The pH Dependence of the Individual Steps in the Glucose Oxidase Reaction

Active site generation of a protonically unstable suicide substrate from a stable precursor: glucose oxidase and dibromonitromethane

Bromonitromethane is an inefficient suicide substrate for glucose oxidase (in contrast to the case of CH(3)CCl=NO(2)(-) and D-amino acid oxidase) because, in the enzyme-substrate encounter step, the required ionization states of enzyme (EH(0)(+), pK(a) approximately 3.5) and substrate (CHBr=NO(2)(-), pK(a) approximately 8.3) cannot be highly populated simultaneously. Because reprotonation of CHBr=NO(2)(-) is rapid at the pH value used for the assay of glucose oxidase, presentation of the enzyme with the preformed anion could not be exploited in this case. We circumvent this difficulty by allowing the enzyme to reductively dehalogenate CHBr(2)NO(2), thereby generating the desired protonically unstable suicide substrate in situ (E(r) + CHBr(2)NO(2) --> E(o) + CHBr=NO(2)(-) + HBr + H(+)). Irreversible inactivation of the enzyme, because of the formation of a dead-end N-5 formylflavin adduct, is more than 100-fold faster when CHBr=NO(2)(-) is generated in situ than when it is externally applied. The remaining competitive fates of CHBr=NO(2)(-) at the active site are protonation and release or oxidation to HCOBr (or HCONO(2)). Strong support for these conclusions comes from (1) the brisk evolution of CH(3)CBr=NO(2)(-) (which is too bulky to act further as an efficient suicide substrate) from the enzyme-catalyzed reductive debromination of CH(3)CBr(2)NO(2), (2) the 1:1 stoichiometry of enzyme inactivation, and (3) the identification of the modified flavin as 5-formyl-1, 5-dihydro-FAD.

Amperometric glucose biosensor based on lipid film

A novel glucose biosensor based on cast lipid film was developed. This model of biological membrane was used to supply a biological environment on the surface of the electrode, moreover it could greatly reduce the interference and effectively exclude hydrophilic electroactive material from reaching the detecting surface. TTF was selected as a mediator because of its high electron-transfer efficiency, and it was incorporated in the lipid film firmly. Glucose oxidase was immobilized in hydrogel covered on the lipid film. The effects of pH, operating potential were explored for the optimum analytical performance by using amperometric method. The response time of the biosensor was less than 20 s, and the linear range is up to 10 mmol l(-1) (corr. coeff. 0.9932) with the detection limit of 2 x 10(-5) mol l(-1). The biosensor also exihibited good stability and reproducibility.

Nature of oxygen activation in glucose oxidase from Aspergillus niger: the importance of electrostatic stabilization in superoxide formation

Glucose oxidase catalyzes the oxidation of glucose by molecular dioxygen, forming gluconolactone and hydrogen peroxide. A series of probes have been applied to investigate the activation of dioxygen in the oxidative half-reaction, including pH dependence, viscosity effects, 18O isotope effects, and solvent isotope effects on the kinetic parameter Vmax/Km(O2). The pH profile of Vmax/Km(O2) exhibits a pKa of 7.9 +/- 0.1, with the protonated enzyme form more reactive by 2 orders of magnitude. The effect of viscosogen on Vmax/Km(O2) reveals the surprising fact that the faster reaction at low pH (1.6 x 10(6) M-1 s-1) is actually less diffusion-controlled than the slow reaction at high pH (1.4 x 10(4) M-1 s-1); dioxygen reduction is almost fully diffusion-controlled at pH 9.8, while the extent of diffusion control decreases to 88% at pH 9.0 and 32% at pH 5.0, suggesting a transition of the first irreversible step from dioxygen binding at high pH to a later step at low pH. The puzzle is resolved by 18O isotope effects. 18(Vmax/Km) has been determined to be 1.028 +/- 0.002 at pH 5.0 and 1.027 +/- 0.001 at pH 9.0, indicating that a significant O-O bond order decrease accompanies the steps from dioxygen binding up to the first irreversible step at either pH. The results at high pH lead to an unequivocal mechanism; the rate-limiting step in Vmax/Km(O2) for the deprotonated enzyme is the first electron transfer from the reduced flavin to dioxygen, and this step accompanies binding of molecular dioxygen to the active site. In combination with the published structural data, a model is presented in which a protonated active site histidine at low pH accelerates the second-order rate constant for one electron transfer to dioxygen through electrostatic stabilization of the superoxide anion intermediate. Consistent with the proposed mechanisms for both high and low pH, solvent isotope effects indicate that proton transfer steps occur after the rate-limiting step(s). Kinetic simulations show that the model that is presented, although apparently in conflict with previous models for glucose oxidase, is in good agreement with previously published kinetic data for glucose oxidase. A role for electrostatic stabilization of the superoxide anion intermediate, as a general catalytic strategy in dioxygen-utilizing enzymes, is discussed.

The soluble alpha-glycerophosphate oxidase from Enterococcus casseliflavus. Sequence homology with the membrane-associated dehydrogenase and kinetic analysis of the recombinant enzyme

The soluble flavoprotein alpha-glycerophosphate oxidase from Enterococcus casseliflavus catalyzes the oxidation of a "non-activated" secondary alcohol, in contrast to the flavin-dependent alpha-hydroxy- and alpha-amino acid oxidases. Surprisingly, the alpha-glycerophosphate oxidase sequence is 43% identical to that of the membrane-associated alpha-glycerophosphate dehydrogenase from Bacillus subtilis; only low levels of identity (17-22%) result from comparisons with other FAD-dependent oxidases. The recombinant alpha-glycerophosphate oxidase is fully active and stabilizes a flavin N(5)-sulfite adduct, but only small amounts of intermediate flavin semiquinone are observed during reductive titrations. Direct determination of the redox potential for the FAD/FADH2 couple yields a value of -118 mV; the protein environment raises the flavin potential by 100 mV in order to provide for a productive interaction with the reducing substrate. Steady-state kinetic analysis, using the enzyme-monitored turnover method, indicates that a ping-pong mechanism applies and also allows the determination of the corresponding kinetic constants. In addition, stopped-flow studies of the reductive half-reaction provide for the measurement of the dissociation constant for the enzyme. alpha-glycerophosphate complex and the rate constant for reduction of the enzyme flavin. These and other results demonstrate that this enzyme offers a very promising paradigm for examining the protein determinants for flavin reactivity and mechanism in the energy-yielding metabolism of alpha-glycerophosphate.

Effects of protein glycosylation on catalysis: changes in hydrogen tunneling and enthalpy of activation in the glucose oxidase reaction

Three glycoforms of glucose oxidase, which vary in their degree of glycosylation and resulting molecular weight, have been characterized with regard to catalytic properties. Focusing on 2-deoxyglucose to probe the chemical step, we have now measured the temperature dependence of competitive H/T and D/T kinetic isotope effects and the enthalpy of activation using [1-2H]-2-deoxyglucose. The D/T isotope effect on the Arrhenius preexponential factor (AD/AT) is 1.47 (+/-0.09), 1.30 (+/-0.10), and 0.89 (+/-0.04) for the 136, 155, and 205 kDa glycoforms, respectively. The value obtained for the 136 kDa glycoform is well above the range expected for semiclassical-classical (no tunneling) reactions (upper limit of 1.22). The abnormal A(D)/A(T) is rationalized by extensive tunneling. The enthalpies of activation are 8.1 (+/-0.4), 11.0 (+/-0.3), and 13.7 (+/-0.3) kcal/mol for the 136, 155, and 205 kDa glycoforms, respectively. Apparently, less glycosylation results in more tunneling and a lower enthalpy of activation. The crystal structure, kinetic analysis, and other studies suggest that the enzyme active site is not conformationally changed by the degree of glycosylation. Hence, the differences among the glycoforms, which indicate that changes in the enzyme polysaccharide envelope lead to a significant change in the nature of the hydrogen transfer step, suggest a dynamic transmission of protein surface effects to the active site.

Reduction of aryl-nitroso compounds by pyridine and flavin coenzymes

1. A systematic kinetic investigation of the reduction of aryl-nitroso compounds by pyridine and flavin coenzymes and their analogs, in enzymatic and nonenzymatic systems, has been reported. 2. Two main groups of nitroso compounds have been investigated, representatives nitroso-benzene and 1-nitroso-2-naphthol; in all enzymatic and nonenzymatic systems, the former was always reduced to phenyl-hydroxyl-amine and the latter to 1-amino-2-naphthol. 3. Pyridine compounds included NADH, APAD-4H2 and DBNA-4H2 in nonenzymatic systems, and liver alcohol dehydrogenase. Flavin compounds included 1,5-dihydrolumiflavin and various forms of reduced 5-ethyl-lumiflavin, in nonenzymatic systems, and the flavoenzymes glucose-oxidase and NADPH-cytochrome P450 reductase. 5. Pyridine coenzymes and their analogs reduced nitroso compounds by a direct hydride transfer, with a primary kinetic isotope of 9.5 +/- 2.2. 6. All flavin compounds (glucose-oxidase and its nonenzymatic analog 1,5-dihydrolumiflavin and NADPH-cytochrome P450 reductase and its analog 5-ethyl-1,5-dihydrolumiflavin) reduced aryl-nitroso compounds with high efficiency (k2 greater than 10(5)M(-1) min(-1)). 7. The flavin compounds have been shown to be much more efficient reductans of nitroso compounds, compared to pyridine coenzymes, both in enzymatic and nonenzymatic systems; the only exception to this rule presented the extremely efficient reduction of p-substituted aryl-nitroso compounds by liver alcohol dehydrogenase.

The oxidative part of the glucose-oxidase reaction

1. Kinetic parameters of the oxidative part of glucose-oxidase reaction have been measured with 16 different electron-acceptors and glucose as a substrate. 2. In each case, the rate-limiting portion of the oxidative part of reaction was the formation of the E-FADH2.Acceptor-complex; this rate was pH-independent around the pH-optimum of the enzyme. 3. In each case, E-FADH2 acceptor-complex was undetectable in the steady-state kinetics, with the exception of cytochrome-c. 4. The rates of redox reactions between various forms of reduced 5-ethyl-lumiflavin and five different electron-acceptors have been examined with a conventional spectrophotometry. In each case, it was found that the reactions proceeded at high rates whenever thermodynamically feasible, and were totally prevented in the opposite case. 5. Molecular oxygen was able to oxidize only the neutral form of 5-ethyl-1,5-dihydrolumiflavin to its radical form, at a moderate rate; all other forms of reduced 5-ethyl-lumiflavin were not oxidized by O2. 6. By the comparison of enzymatic and model redox reactions, it was possible to establish the minimal mechanism of the oxidative part of the glucose-oxidase catalytic cycle.

Enzyme binding to detect carbohydrate expression in tissue sections. 1. Native and cross-linked glucose oxidase

Enzymes may be useful as highly specific histochemical probes to identify and localize macromolecular substrates in tissue sections. We have used glucose oxidase, a double-headed enzyme, to demonstrate beta-glucosyl groups in paraffin sections. Native glucose oxidase has two active sites per molecule. Soluble polymers formed by glutaraldehyde combine many active binding sites on to one molecule. Some of these bind to glucose in tissue sections, leaving others free to react with chromogenic substrate. The intensity of staining is directly related to the concentration of enzyme, duration of incubation with enzyme, temperature and pH. Polymeric forms of enzyme are about 100 times more effective than native. Glucose oxidase, particularly in a polymeric form, appears a simple reagent for the identification of glucose-containing structures. The use of native and polymerized enzymes as a histochemical probe has enormous potential in the analysis of normal tissues and in the detection of aberrant carbohydrate deposition in pathological tissues; this system serves as a useful model.

Biochemistry without oxygen

Published procedures for experimentation under anoxic conditions generally involve specialized apparatus that hinders the easy manipulation of experimental samples. We describe here some procedures that rapidly remove oxygen from experimental solutions, maintain anoxia with simple equipment for long periods of time, and do not interfere with normal sample addition and removal, spectrometric measurements, chromatographic manipulations, and the like. Anoxia can be achieved and maintained by the use of an enzyme system (glucose oxidase, glucose, catalase), or an inorganic oxygen-reducing system (ferrous pyrophosphate), or dithionite. Physical isolation of experimental samples from atmospheric oxygen can be maintained by continuous flushing with treated argon gas and/or by an overlay of heavy mineral oil.

Electrostatic control of enzyme reactions: the mechanism of inhibition of glucose oxidase by putrescine

The interaction of putrescine dihydrochloride with glucose oxidase is reported. At pH 7.65 glucose oxidase is strongly anionic (Z = -80). The pKa of an essential acidic group on the reduced form of the enzyme is extremely sensitive to ionic strength, as predicted by simple electrostatic theory [J. G. Voet, J. Coe, J. Epstein, V. Matossian, and T. Shipley (1981) Biochemistry 20, 7182-7185]. Putrescine dihydrochloride was found to inhibit glucose oxidase at pH 7.65 at a constant ionic strength of 0.05. The kinetics do not obey simple competitive inhibition, however. The data can best be explained by a model in which change in the electrostatic potential of the enzyme on putrescine binding changes the observed pKa of the essential acidic group. The pH dependence of putrescine inhibition supports this interpretation. At I = 0.05, 5 mM putrescine was found to change the pKa of the essential acidic group from 7.6 to 7.1. The shift in the pKa as a function of putrescine concentration at pH 7.7 and I = 0.05 also supports the model presented. The Ka for putrescine to the active form of the enzyme was calculated to be 4.2 mM.

A new peroxidase color reaction: oxidative coupling of 3-methyl-2-benzothiazolinone hydrazone (MBTH) with its formaldehyde azine. Application to glucose and choline oxidases

Hydrogen peroxide in the presence of horseradish peroxidase effects the oxidative coupling of 3-methyl-2-benzothiazolinone hydrazone with its formaldehyde azine to form a tetraazapentamethine dye. The blue chromophore, when formed at pH 3.5 and quenched with acetone or 1 N hydrochloric acid, has an extinction coefficient of 69 +/- 2 or 55 +/- 2 mM-1 cm-1, respectively. This chromogen system has been adapted for enzymatic determinations of hydrogen peroxide and of glucose in the 10- to 45-nmol range and of choline in the 5- to 20-nmol range.

Electrostatic control of enzyme reactions: effect of ionic strength on the pKa of an essential acidic group on glucose oxidase

The dissociation constant of an essential acidic group on the reduced form of glucose oxidase from Aspergillus niger (K4) has been found to be extremely sensitive to ionic strength. Increasing the ionic strength from 0.025 to 0.225 causes a decrease in pK4,obsd of 0.9 pH unit, from 8.2 to 7.3. Analysis of the ionic strength dependence of pK4,obsd, making the assumption that the enzyme is a homogeneously charged impenetrable sphere [Edsall, J. T., & Wyman, J. (1958) Biophysical Chemistry, Vol. 1, pp 282-289, 512-514, Academic Press, New York], predicts that the intrinsic pKa of the acidic group is 6.7 and that the charge on the protein is -78. The enzyme was titrated from its isoelectric point (pH 4.05) to pH 7.7, the pH at which the ionic strength dependence was determined. It was found to have an actual charge at that pH of -77, in remarkable agreement with the theoretical prediction. Thus, glucose oxidase exerts electrostatic control on pK4,obsd as though it were a uniformly charged sphere. The group responsible for pK4,obsd has not been identified. However, its measured delta H degrees obsd of 8.0 kcal mol-1 and delta S degrees obsd of -6.1 cal mol-1 K-1, together with its pKa of 6.7, are consistent with the group being a histidine residue.

Analysis of the coupled transport, reaction, and deactivation phenomena in the immobilized glucose oxidase and catalase system

In a previous paper, the overall or macrokinetics of the immobilized glucose oxidase--catalase system has been presented. In this paper a detailed analysis of the interaction of diffusion and reaction in this system will be presented. The mathematical treatment includes two consecutive reactions with two-substrate kinetics. Furthermore, the deactivation of both enzymes due to the intermediate product peroxide is taken into account. The predicted results suggest that the efficiency of the glucose oxidase reaction depends on the concentration ranges of the two substrates. Furthermore, the external mass-transfer rate may cause a shift from glucose limitation to oxygen limitation. The efficiency of the coupled system is always higher than that predicted for the uncoupled reaction path. The calculations show that the economics of the coupled system depend a great deal on the deactivation of the enzymes.

Effects of immobilization on the kinetics of enzyme-catalyzed reactions. I. Glucose oxidase in a recirculation reactor system

Glucose oxidase from Aspergillus niger was immobilized on nonporous glass beads by covalent bonding and its kinetics were studied in a packed-column recycle reactor. The optimum pH of the immobilized enzyme was the same as that of soluble enzyme; however, immobilized glucose oxidase showed a sharper pH-activity profile than that of the soluble enzyme. The kinetic behavior of immobilized glucose oxidase at optimum pH and 25 degrees C was similar to that of the soluble enzyme, but the immobilized material showed increased temperature sensitivity. Immobilized glucose oxidase showed no loss in activity on storage at 4 degrees C for nearly ten weeks. On continuous use for 60 hr, the immobilized enzyme showed about a 40% loss in activity but no change in the kinetic constant.