Proteins, redox structure-function

Because of the exposed histidine ligands of the [2Fe-2S] cluster, the Rieske is capable of binding quinones in a redox-dependent manner. The variation of the hydrogen bond strength and of the electrostatic properties will control the movement of the catalytic domain of the Rieske protein. Therefore, the function depends on the unique structural and electrochemical properties of the Rieske cluster. 

All these bioelectrocatalytic functions of redox proteins are based on the control and enhancement of the electrical communication between the redox sites of the proteins and the electrode support. This is accomplished by the nano-engineering of the surfaces with covalently anchored proteins, the structural aligmnent of the proteins on the electrodes and the chemical modification of the proteins with redox-active units. Preliminary results suggest that two approaches will play important roles in the future development of bioelectronic systems (i) protein mutagenesis with specific functional amino acid residues that can align the protein on the electrode surface and control the electrical contact with the electrode (ii) the synthesis of de novo proteins with tailored bioelectronic and electrobiocatalytic functions. 

Although the ET processes represented by reactions 3 and 5 are nonphysiologi-cal, they can provide useful information. Thus, by varying the structure of the flavin it is possible to probe the steric and electrostatic environment of a protein redox center [38, 39]. This can be used as a method for assessing functional relationships between members of a homologous series of redox proteins (kinetic taxonomy). It is also possible to use these reactions to test the functional integrity of a redox center in a site-specific mutant [43]. 

The following sections will concentrate on the analytical application of redox proteins and redox enzymes for biosensing. The biomolecule will be briefly introduced, the major route for its direct electric contact to electrodes outlined and the analytical application discussed. The bioelectrochemical studies on structure-function relationship and their role in biological redox processes will not be covered in detail in this review. 

The direct electrochemistry of redox proteins has developed significantly in the past few years. Conditions now exist that permit the electrochemistry of all the proteins to be expressed at a range of electrodes, and important information about thermodynamic and kinetic properties of these proteins can be obtained. More recently, direct electron transfer between redox enzymes and electrodes has been achieved due to the more careful control of electrode surfaces. The need for biocompatible surfaces in bioelectrochemistry has stimulated the development of electrode surface engineering techniques, and protein electrochemistry has been reported at conducting polymer electrodes 82) and in membranes 83, 84). Furthermore, combination of direct protein electrochemistry with spectroscopic methods may offer 85) a novel way of investigating structure-function relationships in electron transport proteins. 

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