Biologically Important Redox Reactions

Microorganisms (or any other organisms) do not perform chemical reactions, they catalyze them and use them for purposes such as deriving 

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Many biologically important redox reactions proceed with reversible hydrogen transfer (formally ... 

What are the reactions of key oxidation-reduction coenzymes Many biologically important redox reactions involve coenzymes, such as NADH and FADH. These coenzymes appear in many reactions as one of the half reactions that can be written for a redox reaction. 

Biologically Important Redox Reactions 411 TABLE 7-12 Control Methods for Sulfide in Carbon Adsorption Columns... 

Many biologically important redox reactions proceed via the N-quaternary pyridine system of nicotinamide (133) in the coenzyme NAD+ with reversible hydrogen transfer to the pyridine-4-position (redox equilibrium NAD 133/NADH 134) ... 

Let us now take a brief look at some important redox reactions of organic pollutants that may occur abiotically in the environment. We first note that only a few functional groups are oxidized or reduced abiotically. This contrasts with biologically mediated redox processes by which organic pollutants may be completely mineralized to C02, HzO and so on. Table 14.1 gives some examples of functional groups that may be involved in chemical redox reactions. We discuss some of these reactions in detail later. In Table 14.1 only overall reactions are indicated, and the species that act as a sink or source of the electrons (i.e., the oxidants or reductants, respectively) are not specified. Hence, Table 14.1 gives no information about the actual reaction mechanism that may consist of several reaction steps. 

Ravindranath and co-workers studied the electrochemical behavior of 5-amino-2-phenyl-4-arylazo-l,2-dihydro-3//-pyrazol-3-one (90UC864) and 5-methyl-4-arylazo-2-(pyridin-2-ylcarbonyl)-2,4-dihydro-3//-pyrazol-3-(Mie (90IJC895). Similar studies were undertaken by Jain and Damodharan of pyrazol-3-ones 408a-f (95CJC176) (Scheme 94). The underlying rationale for this study on the electrochemical reduction of these biologically important pyrazol-3-ones is that it can lead to information on the reaction routes and mechanisms of biological redox reactions. 

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. 

Biological action is very important in Se redox transformations. Rates of abiotic selenium redox reactions tend to be slow, and in soils and sediments, Se(VI), Se(IV), Se(0) and organically bormd Se often coexist (Tokrmaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGratii 1998). Bacteria use Se(VI) and Se(IV) as eleclron acceptors (Blum et al. 1998 Dungan and Frankenberger 1998 Oremland et al. 1989), or oxidize elemental Se (Dowdle and Oremland 1998), and it is likely that most of the important redox transformations are microbially mediated. 

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