Redox reactions in biological systems

Cytochromes.—A review article on the properties and reactions of cytochromes of the higher plants and algae has been published.  

Changes in solvation caused by specific ion binding at some distance from the haem edge are proposed to explain the effect of changing the ionic medium on the oxidation of horse heart cytochrome c(ii) by [Fe(CN)e] at pH 7.0. Binding constants vary from 3 for phosphate to 7 M and 18 for chloride and 

Sakurai and A. Nakahara, Inorg. Chim. Acta, 1979, 34, L243. 

Non-adiabatic multiphonon electron tunnelling theory has been used to interpret kinetic data on a number of redox reactions of cytochrome c and yields a self-exchange rate constant for the protein of 1.7 x 10 s (ATT =4.9 kcal 

Rates for a series of mediators increase with decreasing reduction potential of the mediator until it is less than the potential of the Hb(m)/(n) couple. 

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All organisms use the same pair of pyridine nucleotides as carrier molecules for hydrogen and electrons. Both of these molecules accept hydrogen and electrons in the redox reactions of catabolism and become reduced. The oxidative half-reactions of catabolism generally produce two H+ and two electrons. The nicotinamide ring can accept two electrons and one H+ and, since the second H+ is released into the solution, most redox reactions in biological systems take the form ... 

These studies show that Cu(III)-peptide complexes have relatively low electrode potentials and suggest that Cu(III) may be a far more common oxidation state than had previously been thought possible. Furthermore, the decomposition reactions of Cu(III)-peptides indicate that two-electron transitions to give Cu(I) species are possible. Two-electron redox reactions in biological systems are intriguing because high energy, free radical intermediates are avoided. However, as yet we know very little about possible Cu(I) complexes. This oxidation state is poorly characterized in aqueous solution, and studies with various model complexes are needed. 

A number of metals are involved in redox reactions in biological systems in which the oxidation state of the metals changes. Which of these metals are most likely to take part in such reactions Na, K, Mg, Ca, Mn, Fe, Co, Cu, Zn Explain. 

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

While these calculations provide information about the ultimate equilibrium conditions, redox reactions are often slow on human time scales, and sometimes even on geological time scales. Furthermore, the reactions in natural systems are complex and may be catalyzed or inhibited by the solids or trace constituents present. There is a dearth of information on the kinetics of redox reactions in such systems, but it is clear that many chemical species commonly found in environmental samples would not be present if equilibrium were attained. Furthermore, the conditions at equilibrium depend on the concentration of other species in the system, many of which are difficult or impossible to determine analytically. Morgan and Stone (1985) reviewed the kinetics of many environmentally important reactions and pointed out that determination of whether an equilibrium model is appropriate in a given situation depends on the relative time constants of the chemical reactions of interest and the physical processes governing the movement of material through the system. This point is discussed in some detail in Section 15.3.8. In the absence of detailed information with which to evaluate these time constants, chemical analysis for metals in each of their oxidation states, rather than equilibrium calculations, must be conducted to evaluate the current state of a system and the biological or geochemical importance of the metals it contains. 

A clear avenue of future research is to explore the S-Fe redox couple in biologic systems. Bacterial sulfate reduction and DIR may be spatially decoupled, dependent upon the distribution of poorly crystalline ferric hydroxides and sulfate (e.g., Canfield et al. 1993 Thamdrup and Canfield 1996), or may be closely associated in low-suUate environments. Production of FIjS from bacterial sulfate reduction may quickly react with Fefll) to form iron sulfides (e.g., Sorensen and Jeorgensen 1987 Thamdrup et al. 1994). In addition to these reactions, Fe(III) reduchon may be coupled to oxidation of reduced S (e.g., Thamdrup and Canfield 1996), where the net result is that S and Fe may be cycled extensively before they find themselves in the inventory of sedimentary rocks (e.g., Canfield et al. 1993). Investigation of both S and Fe isotope fractionations produced during biochemical cycling of these elements will be an important future avenue of research that will bear on our understanding of the isotopic variations of these elements in both modem and ancient environments. 

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... 

Electron-transfer reactions, especially long distance electron-transfer reactions in biological systems, will probably be of primary interest. The influence of the media on the rates of electron-transfer reactions is still poorly understood. It has been established that the electron-transfer reactions can occur at quite high rates when the reactants are separated far beyond the distances of collision. It has been observed that if reactants are linked by an aliphatic chain, the rates can be substantially enhanced. Such studies will be especially pursued in redox reactions involving proteins. 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

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