the same parameters in the presence of added complexing agent. The significance of t o b i d in binary (Mn2+ and enzyme) and ternary (Mn2+, enzyme, and PCHOHP) solutions has been discussed previously (Cooperman and Chiu, 1973).
Methods Enzyme solutions (2-5 Kunitz units/ml) of varying composition were incubated at 25" and aliquots were withdrawn at various times and assayed for enzymatic activity using the pH-Stat assay described elsewhere (Cooperman et al., 1973). The aliquots were diluted between 100- and 1000-fold on transfer to the assay medium, the dilution serving as a quench procedure. In all cases the large dilution led to an immediate loss in activity of 15-20%, which may be a surface inactivation effect. All activity measurements are corrected for this
Stability of Enzyme. The stability of the enzyme was tested over a wide range of pH values in order to determine what the suitable pH range would be for studies on the effects of electrophilic reagents. The results are summarized in Figure 1 and are qualitatively similar to those obtained previously by Kunitz (1952). Effect ofPhCx. Enzyme incubated with PhCx showed a n apparent first-order loss in activity (Figure 2) over at least three to four half-lives. The reaction was much faster at pH 8 than at pH 7, which parallels similar studies on ribonuclease (Takahashi, 1968). The pseudo-first-order constant at pH 7, 25", 0.1 M PhGx is 0.082 min-l, so that the calculated secondorder rate constant is 0.82 M-' min-'. This is considerably (5.5 times) faster than'the second-order rate constant of 0.15 M-' rnin-', which can be calculated from Takahashi's data, obtained under identical experimental conditions, for reaction with both ribonuclease and free arginine. PhCx treatment appears to fully inactivate the enzyme. Enzyme incubated with 0.08 M PhGx at pH 7.85 for 75 min (>15 halflives) showed no (
