Oxidoreductase reactions

Kim JE, Wang CJ, Bollag JM (1997) Interaction of reactive and inert chemicals in the presence of oxidoreductases Reaction of the herbicide bentazon and its metabolites with humic monomers. Biodegradation 8 387-392... 

The third largest class of enzymes is the oxidoreductases, which transfer electrons. Oxidoreductase reactions are different from other reactions in that they can be divided into two or more half reactions. Usually there are only two half reactions, but the methane monooxygenase reaction can be divided into three "half reactions." Each chemical half reaction makes an independent contribution to the equilibrium constant E for a chemical redox reaction. For chemical reactions the standard reduction potentials ° can be determined for half reactions by using electrochemical cells, and these measurements have provided most of the information on standard chemical thermodynamic properties of ions. This research has been restricted to rather simple reactions for which electrode reactions are reversible on platinized platinum or other metal electrodes. 

When the pH is specified, each biochemical half reaction makes an independent contribution to the apparent equilibrium constant K for the reaction written in terms of reactants rather than species. The studies of electochemical cells have played an important role in the development of biochemical thermodynamics, as indicated by the outstanding studies by W. Mansfield Clarke (1). The main source of tables of ° values for biochemical half reactions has been those of Segel (2). Although standard apparent reduction potentials ° can be measured for some half reactions of biochemical interest, their direct determination is usually not feasible because of the lack of reversibility of the electrode reactions. However, standard apparent reduction potentials can be calculated from for oxidoreductase reactions. Goldberg and coworkers (3) have compiled and evaluated the experimental determinations of apparent equilibrium constants and standard transformed enthalpies of oxidoreductase reactions, and their tables have made it possible to calculate ° values for about 60 half reactions as functions of pH and ionic strength at 298.15 K (4-8). 

Charges are not shown on electrons because at specified pH, charges are not balanced. It is important to realize that these are not hydrated electrons in water they are electrons that are transfered and they are refered to as formal electrons. The standard transformed reaction Gibbs energy of an oxidoreductase reaction is given by... 

Figure 8.2 Standard transformed Gibbs energies of reaction in kJ mol and Ar Nh at 298.15 K and 0.25 M ionic strength for four oxidoreductase reactions.

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