Glucose, electron donor

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

The anaerobic dechlorination of hexachlorobenzene has been described in anaerobic mixed cultures supplemented with electron donors including lactate, ethanol, or glucose (Holliger et al. 1992) successive and partial dechlorination produced 1,2,4- and 1,3,5-trichlorobenzenes, while the 1,2,3-trichlorobenzene was further dechlorinated. The partial dechlorination of 1,2,3,4-tetra-, 1,2,3,5-tetra-, and pentachlorobenzene has been examined in a methanogenic mixed culture using lactate as electron donor (Middeldorp et al. 1997), and sterile Rhine River sand was needed to maintain dechlorination activity for unresolved reasons. 

Since anaerobic azo dye reduction is an oxidation-reduction reaction, a liable electron donor is essential to achieve effective color removal rates. It is known that most of the bond reductions occurred during active bacterial growth [48], Therefore, anaerobic azo dye reduction is extremely depended on the type of primary electron donor. It was reported that ethanol, glucose, H2/CO2, and formate are effective electron donors contrarily, acetate and other volatile fatty acids are normally known as poor electron donors [42, 49, 50]. So far, because of the substrate itself or the microorganisms involved, with some primary substrates better color removal rates have been obtained, but with others no effective decolorization have been observed [31]. Electron donor concentration is also important to achieve... 

Razo-Flores et al. (1999) studied the fate of 2,4-dinitrotoluene (120 mg/L) in an upward-flow anaerobic sludge bed reactor containing a mixture of volatile fatty acids and/or glucose as electron donors. 2,4-Dinitrotoluene was transformed to 2,4-diaminotoluene (52% molar yield) in stoichiometric amounts until day 125. Thereafter, the amine underwent continued degradation. Approximately 98.5% of the volatile fatty acids in the reactor was converted to methane during the 202-d experiment. 

In principle, glucose oxidase could be oxidized directly at the electrode, which would be the ultimate electron acceptor. However, direct electron transfer between redox enzymes and electrodes is not possible because the FADH2/FAD redox centers are buried inside insulating protein chains (Heller, 1990). If it were not the case, various membrane redox enzymes with different standard potentials would equalize their potentials on contact, thus effectively shorting out the biological redox chains. The electron transfer rate is strongly dependent on the distance x between the electron donor and the electron acceptor. 

While enantioselectivity during reduction of ethyl 3-oxobutanoate by baker s yeast (Saccharomyces cerevisiae) to ethyl (S)-3-hydroxybutanoate was found to exceed 99%, yields did not exceed 50-70% (Chin-Joe, 2000). Elimination of two of three causes, evaporation of substrate and product esters and absorption or adsorption of the two esters by the yeast cells, increased the yield to 85%. Alleviation of hydrolysis of the two esters by yeast enzymes could increase the yield even more. Low supply rates of glucose as an electron donor provided the most efficient strategy for electron donor provision and yielded a high enantiomeric excess of ethyl (S)-3-hydroxybutanoate, low by-product formation and biomass increase, with a low oxygen requirement(Chin-Joe, 2001). 

Reaction 2 has been studied in great detail (28-441 since Baur and Neuweiler (28) in 1927 observed the formation of H202 when they illuminated aqueous zinc oxide suspensions in the presence of glycerin and glucose which in turn were oxidized. Appreciable yields of hydrogen peroxide are detected only when appropriate electron donors, D, are added prior to illumination. This strongly indicates that it is the electron donor, D, which is adsorbed on the catalyst s surface and hence sacrificed via Reaction 4. 

Bacterial reduction in air of Mn02 to Mn and of CrO/ to Cr Several examples of enzymatic reduction of Mn02 to Mn in air have been reported (see Ehrlich, 2002a, pp. 449-55). Glucose and acetate have been shown to be effective electron donors in Mn(IV) reduction. In at least one instance of Mn(IV) reduction, some energy appeared to be conserved in the process (Ehrlich, 1993a, b). 

A number of examples of enzymatic reduction of Cr04 to Cr(III) in air have also been reported (see Ehrlich 2002a, pp. 531 ). Glucose and citrate were found to be effective electron donors. Involvement of the electron transport system in the plasma membrane of Pseudomonas fluor-escens LB300 suggested that some energy may be conserved in this process. 

ZnSe-coated DHP vesicles led to charge separation upon irradiation and to electron transfer to methylviologen in the presence of sacrificial electron donors like glucose or cysteine [85], The process is pH dependent as not current was observed below pH 9, while above this pH the photocurrent was linearly dependent on pH. The anodic photocurrent results from electrohole... 

A promising new area of enzymatic reactions is that of anaerobes. These organisms use redox systems that are not pyridine nucleotide dependent, but can use mcthylvirologen or benzylviologen (commercially available) as mediators. Electron donors can be hydrogen gas, formate, or carbon monoxide rather than glucose." The first new enzyme from this source is a 2-enoatc reductase, effected with stereospecific /rans-addition of hydrogen. An... 

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