Glucose oxidase direct electron transfer

Hu F, Chen S, Wang C, Yuan R, Chai Y, Xiang Y, Wang C. ZnO nanoparticle and multiwalled carbon nanotubes for glucose oxidase direct electron transfer and electro-catalytic activity investigation. J Mol Catal B Enzym 2011 72 298-304. 

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) ... 

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S.Q. Liu and H.X. Ju, Reagentless glucose biosensor based on direct electron transfer of glucose oxidase immobilized on colloidal gold modified carbon paste electrode. Biosens. Bioelectron. 19, 177-183 (2003). 

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In contrast to the mediator-modified electrodes, Degani et al. modified glucose oxidase itself by means of covalently bound ferrocene [4]. After modifying enzymes with ferrocene carboxylic acid, they observed direct electron transfer from the active site of the enzyme to a gold or platinum... 

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. 

J.Z. Xu, J.J. Zhu, Q. Wu, Z. Hu and H.Y. Chen, Direct electron transfer between glucose oxidase and multi-walled carbon nanotubes, Chin. J. Chem., 21 (2003) 1088-1091. 

Although enzymes with deeply embedded active centers do not normally allow significant non-mediated electron transfer [44], under carefully chosen conditions direct electron transfer is possible, and has been reported for glucose oxidase (GOx) at a bare Ag electrode [45] and in graphite paste electrodes [46]. These systems are unusual and the nonmediated mechanism of such long-range electron transfer is not fully understood. 

Figure 28. CVs of GOx /NiOx modified GC electrode at various scan rate in pH 7 PBS, from inner to outer, 10, 20, 30, 40, 50, 60, 70, 80, 90 and lOOmVs 1. Reprinted from Biosensors and Bioelectronics, 22, ASalimi, E. Sharifi, A. NoorBakhash, S. Soltanian, Immobilization of glucose oxidase on electrodeposited nickel oxide nanoparticles Direct electron transfer and electrocatalytic activity,3148,Copy eight (2007), with permission from Elsevier.

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