Protein-mediated electron transfer

Simple iron-sulfur proteins, containing these basic structures are listed in Table 1. They are generally electron-transfer proteins mediating electron exchange between enzymatic systems, with the possible exception of hydrogenase, and aconitase, which might have catalytic activity of their own, as shall be discussed later. 

In summary, electron transfer dynamics, mediated through saturated hydrocarbon bridges and proteins, displays a surprisingly weak distance dependence behaviour (/J = 0.8-12 A 1), compared to that predicted for a pure through-space mechanism (P 3.0 A1). 

Ferredoxins are electron-transfer proteins that can mediate between pyruvate ferredoxin oxidoreductase and hydrogenase. It appears that during the course of the evolution, different types of ferredoxin were recruited for this purpose. In Clostridia, ferredoxins of the 2[4Fe-4S] type are used (Uyeda and Rabinowitz 1971). In T. vaginalis (Chapman et al. 1986) and T. foetus (Marczak et al. 1983), [2Fe-2S] ferredoxins are used. Their axial EPR spectra at g = 1.94,2.02 (Fig. 9.2) resemble those of the ferredoxins that are involved in P450 monooxygenase systems. Similar ferredoxins, with various functions, have been isolated from... 

A preliminary investigation of kinetics and equilibrium of the reactions of electron transfer between mediators and proteins is therefore usehil in order to evaluate the efficiency of the mediation and the quality of the deduced information on the red-ox properties of the protein itself... 

The formation of superoxide is the result of one electron transfer by several coenzymes in ETS, including flavins, flavoproteins, quinones, and iron sulfur proteins. This product has a longer half-life than other intermediates and is toxic to anaerobic bacteria. Peroxidase is formed by two electron transfers and mediated by flavoproteins. Peroxidase is further reduced to the hydroxyl radical with the addition of one electron followed by subsequent reduction to water by the addition of another electron. The oxidative effect of these intermediates can result in the destruction of cells. The aerobic bacteria have enzyme systems such as superoxidase dismutase, peroxidase, and catalase to reduce the toxic levels of these intermediates. 

In biological systems proteins mediate electron transfer. This mediation takes place either through bonds in the protein, or through space, when aromatic amino acid residues are considered to mediate the electron transfer in the space. 

While it is interesting to view strongly adsorbed proteins in the role of relaying electrons to bulk species, there is no clear evidence that the deliberate provision of electron-mediating functionalities at the electrode surface is relevant, at least for the small electron-transfer proteins that we have discussed above. Modified electrodes designed to relay electrons have been described, and results show that the potential of the surface-confined mediator (as would be the case were these to be free reagents [6]) needs to be matched with that of the protein to be addressed in... 

The first MFCs used a mediator, a chemical that transfers electrons from bacteria in the cell to the anode. Mediators include humic acid, methylene blue and thionine. Many mediators are toxic and expensive. In MFCs without mediators the bacteria have electron transfer proteins, such as cytochromes, on their outer membrane that can transfer electrons directly to the anode. 

Mediators were adopted by Theodore Kuwana for use in faradaic electrochemical studies of electron transfer proteins. The electron transfer reactions of a protein/enzyme were coupled to the potential applied to an electrode by having the appropriate mediators present in solution. Initial experiments involved using mediators to conduct indirect coulometric titrations of proteins/enzymes, often using optical absorption spectroscopy at optically transparent electrodes to simultaneously monitor the titration progress. The reaction scheme in its simplest form is illustrated with the equations ... 

This background is provided to show that work done in this laboratory initially on mediated electron protein studies led to the study of direct electron transfer reactions of proteins and enzyme at electrodes. It was a not a large intellectual step to consider direct studies of electron transfer proteins at electrodes, given this background and the work being done in other laboratories in the area of chemically modified electrodes at that time. Still, it was the unexpected that led to actually thinking that an electron transfer protein might react reversibly or quasi-reversibly at an elecflode, as will be described later. 

Dioxygenases are nonheme Rieske-type NAD(P)H-dependent enzymes that introduce both atoms of molecular oxygen into their substrates. The multicomponent enzyme systems are composed of a catalytic oxygenase component and one- or two-electron transfer proteins, including a flavoprotein reductase, and in some cases a ferredoxin that mediates electron transfer from the reductase to the oxygenase component ([3] Figure 17.1). 

The pathway model makes a number of key predictions, including (a) a substantial role for hydrogen bond mediation of tunnelling, (b) a difference in mediation characteristics as a function of secondary and tertiary stmcture, (c) an intrinsically nonexponential decay of rate witlr distance, and (d) patlrway specific Trot and cold spots for electron transfer. These predictions have been tested extensively. The most systematic and critical tests are provided witlr mtlrenium-modified proteins, where a syntlretic ET active group cair be attached to the protein aird tire rate of ET via a specific medium stmcture cair be probed (figure C3.2.5). 

The large size of redox enzymes means that diffusion to an electrode surface will be prohibitively slow, and, for enzyme in solution, an electrochemical response is usually only observed if small, soluble electron transfer mediator molecules are added. In this chapter, discussion is limited to examples in which the enzyme of interest is attached to the electrode surface. Electrochemical experiments on enzymes can be very simple, involving direct adsorption of the protein onto a carbon or modified metal surface from dilute solution. Protein film voltammetry, a method in which a film of enzyme in direct... 

Figure 17.4 Cartoon representation of strategies for studying and exploiting enzymes on electrodes that have been used in electrocatalysis for fuel cells, (a) Attachment or physisorption of an enzyme on an electrode such that redox centers in the protein are in direct electronic contact with the surface, (b) Specific attachment of an enzyme to an electrode modified with a substrate, cofactor, or analog that contacts the protein close to a redox center. Examples include attachment of the modifier via a conductive linker, (c) Entrapment of an enzyme within a polymer containing redox mediator molecules that transfer electrons to/from centers in the protein, (d) Attachment of an enzyme onto carbon nanotubes prepared on an electrode, giving a large surface area conducting network with direct electron transfer to each enzyme molecule.

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