Redox processes, biological mediation

Biologically mediated redox reactions tend to occur as a series of sequential subreactions, each of which is catalyzed by a specific enzyme and is potentially reversible. But despite favorable thermodynamics, kinetic constraints can slow down or prevent attainment of equilibrium. Since the subreactions generally proceed at unequal rates, the net effect is to make the overall redox reaction function as a imidirectional process that does not reach equilibrium. Since no net energy is produced imder conditions of equilibrium, organisms at equilibrium are by definition dead. Thus, redox disequilibrium is an opportunity to obtain energy as a reaction proceeds toward, but ideally for the sake of the organism does not reach, equilibrium. 

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. 

The major in situ process that results in the formation of H202 is undoubtedly photochemical (e.g., 12, 15, 49, 50). Photochemical formation of H202 in fresh and salt waters probably results from the disproportionation of the superoxide ion radical, 02 (8, 9, 15, 51, 52). The kinetics of superoxide disproportionation are well established (53), and its steady-state concentration can be calculated. Because of the known effects of superoxide ion in cells (47), its presence in surface waters may be important in biologically mediated processes. However, other sources, such as biological formation (e.g., 45, 54), redox chemistry (21, 24, 29, 31, 32), wet (e.g., 55) and dry (50, 56, 57) deposition, and surfaces (e.g., 58) may also be important. 

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. 

When comparisons are made between calculations for an equilibrium redox state and concentrations in the dynamic aquatic environment, the implicit assumptions are that the biological mediations are operating essentially in a reversible manner at each stage of the ongoing processes or that there is a metastable steady-state that approximates the partial equilibrium state for the system under consideration. 

Fig. 3. Biologically mediated redox processes. Reprinted from Stumm and Morgan, 1981.

A model of a flavin-based redox enzyme was prepared.[15] Redox enzymes are often flavoproteins containing flavin cofactors flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). They mediate one- or two-electron redox processes at potentials which vary in a range of more than 500 mV. The redox properties of the flavin part must be therefore tuned by the apoenzyme to ensure the specific function of the enzyme. Influence by hydrogen bonding, aromatic stacking, dipole interactions and steric effects have been so far observed in biological systems, but coordination to metal site has never been found before. Nevertheless, the importance of such interactions for functions and structure of other biological molecules make this a conceivable scenario. 

BIOPLUME III is a public domain transport code that is based on the MOC (and, therefore, is 2-D). The code was developed to simulate the natural attenuation of a hydrocarbon contaminant under both aerobic and anaerobic conditions. Hydrocarbon degradation is assumed due to biologically mediated redox reactions, with the hydrocarbon as the electron donor, and oxygen, nitrate, ferric iron, sulfate, and carbon dioxide, sequentially, as the electron acceptors. Biodegradation kinetics can be modeled as either a first-order, instantaneous, or Monod process. Like the MOC upon which it is based, BIOPLUME III also models advection, dispersion, and linear equilibrium sorption [67]. 

The most important biologically mediated reactions are summarized in Table 1.1 together with information about the redox environment these reactions take place in, the organisms that are usually conducting these processes, and what biochemical role these processes play for these organisms. I briefly discuss these reactions, but refer to the following chapters for details (see also Table 1.1). For a discussion of a series of additional reactions (e.g. Oxygen-Limited Autotrophic Nitrification-... 

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). 

Acid atmospheric deposition causes acidification of waters and soils if the neutralization of the acids by weathering is too slow. Biologically mediated redox processes are important in affecting the H balance. Among the redox processes that have a major impact on H" production and consumption are the synthesis and mineralization of biomass. Any uncoupling of linkages between photosynthesis and respiration affects acidity and alkalinity in terrestrial and aquatic ecosystems (Table 15.1). 

Figure 6 shows schematically the aquatic redox cycle of iron. Under the conditions usually encountered in natural aquatic systems, the reduction of iron(III) is accompanied by dissolution and the oxidation of iron(II) by precipitation. Reductive dissolution of iron(III) hydroxides occurs primarily at the sediment-water interface under anoxic conditions in the presence of reduct-ants, such as products of the decomposition of biological material or exudates of organisms. Reductive dissolution of iron(III) hydroxides, however, can also occur in the photic zone in the presence of compounds that are metastable with respect to iron(III), that is, compounds that do not undergo redox reactions with iron(III) unless catalyzed by light. The direct biological mediation of redox processes may also influence the redox cycles of iron (Arnold et al., 1986 Price and Morel, Chapter 8, this volume). Dissolved oxygen is usually the oxidant of... 

The potential of pulse radiolysis for studying biological redox processes, particularly of macromolecules, was recognized rather early. It was initially employed for investigating radiation-induced damage and, later on, as an effective tool for resolving electron transfer processes to and within proteins. Cytochrome c, a well-characterized electron-mediating protein, was the first to be... 

The variety of electron transfer reactions described above for PQ2+ and PQ + obviously provide some (model compound) information related to the possible mechanism of action of PQ2+ in herbicidal applications. An even bigger and more applicable range of electron transfer processes involving PQ2+ and PQ + is provided by studies of the action of paraquat as a mediator (i.e. electron-transfer bridge), or terminal electron acceptor, in redox processes involving biologically active compounds (see for example Steckham and Kuwana, 1974 Krasnovsky, 1972). Whilst such studies properly reside outside the scope of this review, there arc two aspects of bipyridylium ion... 

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