Biofuel cells redox proteins

Willner and coworkers have extended this approach to electron relay systems where core-based materials facilitate the electron transfer from redox enzymes in the bulk solution to the electrode.56 Enzymes usually lack direct electrical communication with electrodes due to the fact that the active centers of enzymes are surrounded by a thick insulating protein shell that blocks electron transfer. Metallic NPs act as electron mediators or wires that enhance electrical communication between enzyme and electrode due to their inherent conductive properties.47 Bridging redox enzymes with electrodes by electron relay systems provides enzyme electrode hybrid systems that have bioelectronic applications, such as biosensors and biofuel cell elements.57... 

Methods to electrically wire redox proteins with electrodes by the reconstitution of apo-proteins on relay-cofactor units were discussed. Similarly, the application of conductive nanoelements, such as metallic nanoparticles or carbon nanotubes, provided an effective means to communicate the redox centers of proteins with electrodes, and to electrically activate their biocatalytic functions. These fundamental paradigms for the electrical contact of redox enzymes with electrodes were used to develop amperometric sensors and biofuel cells as bioelectronic devices. 

Direct, unmediated electrochemistry of redox enzymes has interested many researchers in several aspects. Understanding of the thermodynamics, kinetics, stoichiometry, and interfacial properties of redox enzymes is obviously important. The most attractive aspect, however, is the use of enzyme electrodes as novel electrochemical biosensors and their applications to bioreactors and biofuel cells. Although the observation of direct electrochemistry of small redox proteins has become almost commonplace as the consequence of extensive research over the past decade, the corresponding study with larger redox enzymes has proved more elusive. The difficulty lies mainly in that the redox centers are located sufficiently far from the outermost... 

The need to improve the electrical communication between redox proteins and electrodes, and the understanding that the structural orientation at the molecular level of redox proteins and electroactive relay units on the conductive surfaces is a key element to facilitate ET, introduced tremendous research efforts to nano-engineer enzyme electrodes with improved ET functionalities. The present chapter addresses recent advances in the assembly of structurally aligned enzyme layers on electrodes by means of surface reconstitution and surface crosslinking of structurally oriented enzyme/cofactor complexes on electrodes. The ET properties of the nano-structured interfaces is discussed, as well as the possible application of the systems in bioelectronic devices such as biosensors, biofuel cell elements or optical and electrical switches. 

Studies of electrochemical reactions of redox proteins have attracted widespread interest and attention. Such studies can yield important information about not only intrinsic thermodynamic and kinetic properties of redox proteins, but also structural properties, such as binding characteristics of proteins at specific types of electrode surfaces and the orientational requirements for electron transfer between the protein and the electrode. The results are useful for the development of biosensors, biofuel cells, and biocatalysts. In addition, the information obtained from these studies can contribute to an understanding of the physiological implications of biological electron transfer reactions, because many electron transfer proteins are located at, or close to, charged membranes and are thus subject to large electric field effects that are similar to those near an electrode surface. 

The structure and physicochemical properties of the enzymes which have been used to date to promote electrochemical reactions are briefly outlined. Methods of their immobilization are described. The status of research on redox transformations of proteins and enzymes at the electrode-electrolyte interface is discussed. Current concepts on the ways of conjugation of enzymatic and electrochemical reactions are summarized. Examples of bioelectrocatalysis in some electrochemical reactions are described. Electrocatalysis by enzymes under conditions of direct mediatorless transport of electrons between the electrode and the enzyme active center is considered in detail. Lastly, an analysis of the status of work pertaining to the field of sensors with enzymatic electrodes and to biofuel cells is provided. 

Microfluidk Fuel Cells, Fig. 2 Anodic half-cell of a biofuel cell based on biocatalytic Mizyme proteins, fuel, and redox mediators in solution... 

DET was also evidenced at SWCNT electrodes for other redox proteins such as hemoglobine [15], cytochrome C [11], microperoxydases [16] or catalases[17]. The ability of CNTs to achieve DET with enzymes has lead to the design of novel biofuel cell electrodes. 

Direct electron transfer (DET) means an exchange of electrons between the cofactor of a redox-active enzyme (oxidoreductase) or a redox protein and an electrode (transducer) in the absence of redox mediators. DET is rarely observed and reported for only few redox proteins and redox enzymes. DET is of interest for fundamental studies on electron transfer in proteins and enzymes and for the development of sensitive and specific biosensors, robust biofuel cells and heterogeneous bioelectrosynthesis. 

The field of electrochemical studies of redox proteins from its theory to application has been extensively reviewed and documented since its inception in 1970s [4-7, 12-19]. The last few years have seen a tremendous development in this field ranging from the diversity of proteins (small to large, monomeric to multimeric, cytoplasmic to membrane bound, simple to complex) that could possibly be studied electrochemically to their efficient immobilisation in a variety of matrices especially nanomatrices. Major advances have also been made in the areas of characterisation of fabricated bioelectrodes and their applications for studies of redox mechanism or as biosensors or biofuel cells. This chapter will focus mainly on the aforementioned recent developments in the field of electrochemical studies of redox proteins with a brief discussion over the theory and instmmentation required to understand the context. 

NoU T, NoU G. Strategies for wiring redox-active proteins to electrodes and applications in biosensors, biofuel cells and nanotechnology. Chem Soc Rev 2011 40 3564-3576. 

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