Biological redox mediator

Biological redox mediation chain involving cytochrome P450... 

In some cases, small biological redox partner proteins such as heme-containing cytochromes, ferredoxins comprising an iron-sulfur cluster, or azurin with a mononuclear Cu site have been used as natural mediators to facilitate fast electron exchange with enzymes. A specific surface site on the redox protein often complements a region on the enzyme surface, and enables selective docking with a short electron tunneling... 

N2, or CO2 when the proper enzyme is present as a catalyst.(53) The use of surface-confined, fast, one-electron, outer-sphere redox reagents like those derived from or III as redox mediators for biological reagents would seem to represent an excellent approach to the equilibration of the electrode with the biological reagents. 

Though accelerating effect of redox mediators is proved, differences in electrochemical factors between mediator and azo dye is a limiting factor for this application. It was reported that redox mediator applied for biological azo dye reduction must have redox potential between the half reactions of the azo dye and the primary electron donor [37], The standard redox potentials for different azo dyes are screened generally between -430 and -180 mV [47],... 

Albuquerque MGE, Lopes AT, Serralheiro ML et al (2005) Biological sulphate reduction and redox mediator effects on azo dye decolourisation in anaerobic-aerobic sequencing batch reactors. Enzyme Microb Technol 36 790-799... 

Anaerobic bio-reduction of azo dye is a nonspecific and presumably extracellular process and comprises of three different mechanisms by researchers (Fig. 1), including the direct enzymatic reduction, indirect/mediated reduction, and chemical reduction. A direct enzymatic reaction or a mediated/indirect reaction is catalyzed by biologically regenerated enzyme cofactors or other electron carriers. Moreover, azo dye chemical reduction can result from purely chemical reactions with biogenic bulk reductants like sulfide. These azo dye reduction mechanisms have been shown to be greatly accelerated by the addition of many redox-mediating compounds, such as anthraquinone-sulfonate (AQS) and anthraquinone-disulfonate (AQDS) [13-15],... 

The acceleration mechanism of redox mediators are presumed by van der Zee [15]. Redox mediators as reductase or coenzymes catalyze reactions by lowering the activation energy of the total reaction. Redox mediators, for example, artificial redox mediators such as AQDS, can accelerate both direct enzymatic reduction and mediated/indirect biological azo dye reduction (Fig. 3). In the case of direct enzymatic azo dye reduction, the accelerating effect of redox mediator will be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the azo dye. Possibly, both reactions will be catalyzed by the same nonspecific periplasmic enzymes. In the case of azo dye reduction by reduced enzyme cofactors, the accelerating effect of redox mediator will either be due to an electron shuttle between the reduced enzyme cofactor and redox mediator or be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the coenzymes. In the latter case, the addition of redox mediator simply increases the pool of electron carriers. 

It is known that the decolorization rate of azo dyes is increased by using redox mediators, which speed up the reaction rate by shuttling electrons from the biological oxidation of primary electron donors or from bulk electron donors to the electron-accepting azo dyes [21, 31, 40]. But continuous dosing of the dissolved redox mediators implies continuous expenses related to procurement of the... 

In a word, these studies explore a great improvement of the redox mediator application and the new bio-treatment concept for biological treatment. 

To reach the reductive step of the azo bond cleavage, due to the reaction between reduced electron carriers (flavins or hydroquinones) and azo dyes, either the reduced electron carrier or the azo compound should pass the cell plasma membrane barrier. Highly polar azo dyes, such as sulfonated compounds, cannot pass the plasma membrane barrier, as sulfonic acid substitution of the azo dye structure apparently blocks effective dye permeation [28], The removal of the block to the dye permeation by treatment with toluene of Bacillus cereus cells induced a significant increase of the uptake of sulfonated azo dyes and of their reduction rate [29]. Moreover, cell extracts usually show to be more active in anaerobic reduction of azo dyes than whole cells. Therefore, intracellular reductases activities are not the best way to reach sulfonated azo dyes reduction the biological systems in which the transport of redox mediators or of azo dye through the plasma membrane is not required are preferable to achieve their degradation [13]. 

Electron-transfer in biological systems takes place through the mediation of a number of proteins, which contain a variety of active sites such as heme, Fe—S, Cu, and flavin. These active sites are protected from the solvent by a hydrophobic environment created by the peptide chain 48). The redox potential of a biological redox couple in vivo lies, for the most part, between —0.5 and +0.85 V. The former and latter potentials correspond to the redox potentials of H20/H2 and H20/02 respectively 49). 

Fultz, M.L. and R.A. Durst. 1982. Mediator compounds for the electrochemical study of biological redox systems A compilation. Anal. Chim. Acta 140, 1-18. 

Redox mediators provide an alternative to dissolved oxygen measurements for monitoring of respiration or photosynthesis. The mechanism is based on the reducing activity of metabolising biological whole cells ... 

Flavin-dependent le -transfer in enzymes and chemical model systems can he differentiated from 2e -transfer activities, i.e., (de)hydrogenation and oxygen activation, by chemical structure and dynamics. For le -transfer, two types of contacts are discussed, namely outer sphere for interflavin and flavin-heme and inner sphere for flavinr-fenedoxin contacts. Flavin is the indispensable mediator between 2e - and le -transfer in all biological redox chains, and there is a minimal requirement of three cooperating redox-active sites for this activity. The switch between 2e - and le -transfer is caused by apoprotein-dependent prototropy between flavin positions N(l)/0(2a) and N(5) or by N(5)-metal contact. 

The use of reversible redox systems in indirect electrolytic reactions may be of significance, especially in biological systems. Many of these systems comprise large molecules which diffuse slowly to the electrode and may lose some of their structural features on contact with the electrode, thereby becoming biologically inactive. In a number of cases this difficulty may be circumvented by using a suitable mediator to transfer the electrons between the biological redox system and the electrode. 

Relatively simple syntheses for the majority of macrobicyclic complexes, compared with conventional techniques for the preparation of macrocyclic compounds, have made such complexes attractive not only for research, but also for practical application as electron carriers, catalysts for electro- and photochemical processes, and some other purposes (e.g., protein redox titrants, biological electrochemical mediators, and ionophore and electrode modifiers). 

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