Abstract
This review summarizes the fundamentals of the phenomenon of electron transfer (ET) reactions occurring in redox enzymes that were widely employed for the development of electroanalytical devices, like biosensors, and enzymatic fuel cells (EFCs). A brief introduction on the ET observed in proteins/enzymes and its paradigms (e.g., classification of ET mechanisms, maximal distance at which is observed direct electron transfer, etc.) are given. Moreover, the theoretical aspects related to direct electron transfer (DET) are resumed as a guideline for newcomers to the field. Snapshots on the ET theory formulated by Rudolph A. Marcus and on the mathematical model used to calculate the ET rate constant formulated by Laviron are provided. Particular attention is devoted to the case of glucose oxidase (GOx) that has been erroneously classified as an enzyme able to transfer electrons directly. Thereafter, all tools available to investigate ET issues are reported addressing the discussions toward the development of new methodology to tackle ET issues. In conclusion, the trends toward upcoming practical applications are suggested as well as some directions in fundamental studies of bioelectrochemistry.
Highlights
Redox enzymes are defined as proteins that facilitate biological electron transfer (ET) processes, acquitting for multiple essential biological functions like photosynthesis, respiration, nucleic acid biosynthesis, etc. [1,2,3]
As the first experiment, we might consider determining the formation of the expected product, which does not give any confirmation for the ET mechanism, but it only tells that the reaction is taking place
Close to the heme groups of the cytochrome domain interact with the grafted anthracene due to the π-π interactions with the aromatic side chains of the amino acids present in the hydrophobic pocket of fructose dehydrogenase (FDH) [179,180]. This interaction results in the oriented deposition of the FDH molecules facilitating redox transformations for both heme groups according to the direct electron transfer (DET) mechanism
Summary
Redox enzymes are defined as proteins that facilitate biological electron transfer (ET) processes, acquitting for multiple essential biological functions like photosynthesis, respiration, nucleic acid biosynthesis, etc. [1,2,3]. Redox cofactors exhibit different formal potentials (E◦ ) spread over a potential window of approximately 1.5 V [3,7,8,9,10,11,12,13,14,15], which is wider, especially compared to the water thermodynamic stability window, Figure 1, considering hydrogen ions reduction to molecular hydrogen (E0 ’2H+ /H2 = −0.41 V vs standard hydrogen electrode (SHE) at pH 7) and oxygen reduction to water (E0 ’O2/H2O = +0.82 V vs SHE at pH 7) normally occurring in biological systems [16,17].
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