Oxygen reduction enzymes

In 2011, alcohol dehydrogenase (ADH) was used as a model enzyme coupled with poly (MG) for NADH reoxidation in the construction of a three-dimensional BFC with ethanol as fuel [103]. In combination with an air-breathing/gas difhision cathode (using laccase as an oxygen reduction enzyme), a BFC was fabricated that was able to successfully exploit ethanol oxidation by an NAD -dependent ADH, immobilized by entrapment in a multiwaUed CNT (MWCNT)/chitosan matrix [106]. The feasibility and reproducibUity of the resulting BFC were demonstrated in 2008 with a series of standardized multilaboratory experiments [96]. 

White-Stevens RH, H Kamin (1972a) Studies of a flavoprotein, salicylate hydroxylase. I. Preparation, properties, and the uncoupling of oxygen reduction from hydroxylation. J Biol Chem 247 2358-2370. White-Stevens RH, H Kamin, QH Gibson (1972b) Studies of a flavoprotein, salicylate hydroxylase II Enzyme mechanism. J Biol Chem 247 2371-2381. 

It is important to compare the catalytic properties of Prussian blue with known hydrogen peroxide transducers. Table 13.2 presents the catalytic parameters, which are of major importance for analytical chemistry selectivity and catalytic activity. It is seen that platinum, which is still considered as the universal transducer, possesses rather low catalytic activity in both H202 oxidation and reduction. Moreover, it is nearly impossible to measure hydrogen peroxide by its reduction on platinum, because the rate of oxygen reduction is ten times higher. The situation is drastically improved in case of enzyme peroxidase electrodes. However, the absolute records of both catalytic activity... 

In the 02-saturated buffer solution, an increase in reduction peak was observed, accompanied by a decrease in the oxidation peak current, which demonstrates that GOD in the film nicely catalyzes the oxygen reduction. When glucose is added to the 02-saturated buffer solution, the reduction peak current decreases with the increase of the glucose concentration. (3-(D)-glucose is the substrate of GOD and it will react with the enzyme and decrease the concentration of the oxidized form of GOD on the electrode surface ... 

Under aerobic conditions, aerobic bacteria has so far been only found in studies capable of reducing azo compounds and produce aromatic amines by specific oxygen-catalyzed enzymes called azo reductases. These aerobic bacteria could grow with mostly simple azo compounds as sole source of carbon and energy and under strict aerobic conditions by using a metabolism that started with reductive cleavage of the azo linkage. 

Redox enzymes catalyze either oxidation or reduction of corresponding substrates. However, these enzymes cannot be regenerated by themselves if neither electron acceptor nor donor is associated. An oxidase, for instance, accepts an electron from corresponding substrate to be oxidized. The enzyme remains in reduced form when the electron cannot be transferred to such an electron acceptor as oxygen. Redox enzymes are thus generally associated by either electron acceptor or donor for their regeneration as shown in Fig.5. 

There has long been interest in investigating Fe- and Co-based catalysts for oxygen reduction because of their role as highly effective enzymes for oxygen transport and conversion in biological systems. More recently, additional interest has been centered on alternative precious metals, metal oxides, and metal carbides and nitrides as possible oxygen reduction catalysts. 

Different chemical environments surrounding the T1 copper result in different redox potentials. Fungal laccases demonstrate the highest potential, close to the equilibrium potential of oxygen reduction in their respective pH regions (see Table 1). Laccases, however, are anion sensitive, with deactivation involving dissociation of T2 copper from the active site of the enzyme. Alternative copper oxidases such as bilirubin oxidase and ceruloplasmin ° ... 

This electrode is unique in that the bilirubin oxidase is active at neutral pH, whereas the laccase cited above is not, even though the redox potential of laccase is somewhat higher. Additionally, the bilirubin oxidase is much less sensitive to high concentrations of other anions such as chloride and bromide, which deactivate laccase. It was shown that mutations of the coordination sphere of bilirubin oxidase led to an increased redox potential of the enzyme, which increased current density and reduced current decay to 5%/day over 6 days at 300 rpm. The latter improvement was attributed to improved electrostatic attraction between the enzyme and the redox polymer. An electrode made with high-purity bilirubin oxidase and this redox polymer has recently been shown to outperform a planar platinum electrode in terms of activation potential and current density of oxygen reduction. ... 

There are several demonstrations that cytochrome cdi catalyzes the reduction of molecular oxygen to water. Exactly how the enzyme catalyzes this reaction is of some interest, because the crystal structure shows that the catalytic center is mononuclear and expected to handle one electron at a time. If we assume that electron transfer between subimits cannot occur, then only two of the four electrons required for reduction of one oxygen atom can obviously be stored on one subimit of the enz5une before reduction of oxygen commences. Thus, it might be anticipated that some intermediates of oxygen reduction are relatively long-lived. 

The enteric bacterium Enterobacter cloacae produces a nitroreductase that reduces nitrofurans, nitroimidazoles, nitrobenzene derivatives, and quinones (Bryant DeLuca, 1991). This oxygen-insensitive enzyme has been purified and is known to require FMN to transfer reducing equivalents from NAD(P)H to the nitroaromatic compounds, TNT being the preferred substrate. Aerobically, this enzyme reduces nitrofurazone through the hydroxylamine intermediate, which then tautomerizes to yield an oxime end-product. Anaerobically, however, the reduction proceeds to the fully reduced amine adduct. When E. cloacae was grown in the presence of TNT, the nitroreductase activity increased five- to tenfold. 

The characteristics of intermediates MMOH-Q and RNR R2-X continue to converge. The radiolytic reduction of MMOH-Q affords an Fe(III)/Fe(I V) form designated Qx, which has Mossbauer properties similar to RNR R2-X [25]. This work and the recent EXAFS characterization of MMOH-Q and RNR R2-X [26-28], revealing that the two intermediates have very similar core structures, provide strong support that the 02 reaction chemistry in these two enzymes is analogous. This notion very likely extends to the other diiron oxygen activating enzymes. 

The phenomenon of bioelectrical catalysis with direct electron transfer from electrode to enzyme active site was primarily observed in the study of electrochemical oxygen reduction in the presence of a copper-containing oxidase - laccase, adsorbed on electrodes of different origins. This work was developed with peroxidase and hydrogenase application as the working components [2],... 

Fig. 14.25. (A). The scheme of enzyme adsorption of the electrode with different lipid interlayers. (B). The relative oxygen reduction rate vs. the distance between the electrode and the enzyme for lactase adsorbed (1) on soot, (2) on cholesterol, (4) on lecithin. The curves a and b are calculated for barrier heights of 4 and 5 eV, respectively. (Reprinted from J. O M Bockris, M. Szklonzyk, and Szucs, in Electropharmacology, G. M. Eckert, F. Gutmann, and H. Keyzar, eds., Figs. 25,29, 30,1990. Reproduced with permission of CRC Press.)...

There is evidence from other areas of biochemistry that lends support to the present model. Thus, Rotenberg (1988) correlated the number of cytochrome c sites with the number of ATP synthesis sites. This supports the speculation that cytochrome c can serve as a site on an enzyme electrode, electrodically supplying cathodically active groups for oxygen reduction. Correspondingly, Tsong (1994) has shown that when an anode and cathode are in contact with suspended mitochondrial particles, the rate of ATP formation increases exponentially with the change in potential. 

COX is an extensive membrane-bound ensemble in which cytochrome a3 and Cub cooperate to form an oxygen reduction site and reduced cytochrome c containing iron(II) is oxidized through proximity to Cua- As a consequence of this reaction four protons are pumped across the membrane to set up a potential that helps to power ATP synthase. In plants, and some bacteria, the latter enzyme is serviced by another complex, membrane-bound protein ensemble, photosystem II. 

Figure 3.4.10 Simplified scheme for C>2-tolerant hydrogenases that show both hydrogen oxidation and oxygen reduction activity [101, 140], The Ni-SIa state (center) reacts with H2 and forms the Ni-C state (left) that returns (via Ni-R, not shown) to Ni-SIa thereby 2 H+ and 2e are released. Upon 02 binding to Ni-SIa, one electron from Ni(II) and three from the FeS clusters (two from the proximal cluster ) are delivered quickly, together with 3 H+, to reduce oxygen to water. OH- remains in the active side bridging Ni and Fe in the Ni-B state (right). A key role is played by the novel proximal [4Fe-3S] cluster. Efficient electron delivery from the other FeS clusters also assures rapid reactivation of the enzyme from the oxidized state.

Electrochemistry of respiration — The function of the enzymes in the mitochondrial respiratory chain is to transform the energy from the redox reactions into an electrochemical proton gradient across the hydrophobic barrier of a coupling membrane. Cytochrome oxidase (EC 1.9.3.1, PDB 20CC) is the terminal electron acceptor of the mitochondrial respiratory chain. Its main function is to catalyze the reaction of oxygen reduction to water using electrons from ferrocytochrome c 4H+ + 02 + 4e 2H20. This reaction is exother-... 

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