Glucose oxidase protonated

In spite of its importance and popularity, the fine details of the P-D-glucose oxidase mechanism are not completely known. The proposed model (Fig. 2.14) includes both the catalase cascade and the protonation equilibria (Caras et al., 1985b). The pH-dependent reaction term corresponding to this model is quite complex. 

There are several species in this reaction that can be used for electrochemical sensing. Detection of proton released from the gluconic acid was used in the poten-tiometric glucose electrode (Section 6.2.1). The amperometric sensor can be based on oxidation of hydrogen peroxide, on reduction of oxygen, or on the oxidation of the reduced form of glucose oxidase itself. 

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

Lowe et al. (1983) utilized the detection of protons produced during glucose oxidation by an optoelectronic sensor. Glucose oxidase was coupled to a transparent cellophane membrane together with the pH sensitive triphenylmethane dye bromocresol green. The membrane was... 

Repeat of the same land of experiments at other pHs shows that the rate constant 3 varies with pH in a sigmoid manner, as shown in Fig. 5 (whereas k2 and red practically insensitive to this factor). Figure 5 also displays the pH-dependent values ofks obtained with other cosubstrates than ferrocene methanol [35, 36], The dependency of k upon pH is due to the fact that, as with many redox enzymes, the prosthetic group of glucose oxidase does not merely exchange electrons with the cosubstrate but both electrons and protons. Analysis of the... 

The products of the reaction they catalyze may inhibit many enzymes through Michaelis-Menten kinetic retroaction. Protons, which are involved as products or reactants in a number of cases, may also influence the enzymatic kinetics. The course of the reaction may therefore be altered by the attending production or depletion of protons. It is thus interesting to examine whether these phenomena may be revealed by the effect they might have on the electrochemical responses of immobilized enzyme films under appropriate conditions [92]. A first clue of the existence of such inhibition effects is the observation of hysteresis behaviors of the type shown in Fig. 18(a) where data obtained with 10 glucose oxidase monolayers with ferrocene methanol as cosubstrate have been taken as example. In the absence of inhibition, the forward and reverse traces should be exactly superimposed. Hysteresis increases to the point of making a peak appear on the forward trace as the scan rate decreases and as the concentration of the buffer decreases, as illustrated in Fig. 18c, c , c , c by comparison with Fig. 18(a). 

If the enzymatic reaction produces protons, as is often the case with oxidases, the local variations of pH due to insufficient buffering may locally modify the rate of the various reactions involved and therefore influence the electrochemical response of the system. For example, in the case of glucose oxidase with ferrocene methanol as cosubstrate, the main effect of pH is on 3 [35] ... 

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