Standard reduction potential biochemical

Furtmuller PG, Amhold J, Jantschko W et al (2005) Standard reduction potentials of all couples of the peroxidase cycle of lactoperoxidase. J Inorg Biochem 99 1220-1229... 

Standard reduction potentials for reactions in biochemical systems are normally determined at pH = 7. From the discussion above, it can be seen that the corresponding potentials for the hydrogen and oxygen reduction reactions at this pH value are... 

This is arbitrarily assigned a standard reduction potential Eo= 0.0 V. At the biochemical standard state of pH 7, the hydrogen half-cell has an Eq = —0.421 V. 

Table 18.6 lists formal potentials for common protein electron transfer reactions in biologically related systems. Table 18.7 lists standard reduction potentials for biochemical reduction reactions. 

Many complex biochemical reactions are electron-transfer processes, and as such have a standard reduction potential. For example, nicotinamide adenine dinucleotide (NAD" ") accepts a proton and two electrons to become NADH ... 

When a biochemical half-reaction involves the production or consumption of hydrogen ions, the electrode potential depends on the pH. When reactants are weak acids or bases, the pH dependence may be complicated, but this dependence can be calculated if the pKs of both the oxidized and reduced reactants are known. Standard apparent reduction potentials E ° have been determined for a number of oxidation-reduction reactions of biochemical interest at various pH values, but the E ° values for many more biochemical reactions can be calculated from ArG ° values of reactants from the measured apparent equilibrium constants K. Some biochemical redox reactions can be studied potentiometrically, but often reversibility cannot be obtained. Therefore a great deal of the information on reduction potentials in this chapter has come from measurements of apparent equilibrium constants. 

Since tables of standard apparent reduction potentials and standard transformed Gibbs energies of formation contain the same basic information, there is a question as to whether this chapter is really needed. However, the consideration of standard apparent reduction potentials provides a more global view of the driving forces in redox reactions. There are two contributions to the apparent equilibrium constant for a biochemical redox reaction, namely the standard apparent reduction potentials of the two half-reactions. Therefore it is of interest to compare the standard apparent reduction potentials of various half reactions. 

An enzyme-catalyzed redox reaction can be divided into two half-reactions, one producing electrons and the other consuming electrons. The standard apparent reduction potentials Fr° and E ° for the two half-reactions in an enzyme-catalyzed redox reaction at a specified pH and ionic strength determine E ° for the overall reaction, which is positive for a reaction that can occur spontaneously. A biochemical redox reaction at a specified pH can be represented schematically by... 

Figure 9.2 The pH dependence of the standard apparent reduction potentials at 298.15 K and 0.25 M ionic strength of the following biochemical half reactions, starting at the top of the ordinate [With permission from R. A. Alberty, Biophys. Chem. 389, 94-109 (2001). Copyright Academic Press.] ...

Module[ energy),( Calculates the standard apparent reduction potential of a half reaction at specified pHs and ionic strengths for a biochemical half reaction typed in the form nadox+de==nadred. The names of the reactants call the corresponding functions of pH and ionic strength, nu is the number of electrons involved. pHlist and islist can be lists. ) energy = Solve[eq, de] ... 

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). 

Apparent equilibrium constants for biochemical reactions can readily be calculated from standard apparent reduction potentials as illustrated here for... 

R. A. Alberty, Calculation of standard transformed formation properties of biochemical reactants and standard apparent reduction potentials, Arch. Biochem. Biophys. 358, 25-39 (1998). 

R. A. Alberty, Standard apparent reduction potentials of biochemical half reactions and thermodynamic data on the species involved, Biophys. Chem., 111,115-122 (2004). 

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