Biological systems redox processes

The electron-exchange properties of melanins have been studied with a number of especial reagents in order to understand the reaction mechanism as well as the role of melanin redox properties in biological systems. The processes have been found to be strongly irradiation dependent (both by visible and UV light). Thus, nitroxide radicals are reversibly reduced by melanins in the dark, and the redox equilibria are altered on irradiation (233). Similarly, other processes in living systems... 

Nowadays, studies of direct electrochemistry of redox proteins at the electrodesolution interface have held more and more scientists interest. Those studies are a convenient and informative means for understanding the kinetics and thermodynamics of biological redox processes. And they may provide a model for the study of the mechanism of electron transfer between enzymes in biological systems, and establish a foundation for fabricating new kinds of biosensors or enzymatic bioreactors. 

Obviously the redox poise in biological systems is very important and the movement of selenium through this process has been investigated for denitrifiers such as Paracoccus denitrificans,159 a specialized selenate-respiring bacterium Thauera selenatis which used selenate as the sole electron acceptor,160,161 and phototrophic bacteria which produced different reduced forms of selenium when amended with either selenite or selenate and even added insoluble elemental Se.162 As noted above, Andreesen has commented on the importance of redox active selenocysteines135 and Jacob et al.136 note the importance of the thioredoxin system to redox poise. 

As mentioned previously, siderophores must selectively bind iron tightly in order to solubilize the metal ion and prevent hydrolysis, as well as effectively compete with other chelators in the system. The following discussion will address in more detail the effect of siderophore structure on the thermodynamics of iron binding, as well as different methods for measuring and comparing iron-siderophore complex stability. The redox potentials of the ferri-siderophore complexes will also be addressed, as ferri-siderophore reduction may be important in the iron uptake process in biological systems. 

In biological systems the movement of an electron from a donor to an acceptor site is a quite common and apparently simple event. In reality, however, electron transfers over distances greater than 10 A are frequently necessary, which means that the electron transfer can be a slow process. In fact, according to the Marcus theory, the rate constant for electron transfer between two redox sites, ket (s-1), is given by 5... 

The oxidation/reduction of redox cofactors in biological systems is often coupled to proton binding/release either at the cofactor itself or at local amino acid residues, which provides the basic mechanochem-ical part of a proton pump such as that foimd in cytochrome c oxidase (95). Despite a thermodynamic cycle that provides that coupling of protonation of amino acids to the reduction process will result in a 60 mV/pH decrease unit in the reduction potential per proton boimd between the pAa values in the Fe(III) and Fe(II) states, the essential pumping of protons in the respiratory complexes has yet to be localized within their three-dimensional structures. 

Since the initial observation of flavin radical species by Michaelis and coworkers the involvement of flavins in one-electron oxidation-reduction processes in biological systems has occupied the attention of workers in the field of redox enzymology up to the present time. Flavin coenzymes occupy a unique role in biological oxidations in that they are capable of functioning in either one-electron or two-electron transfer reactions. Due to this amphibolic reactivity, they have been termed in a recent review to be at the crossroads of biological redox processes. 

The need for biological mediation of most redox processes encountered in natural waters means that approaches to equilibrium depend strongly on the activities of the biota. Moreover, quite different oxidation-reduction levels may be established within biotic microenvironments than those prevalent in the over-all environment diffusion or dispersion of products from the microenvironment into the macroenvironment may give an erroneous view of redox conditions in the latter. Also, because many redox processes do not couple with one another readily, it is possible to have several different apparent oxidation-reduction levels in the same locale, depending upon the system that is being used as reference. 

In recent years there has been a considerable amount of research on transition metal complexes due to the large number of potential or already realized technical applications such as solar energy conversion through photo-redox processes, optical information and storage systems, photolithographic processes, etc. Moreover, metal complexes are also of considerable importance in biology and medicine. Most of these applications are directly related to the electronic and vibronic properties of the ground and lowest excited states. 

The oxidation of carbohydrate involves the making and breaking of C-H bonds. This is a two electron process, and the most efficient way of achieving the oxidation will use a two electron oxidant. In many biological systems one of the key redox steps involves the two-electron conversion of a quinone to a hydroquinone (Fig. 10-6). 

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