Action Mechanism of Glucose Oxidase of Aspergillus niger. By SATOSHI NAKAMURA and YASUYUKI OGURA

Transcription

1 The Journal of Biochemistry, Vol. 63, No. 3, 1968 Action Mechanism of Glucose Oxidase of Aspergillus niger By SATOSHI NAKAMURA and YASUYUKI OGURA (From the Department of Biophysics and Biochemistry, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo) (Received for publication, August 3, 1967) 1. The reaction of glucose oxidase [EC , Ĉ-d-glucose: oxygen oxidoreductase] obtained from Aspergillus niger was investigated by overall reaction kinetics as well as by the "stopped flow" and "rapid flow" methods. The experiments were carried out using n-glucose and 2- deoxy-d-glucose as substrates at ph 5.5 and at different temperatures (15, 20, 25, and 30 C). 2. The difference absorption spectrum of the enzyme (steady state minus reduced level), which was obtained by the "stopped flow" method, was the same as that of the oxidized minus reduced level. No ESRsignal was observed within 5 msec. after mixing the oxidized enzyme solution with n-glucose or the reduced form with molecular oxygen. 3. When n-glucose or 2-deoxy-D-glucose was used as substrate, the value of kmaxred, the rate constant for reduction of the FAD moiety of oxidized enzyme at a sufficient concentration of the substrate, was almost the same as that of respective V /e obtained by overall reaction kinetics, (V,,,/e: the maximum velocity per unit enzyme concentration). The value of K,a at 25 C for 2-deoxy-D-glucose was the same as that for n-glucose at the same temperature. 4. The following scheme for the action mechanism of the glucose oxidase of Aspergillus niger was proposed to account for the data obtained. where E r stands for the oxidized form of the enzyme, EoxS the enzyme (oxidized form)-substrate compound, Ered the reduced form of the enzyme, S the substrate (D-glucose or 2-deoxy-D-glucose) and P the product (ĉlactone). In a previous paper (1), NAKAMURA, T. and OGURA reported kinetic studies on the reaction catalyzed by glucose oxidase [EC ] of Penicillium amagasakiense, using n- glucose as substrate, and proposed the following scheme for the mechanism of this enzyme reaction : Later, GIBSON et al. (2) and BRIGHT and GIBSON 308 (3) postulated the following reaction scheme for the glucose oxidase of Aspergillus niger acting upon n-glucose. where EredP represents the enzyme (reduced form)-product compound and E.H202 the enzyme (oxidized form)-hydrogen peroxide compound. However, for the reaction using 2-deoxy-D-glucose (2) or 1-deuterated-n-glucose

2 Mechanism of Glucose Oxidase 309 (3) as substrate, they proposed the same scheme as that propounded by NAKAMURA and OGURA. To explain this discrepancy, we investi gated the reaction of glucose oxidase of Asp ergillus niger by overall reaction kinetics and by the "stopped flow " and " rapid flow " methods. The conclusion we have arrived at was that when D-glucose or 2-deoxy-D-glucose was used as substrate the reaction proceeds in the same manner as it was the case with the enzyme of Penicillium amagasakiense (1) acting upon D-glucose. obtain the value of kred, the apparent rate constant of the reduction-reaction of the FAD moiety, " stopped flow" experiments were performed under aerobic or anaerobic conditions. In the experiments performed under anaerobic conditions, both the enzyme and the substrate solutions were deoxygenated by repeated evacuation and flushing with nitrogen gas, and then the solutions were introduced separately into the reservoirs of the flow apparatus using an injection syringe. The gas phase in the reservoirs was a con tinuous flow of O2 free nitrogen gas. The values of krea and kox (the rate constant of the oxidationreaction of the reduced FAD moiety with molecular oxygen) were computed by Chance's formula (7, 8) from a trace obtained by the " stopped flow " method : MATERIALS AND METHODS Glucose Oxidase-The enzyme of Aspergillus niger was purified from crude samples supplied by Kyowa Hakko Kogyo Co., Ltd. by the method described previously (4). The enzyme samples used in the present ex periments were homogeneous on electrophoresis and ultracentrifugation. The concentration (at) of the enzyme was expressed in terms of that of bound FAD, which was measured spectrophotometrically using a value for the extinction coefficient of 10.8 mm -1 cm -1 at 452mƒÊ (4). Chemical Reagents-The reagents used were of the highest grade of purity, and were used without further purification unless otherwise stated. The substrate concentration in the reaction mixture was expressed as the total concentration added although the enzyme is known to react only with the a-form of hexose (5). Measurements of Oxygen Uptake-The rate of the overall reaction was measured by the consumption of molecular oxygen dissolved in the reaction medium with a Clark oxygen electrode (Yellow Springs Instru ment Co.). The temperature of the reaction medium was controlled by circulating water of a constant tem perature through a jacket around the reaction vessel. The rate of the overall reaction (vie) was expressed as the oxygen uptake per second per unit enzyme con centration. Flow Experiments-The rate of oxidation or re duction of the flavin moiety of the enzyme was determined from the change in optical density at 452 mp by the " rapid flow " and " stopped flow " methods using a flow apparatus equipped with a temperaturecontrolled jacket similar to that originally designed by CHANGE and LEGALLAIS (6). The lower limit of the reaction-time in the continuous flow experiments was 2.4 msec. The output of the photomultiplier was amplified and recorded on a San-ei oscillograph, model ER-101, or a Riken Denshi recorder, model SP-J. To The value of kox was also calculated from a trace ob tained by the "rapid flow" method (9) : Pmax : Maximum concentration of oxidized form of FAD obtained from the peak height of the OD-time curve (see Fig. 4). t1/z, off : Time interval from the middle point of increase to the middle point of decrease of the ODtime curve. e : Total concentration of the enzyme added. et: Concentration of the oxidized form of the enzyme produced t seconds after the reduced form was mixed with molecular oxygen. Electron Spin Resonance (ESR) Measurements-The reaction mixtures were frozen by an ultra-rapid freezing technique with a flow system, which was essentially the same as that reported by BRAY (10) and ESR was measured with a Varian ESR spectrometer, model E-3. A solution of the oxidized enzyme was mixed with n-glucose or that of reduced enzyme with mole cular oxygen, and then the mixture was frozen by blowing it into isopentane cooled with liquid nitrogen. The time required for the mixture to reach the surface of the isopentane was 3 msec. RESULTS Rate of Overall Reaction-The overall reac tion was followed by measuring oxygen uptake under various conditions, that is, in the pre sence of various concentrations of the substrate and oxygen at different temperatures (15, 20, 25 and 30 C). Fig. 1 shows the relationships between e/v and 1/[S] obtained at 25 C in the presence of known concentrations of oxygen

3 310 S. NAKAMURA and Y. OGURA FIG, 1. Plots of e/v vs. 1/[S], and of e/vap vs. 1/[O2], with n-glucose as substrate. Enzyme concentration : 0.038µM. Oxygen con centration: 110 µm (U), 130 µm (.), 165 µm (0), 220µM (A), and (0). Buffer: 0.05M acetate (ph 5.5). Temperature : 30 C. when n-glucose was used as a substrate. As may be seen in this figure, the relationships thus obtained gave straight lines, which ran parallel with each other. The apparent maximum velocities (Vap) for n-glucose at known concentrations of oxygen were obtained from the intercept on the ordinate, and the value of the true maximum velocity (V,a), which should be obtained at sufficient con centrations of both substrate and oxygen, was estimated by plotting the reciprocals of the Vap-values against those of the concentration of oxygen. Although the data are not pre sented here, the relationships between e/v and 1/[02] at various concentrations of n-glucose were also expressed by straight parallel-lines. The facts that the lines of e/v vs. 1/[02] or e/v vs. 1/[S], which were obtained at various given concentrations of the substrate or of FIG. 2. Plots of e/v vs. 1/[O2] and of e/yap vs. 1/[S], with 2-deoxy-o-glucose as substrate. Enzyme concentration: 0.17ƒÊM. Oxygen con centration: 260ƒÊM. 2-Deoxy-n-glucos concentration: 10mM( œ), 12.5 mm ( ), 16.7 mm ( ), 25 mm ( ), and 50 mm ( ü). Buffer; 0.05M acetate (ph 5.5). Temperature : 25 Ž. oxygen, respectively, ran parallel with each other, may exclude the possibility of forma tion of a ternary complex, such as an enzymesubstrate-oxygen compound, in this enzyme reaction. Fig. 2 (left) shows the relationships between e/v and 1/[O2] at different concentrations of the substrate, 2-deoxy-D-glucose. The relationships obtained were also expressed by straight parallel-lines. From a plot of the reciprocal values of the " apparent maximum velocities (Vap) at a sufficient oxygen concentration and various known concentrations of substrate against the reciprocals of the substrate con centration, the true maximum velocity and the Michaelis constant (K.) at the sufficient oxygen concentration were obtained. These values are summarized in Table I. It is inter esting that the value of Km at 25 C for 2-deoxy Kinetic constants obtained from overall reaction kinetics. I 1) Values of B are expressed in reciprocals to facilitate comparison with the kox, values obtained by the flow method.

4 Mechanism of Glucose Oxidase 311 D-glucose was almost identical with that for D-glucose at the same temperature. This finding suggests that the enzyme has the same affinity for the two substrates, which implies that the hydroxyl group linked to the C(2) of hexose has no effect on the binding of the substrate with the enzyme molecule as sug gested by PAZUR and KLEPPE (11). The data obtained by overall reaction kinetics using D-glucose or 2-deoxy-D-glucose as substrate are expressed by the following experimental equation : where A and B are constants. The magnitude of these values obtained at different temper atures are listed in Table I. GIBSON et al. (2) also reported relationships similar to those described here, but their values of V/e for D-glucose and 2-deoxy-D-glucose were 5-4 and 2.5 times greater than ours, and their K,, value for D-glucose was a little larger than that obtained in our experiment. The K. -value they obtained for 2-deoxy-D-glucose was in good agreement with ours. Absorption Spectrum of the Enzyme in the Steady State-Using a flow apparatus, studies were made by the "stopped flow" method of whether any transient intermediate would appear at the steady state of the reaction. The details of the method used were reported previously (1). The solution of the enzyme reduced by 5 MM n-glucose was mixed with an equal volume of buffer solution containing 520 ƒêm oxygen. As shown in Fig. 3, the differ ence absorption spectrum of the enzyme (steady state minus reduced level) was completely the same as that of the oxidized minus reduced form of the enzyme. No evidence was obtained to indicate the appearance of an intermediate, such as a semiquinone or a charge transfer type compound, at the steady state of the reaction, although a semiquinone species of the enzyme appeared on anaerobic titration with sodium dithionite (4). This result was in good agreement with that obtained pre viously using the glucose oxidase of Penicillium amagasakiense (1). ESR Measurements-ESR measurements were performed to see whether a semiquinone FIG. 3. Difference absorption spectrum of the flavin moiety (steady state minus reduced level) measured by the " stopped flow " method. Enzyme solution previously reduced with 5 mm n-glucose was mixed with a buffer solution containing 260ƒÊm molecular oxygen using a flow apparatus. Enzyme concentration : 1.8 ƒêm. Buffer : 0.05 m ace tate tate (ph 5.5). Temperature : 25 C. species of the FAD moiety appears in the pre-steady state of the oxidation-reaction of the reduced enzyme with molecular oxygen or in the reduction-reaction of the oxidized form with D-glucose. To obtain the initial state of the reduction process of the enzyme, a solution of oxidized enzyme (67,ƒÊm) was mixed with an equal volume of D-glucose (5 mm) using a flow apparatus under 02-free conditions and then frozen within 5 msec. by introducing the mixture into isopentane cooled with liquid nitrogen. To obtain the initial state of the oxidation process of the FADH2 moiety, an enzyme solution (56 ƒêm), which had been reduced by 8 MM D-glucose, was mixed with a buffer solution containing 240,ƒÊm oxygen and frozen within 5 msec. as des cribed above. Both samples were subjected to ESR measurements at 77 K. No ESR-signal, which would indicate the appearance of a semiquinone species of the FAD moiety, was detected in either experiment. Therefore, it seems that no semiquinone species is formed in this enzyme reaction or that, even if a semiquinone is formed in this reaction, its life-time is so short that its detection by the present methods is impossible. Rate of Oxidation of the Enzyme with Mole cular Oxygen-Using a flow apparatus, a solu tion of the reduced form of the enzyme was mixed with a buffer solution containing a-

5 312 S. NAKAMURA and Y. OGURA known concentration of molecular oxygen and the change of optical density at 452 mp was traced. Fig. 4 shows a typical record on oscillograph paper obtained at ph 5.5 and 30 Ž. The value of k, the rate constant of the reaction between the reduced enzyme (Ered) and molecular oxygen, was computed FIG. 4. Oscillograph trace of change of optical density at 452mu obtained in a flow experiment. Enzyme concentration: 2.17 ƒêm. n-glucose concentration : 6.7 mm. Oxygen concentration : 117,ƒÊm. Buffer: 0.05 M acetate (ph 5.5). Temperature: 30 Ž. using Chance's formula (8) from the traces obtained by the "stopped flow" or "rapid flow" method. Table II shows the values of k x obtained at ph 5.5 and 25 Ž in the pre sence of different concentrations of D-glucose. As expected, the values obtained were almost constant within the limits of experimental error, and were independent of the concen tration of substrate added. Tables III and IV show the values of kox obtained at ph 5.5 and 25 Ž in the presence of different concentrations of oxygen. As may be seen in the tables, the reaction between the enzyme reduced with D-glucose or 2-deoxy-D-glucose and molecular oxygen was found to follow simple second order kinetics in the range of oxygen con centration from 55 ƒêm to 430ƒÊM. The value of k x of the reaction between the enzyme reduced with 0.04 M n-galactose and molecular oxygen was found by the "rapid flow" method to be 11.2X 105 m-1-sec-1 at 25 Ž. This value is in good accordance with those listed above. It is worthy of note that the k_- values were identical irrespective of the kind of substrate used. The k..-values at different temperatures are listed in Table VII. These values were identical with the 1/B-values at the same temperatures, which were obtained from the II Effect of D-glucose concentration on koa-values. Enzyme concentration : 2.0 ƒêm. Oxygen concentration : 130 ƒêm. Buffer : 0.05 M acetate (ph 5.5). Tem perature: 25 Ž. 1) Values were obtained by the "rapid flow" method. 2) Values were obtained by the " stopped flow" method. Effect of oxygen concentration on kox values with D-glucose as substrate. Measurements were made by the "rapid flow" method at 25 Ž. Enzyme concentration : 1.5 ƒêm. o- Glucose concentration : 5 mm. Buffer : 0.05 M acetate (ph 5.5). III

6 Mechanism of Glucose Oxidase 313 Temperature Effect of oxygen concentration on the kox-values with 2-deoxy-D-glucose as substrate. Enzyme concentration : 2.l µm. 2-Deoxy-D-glucose concentration : 20 mm. Buffer : 0.05 M acetate (ph 5.5). : 25 Ž. IV 1) Values were obtained by the "stopped flow" method. 2) Values were obtained by the "rapid flow" method. V Effect of oxygen concentration on krea-values. Reactions were measured by the " stopped flow " method. Buffer : 0.05 M acetate (ph 5.5). Enzyme concentration : 2.1 µm. n-glucose concentration : 5 mm. Temperature : 25 Ž. overall reaction kinetics (see Table I), but they were a little smaller than those reported by GIBSON et al. (2). Rate of Reduction of the Enzyme by the Sub strates-the value of kred, the apparent rate constant of the reduction-reaction of the FAD moiety by the substrate, was computed using CHANCE'S formula (8) from a trace recorded by the "stopped flow" method (see Fig. 4). In Table V, the kred-values obtained in the presence of 5 mm D-glucose and various con centrations of oxygen are listed. These values were identical at a given concentration of the substrate, within the limits of experimental error, regardless of the concentration of oxygen. This result seems to indicate that, under the experimental conditions used, the process of reduction of the FAD moiety caused by the substrate is a rate-limiting step in the overall FIG. 5. Plot of reciprocals of kred-values against those of D-glucose concentrations. Enzyme concentration : 2.0/ƒÊm. Initial oxygen concentration : 130ƒÊm. Buffer : 0.05 M acetate (ph 5.5). Temperature : 25 Ž. FIG. 6. Plot of reciprocals of kred-values against those of 2-deoxy-D-glucose concentrations. Enzyme concentration: 2.13ƒÊM. Initial oxygen concentration: 130ƒÊM. Buffer : 0.05 M acetate (ph 5.5). Temperature : 25 Ž.

7 314 S. NAKAMURA and Y. OGURA reaction. To obtain further information, "stopped flow" experiments were carried out in the presence of various concentrations of substrate. Figs. 5 and 6 show plots of the reciprocal values of kred against those of sub strate concentration. When D-glucose and 2- deoxy-d-glucose were used as substrates, both plots were expressed by straight lines, and the intercepts of the curves on the ordinate were not zero, as may be seen in these figures. Similar results could be obtained at temper atures 15 C to 30 C. In contrast with our data, GIBSON et al. (2) reported that the l/kred vs. 1/[S] curve obtained using D-glucose as a substrate passed through the origin (zero point) when the experiments were carried out under anaerobic conditions. In our study, it was also checked whether or not the values of kred obtained under anaerobic conditions are identical with those measured under aerobic conditions. Before measurements, the solution of enzyme (oxidized form) and that of D-glucose in the two reservoirs of the flow apparatus were made 02-free. Since the oxidation of the reduced FAD moiety caused by a small amount of dissolved oxygen cannot be neglected experi mentally, 50 ƒêm AgNO3,* which is a potent inhibitor of the oxidation of the FADH2 moiety but not of the reduction of the FAD moiety, was added to the reaction medium. Fig. 7 shows oscillograph traces obtained on mixing the solution of enzyme (oxidized form) with various concentrations of D-glucose under anaerobic conditions. Assuming that the re duction-reaction of the FAD moiety follows the first order kinetics with respect to the concentration of the oxidized enzyme in the presence of a given concentration of D-glucose, the value of kred was computed from the time for the initial concentration of Eox, to be re duced by half, which was measured from the trace-curve. The values of kred obtained at different n-glucose concentrations under anaerobic conditions were compared with those measured under aerobic conditions. The data are listed in Table VI. As may be seen in the table, it seems that there is no marked differ ence between the kred-values obtained by these two methods. The data illustrated in Figs. 5 and 6, therefore, seem to indicate that an enzyme (oxidized form)-substrate complex (E,S) is formed as an intermediate in the reduction of the enzyme by the substrate. This infer rence, it should be noticed, is based on the assumption that the absorption spectrum of the FAD moiety of the E,S compound is the FIG. 7. Oscillograph trace of reduction of FAD moiety caused by D-glucose. Experiments were carried out by the "stopped flow" method under anaerobic conditions. Arrows in the figure indicate the time when flow was stopped. (A) Enzyme concentration: 2.0.ƒÊm, and n-glucose concentration -.,5 mm. (B) Enzyme concentration : 4 (LM, and a-glucose concentration : 10 mm. (C) Enzyme concentration: 4 ƒêm, and n-glucose concentration : 15 mm. 50 ƒêm AgNO3 was added previously in each glucose solution. Buffer : 0.05 M acetate (ph 5.5). Tem perature: 15 C. * The details on this inhibitor will be reported later.

8 Mechanism of Glucose Oxidase 315 Values of kred measured under anaerobic conditions. Buffer : 0.05 mi acetate (ph 5.5). Temperature : 15 C. VI 1) 5 X 10-5 mt AgNO, was added to substrate solution. 2) Enzyme concentration : 2 gm. 3) Enzyme concentration : 4 fem. VII Kinetic constants obtained by the flow methods. of sufficient concentrations of substrate and oxygen. DISCUSSION Since the values of kmaxred a and k,, were identical with those of Vm/e and 1/B,respec tively, the following scheme is proposed for the mechanism of the reaction catalyzed by the oxidase from Aspergillus niger, acting upon either D-glucose or 2-deoxy-D-glucose: same as that of the oxidized form of the enzyme. In Table VII, the maximum velo cities (k ed ) of reduction of the FAD moiety at various temperatures, which should be ob tained at sufficient concentrations of both substrate and oxygen, are summarized together with the k,.-values. The value of k Cd at 25 C of glucose oxidase obtained from Penicillium,; amagasakiense was 550 sec-1 (I), a value which is somewhat larger than the corresponding value obtained for the Aspergillus-enzyme used in the present study. It is worthy of note that the values of kmaxred obtained for this Asp ergillus-enzyme at different temperatures were, within the limits of experimental error, almost identical with those of Vm/e at res pective temperatures. This indicates that the reaction, EoxS --' Ered+P, is a rate-limiting step in the overall reaction in the presence where k's are the rate constants of the reac tions indicated. In the present study, no evidence was obtained to indicate the form ation of E,,,H2O2, an enzyme (oxidized form)- hydrogen peroxide compound, in the enzyme reaction. The formation of EredP, the enzyme (reduced form)-product compound, cannot be excluded completely, but even if it is formed, the dissociation of EredP into E,ed and P may occur much more rapidly than the reaction, E,S - EredP. In contrast to the scheme proposed by GIBSON et al., the present reaction mechanism could explain not only the data on the oxidation of D-glucose, but also those on the oxidation of 2-deoxy-D-glucose. The reaction scheme proposed in the present study is essentially the same as that reported pre viously for the enzyme obtained from Penicillium

9 316 S. NAKAMURA and Y. OGURA amagasakiense (1), although the values of km ax and kox in the two cases were somewhat dif ferent. Based on the above scheme the reci procal of the rate of reaction at the steady state can be expressed as follows : and from ours, seem to suggest that the properties of the two Aspergillus-enzymes are somewhat different. The authors wish to express their gratitude to Prof. H. Tamiya for his valuable criticism in this work. Thanks are also due to Kyowa Hakko Kogyo Co., Ltd. for the supply of crude glucose oxidase preparation, and to Marubun Co., Ltd. for the use of a Varian ESR-spectrometer, model E-3. The finding that the K values for D- glucose and 2-deoxy-D-glucose at a sufficient concentration of oxygen were identical, while the values of V for these two substrates were markedly different, seems to indicate that the Michaelis constant of this enzyme reaction is practically the dissociation constant of the EoxS compound. As reported by KUSAI (12), the reaction of our Aspergillusenzyme was inhibited by mercuric compounds, such as p-chloromercu ribenzoate and phenylmercuric acetate. SWOBODA and MASSEY (13) and KLEPPE (14) reported, on the other hand, that the reaction of the enzyme of Aspergillus niger used by them was not inhibited by the mercurials described above. These facts together with the fact that the values of the kinetic constants ob tained by GIBSON et al. were markedly different REFERENCES (1) T. Nakamura, and Y. Ogura, J. Biochem., 52, 214 (1962) (2) Q.H. Gibson, B.E.P. Swoboda, and V. Massey, J. Biol. Chem., 239, 3927 (1964) (3) H.J. Bright, and Q.H. Gibson, J. Biol. Chem., 242, 994 (1967) (4) S. Nakamura, and S. Fujiki, J. Biochem., 63, 51 (1968) (5) R. Bently, and A. Neuberger, Biochem. J., 45, 584 (1949) (6) B. Chance, and V. Legallais, Rev. Sci. Intr., 22, 627 (1951) (7) B. Chance, J. Biol. Chem., 151, 553 (1943) (8) B. Chance, Arch. Biochem. Biophys., 71, 130 (1957) (9) B. Chance, Advances in Enzymol., 12, 153 (1951) (10) R.C. Bray, Biochem. J., 81, 189 (1961) (11) J.H. Pazur, and K. Kleppe, Biochemistry, 3, 578 (1964) (12) K. Kusai, Ann. Rept. Sci. Works. Fac. Sci., Osaka Univ., 8, 43 (1960) (13) B.E.P. Swoboda, and V. Massey, J. Biol. Chem., 240, 2209 (1965) (14) K. Kleppe, Biochemistry, 5, 139 (1966)