Midpoint potentials

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

Fig. 9. Discharge and charging curves for a sintered iron electrode at a constant current of 0.2 A where the apparent geometrical surface area is 36 cm and porosity is 65%. A and B represent the discharging and charging regions, respectively. Overall electrode reactions, midpoint potentials, and, in parentheses, theoretical potentials at pH 15 ate Al, n-Fe + 2 OH Fe(OH)2 + 2, 0.88 V (1.03 V) B, Fe(OH)2 FeOOH + H+ +, 0.63 V (0.72 V) C,...

MR Gunner, B Homg. Electrostatic control of midpoint potentials m the cytochrome subunit of the Rhodopseudomonas viridis reaction center. Proc Natl Acad Sci USA 88 9151-9155, 1991. 

When the fully conserved residue Thr 140, which is packed against the Pro loop, was substituted by Gly, His, or Arg in Rhodobacter capsulatus, the midpoint potential of the Rieske cluster was decreased by 50-100 mV, the cluster interacted with the quinone pool and the bci complex had 10-24% residual activity but the Rieske cluster was rapidly destroyed upon exposure to oxygen (49). In contrast, the residual activity was <5%, the cluster showed no interaction with the quinone pool, and the interaction with the inhibitor stigmatellin... 

Fig. 17. Cyclic voltammogram of the water-soluble Rieske fragment from the bci complex of Paracoccus denitrificans (ISFpd) at the nitric acid modified glassy carbon electrode. Protein concentration, 1 mg/ml in 50 mM NaCl, 10 mM MOPS, 5 mM EPPS, pH 7.3 T, 25°C scan rate, 10 mV/s. The cathodic (reducing branch, 7 < 0) and anodic (oxidizing branch, 7 > 0) peak potentisds Emd the resulting midpoint potential are indicated. SHE, standEU d hydrogen electrode.

Mossbauer spectroscopy of AvF clearly demonstrated the presence of P clusters (174). The EPR spectra of dithionite-reduced VFe proteins are complex, indicating the presence of several paramagnetic species. Avl exhibits broad EPR signals, with g values of 5.8 and 5.4 integrating to 0.9 spins per V atom, which have been assigned to transitions from the ground and first excited state of a spin S = system (175). EPR data for AcF are more complex, with g values at 5.6, 4.3, and 3.77 that appear to arise from a mixture of S = species (176). The signals were associated with a midpoint potential of... 

Redox titrations of the gray form monitored by optical spectroscopy indicate midpoint potentials of approx. +4 and -1-240 mV for centers I and II, respectively. 

Midpoint potential values are useful quantitites for defining the role of the various centers in the system. In some instances, these values have even been used to predict the location of the centers in the electron transfer chain, assuming that the potential increases along the chain from the electron donor to the electron acceptor. In several oxidoreductases, however, the measured potential of some centers was found to be clearly outside the range defined by the donor and the acceptor, which raised an intriguing question as to their function. This was observed, for instance, in the case of the [4Fe-4S] (Eni = -320 mV) center in E. coli fumarate reductase (249), the [3Fe-4S] + (Era = -30 mV) center in D. gigas hydrogenase (207), and the low-potential [4Fe-4S] + + (E, = 200 and -400 mV) centers in E. 

Midpoint potential at pH 7 vs. SCE taken from handbook of Biochemistry and Molecular Biology, p. 122, Boca Raton, Florida, CRC Press (1975)... 

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

FIG. 3 The midpoint potential, E, of procaine at 0.1 M TPenATPB (NB)-O.l M LiCl, lOmM buffer (W) interface plotted against pH. 

In other words, if we subject a homogeneous solution of A- to an electrochemical potential E, then the amplitude of the EPR spectrum from this (possibly frozen) solution will be given by Equation 13.12. If we make samples for several different values of E, then their collective EPR amplitudes make a graph of /red versus E that will define the value of the unknown E°, the standard reduction potential (biochemists call this the midpoint potential) of the XieA/Xox couple. 

Dutton, P.L. 1978. Redox potentiometry determination of midpoint potentials of oxidation-reduction components of biological electro-transfer systems. Methods in Enzymology 54 411 135. 

The so-called midpoint potential, Em, of protein-bound [Fe-S] clusters controls both the kinetics and thermodynamics of their reactions. Em may depend on the protein chain s polarity in the vicinity of the metal-sulfur cluster and also upon the bulk solvent accessibility at the site. It is known that nucleotide binding to nitrogenase s Fe-protein, for instance, results in a lowering of the redox potential of its [4Fe-4S] cluster by over 100 mV. This is thought to be essential for electron transfer to MoFe-protein for substrate reduction.11 3... 

The cytochrome (by virtue of its ability to accept and donate electrons during its function in electron transport) can exist in either the oxidised or the reduced state. In reduced-minus-oxidised difference spectra, it has absorption maxima at 426, 530 and 558 nm, typical of many b-type cytochromes. The ease with which the cytochrome can accept and donate electrons is expressed by its redox (reduction-oxidation) potential, which is measured in millivolts. Unlike most mammalian b cytochromes, which have much higher midpoint potentials, that of the cytochrome of the NADPH oxidase is -245 mV. Be-... 

Flavoprotein dehydrogenases usually accept electrons from reduced pyridine nucleotides and donate them to a suitable electron acceptor. The oxidation-reduction midpoint potential of the FAD of the oxidase has been determined by ESR spectroscopy and shown to be -280 mV. The NADP+/ NADPH redox potential is -320 mV and that of the cytochrome b is -245 mV hence, the flavin is thermodynamically capable of accepting electrons from NADPH and transferring them to cytochrome b. As two electrons are transferred from NADPH, although O2 reduction requires only one electron, the scheme of electron transfer shown in Figure 5.8 has been proposed by Cross and Jones (1991). 

Figure 5.8. Proposed scheme of electron transport in neutrophils. In this scheme, electrons are transferred from NADPH — FAD—> cytochrome b —> 02. The values shown in the boxes are the redox midpoint potentials for each of the constituents of the pathway. NADPH donates two electrons upon oxidation, but single electrons are transferred from the flavin to the cytochrome b via the formation of FADH-. Source Redrawn from Cross and Jones (1991).

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