Electron transfer proteins novel

An appreciation of the basic parameters of electron tunneling theory and a survey of the values of these parameters in natural systems allows us to grasp the natural engineering of electron transfer proteins, what elements of their design are important for function and which are not, and how they fail under the influence of disease and mutation. Furthermore, this understanding also provides us with blueprint for the design of novel electron transfer proteins to exploit natural redox chemistry in desirable, simplified de novo synthetic proteins (Robertson et al., 1994). 

Three general approaches may be used to alter the redox properties of a protein (1) modification of existing electron transfer proteins, (2) introduction of a new redox center into an existing protein, and (3) design of novel redox proteins. All approaches have been implemented at some level and are briefly discussed in this section. 

The second approach embodies perhaps the most exciting frontier in present-day protein chemistry, namely, the possibility of designing new proteins of defined stmcture. Novel proteins based on a-helical bundles have already been synthesized and characterized (89-97). This structural motif is of special interest since the cofactors of several classes of electron transfer proteins are associated with helical bundles. Although designed electron-transfer proteins apparently have yet to be created, the extension of this approach to the development of novel redox proteins seems inevitable. The successful construction of designed proteins with prespecified electrochemical properties will provide the ultimate test of our understanding of the principles of protein folding and redox control. 

In this chapter, a novel interpretation of the membrane transport process elucidated based on a voltammetric concept and method is presented, and the important role of charge transfer reactions at aqueous-membrane interfaces in the membrane transport is emphasized [10,17,18]. Then, three respiration mimetic charge (ion or electron) transfer reactions observed by the present authors at the interface between an aqueous solution and an organic solution in the absence of any enzymes or proteins are introduced, and selective ion transfer reactions coupled with the electron transfer reactions are discussed [19-23]. The reaction processes of the charge transfer reactions and the energetic relations... 

The first reports on direct electrochemistry of a redox active protein were published in 1977 by Hill [49] and Kuwana [50], They independently reported that cytochrome c (cyt c) exhibited virtually reversible electrochemistry on gold and tin doped indium oxide (ITO) electrodes as revealed by cyclic voltammetry, respectively. Unlike using specific promoters to realize direct electrochemistry of protein in the earlier studies, recently a novel approach that only employed specific modifications of the electrode surface without promoters was developed. From then on, achieving reversible, direct electron transfer between redox proteins and electrodes without using any mediators and promoters had made great accomplishments. 

Conversely, controlled immobilization of enzymes at surfaces to enable high-rate direct electron transfer would eliminate the need for the mediator component and possibly lead to enhanced stability. Novel surface chemistries are required that allow protein immobilization with controlled orientation, such that a majority of active centers are within electrontunneling distance of the surface. Additionally, spreading of enzymes on the surfaces must be minimized to prevent deactivation due to irreversible changes in secondary structure. Finally, structures of controlled nanoporosity must be developed to achieve such surface immobilization at high volumetric enzyme loadings. 

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. 

Novel biointerfaces of dendrimers with heme proteins have been reported. Such studies have helped to accomplish direct electron transfer between the biomolecule and electrode. Several attempts have been made to widen the scope of the dendrimer-based electrochemical biosensors for environmental monitoring. Analytes like glutamate, ethanol and several pesticides have been detected. Exclusive studies to detect hydrogen peroxide at dendrimer-modified electrodes have also been made. 

Now that it is substantiated that the [2Fe 2S] domain of the Rieske iron-sulfur protein is not static but moves between domains of cytochrome-c, and cytochrome-/ subunits, and that it is likely that such movement may provide a novel mechanism to allow catalysis of all the reactions involved in the oxidation of hydroquinone at the Qo site and the subsequent bifurcated pathway of electron transfer. It has been found that during the movement, the mobile [2Fe 2S] domain retains essentially the same tertiary structure, and the anchoring N-terminal tail of the R-ISP molecule remains in the same fixed position. The movement occurs through an extension of a helical segment in the short linking span. 

The chapters in this volume offer overviews of electronic properties, electron transfer and electron-proton coupled charge transfer of biological molecules and macromolecules both in the natural aqueous solution environment and on metallic electrode surfaces, where the electrochemical potential controls biomolecular function. Redox metalloproteins and DNA-based molecules are primary targets, but amino acid and nucleobase building blocks are also addressed. Novel enviromnents where proteins and DNA-based molecules are inserted in metallic nanoparticle hybrids or in situ STM configurations are other focus areas. 

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