Redox coupling biological electron transfer

Major advances have been made in recent years in the field of redox enzy-mology. In part this has been attributable to the wealth of structural information acquired for redox systems, principally by X-ray methods. The successful application of electron transfer theory to redox proteins has also put the study of biological electron transfer onto a sound theoretical platform. Coupled with the ability to interrogate mechanism by site-directed mutagenesis, spectroscopic and transient kinetic methods these developments have contributed to the major expansion seen in recent years of research activity in the field of biological electron and radical transfer. 

Ferredoxins and Rieske proteins employ a (Fe )2/Fe Fe redox couple for biological electron transfer reactions. Within the protein, the two iron atoms are rendered inequivalent, even in the hilly oxidized (Fe )2 state, by the surrounding protein environment Within a synthetic cluster, however, both iron atoms are typically equivalent, as may be expected from the symmetry of the overall complex. Table 4 shows reduction potentials for selected analog clusters. 

The prerequisites for a DET can be derived from Marcus Theory [27,28]. The highly specific and directional protein-mediated electron transfer in biological systems is governed by factors such as the distance and the bonds between the redox centres, the redox-potential difference between donor and acceptor, an appropriate association of the redox couple and protein-structure dynamics coupled with electron transfer [24,27,29]. 

Along this line, the combination of electrochemistry and IR spectroscopy has been extremely successful in the identification and unraveling of molecular mechanisms of biological electron transfer and of catalysis coupled to redox reactions. 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

Heterogeneous electron reactions at liquid liquid interfaces occur in many chemical and biological systems. The interfaces between two immiscible solutions in water-nitrobenzene and water 1,2-dichloroethane are broadly used for modeling studies of kinetics of electron transfer between redox couples present in both media. The basic scheme of such a reaction is... 

Pteridone redox couples (226-225 and 229-228) have been used as mediators in electron-transfer reactions to large biological molecules.363... 

Tris(l,2-bis(dimethylphosphino)ethane)rhenium(I), [Re(DMPE)3]+ is a simple, symmetrical cation which contains three identical bidentate phosphine ligands. This complex provides a Re(II/I) redox couple with properties that are very convenient for the study of outer-sphere electron transfer reactions.1 Specifically, this couple is stable in both alkaline and acidic media and it exhibits a reversible, one-equivalent redox potential in an accessible region [ °,(II/I) = 565 mV vs. NHE]. Moreover, this complex has been used to obtain information about the biological mechanism of action of 186Re and l88Re radiopharmaceuticals.2,3... 

Electron-transfer in biological systems takes place through the mediation of a number of proteins, which contain a variety of active sites such as heme, Fe—S, Cu, and flavin. These active sites are protected from the solvent by a hydrophobic environment created by the peptide chain 48). The redox potential of a biological redox couple in vivo lies, for the most part, between —0.5 and +0.85 V. The former and latter potentials correspond to the redox potentials of H20/H2 and H20/02 respectively 49). 

The theory of electron transfer in chemical and biological systems has been discussed by Marcus and many other workers 74 84). Recently, Larson 8l) has discussed the theory of electron transfer in protein and polymer-metal complex structures on the basis of a model first proposed by Marcus. In biological systems, electrons are mediated between redox centers over large distances (1.5 to 3.0 nm). Under non-adiabatic conditions, as the two energy surfaces have little interaction (Fig. 5), the electron transfer reaction does not occur. If there is weak interaction between the two surfaces, a, and a2, the system tends to split into two continuous energy surfaces, A3 and A2, with a small gap A which corresponds to the electronic coupling matrix element. Under such conditions, electron transfer from reductant to oxidant may occur, with the probability (x) given by Eq. (10),... 

These corrected values for the pKA of HNO (>11) and reduction potential of NO (< —0.7 V) demonstrate that HNO, rather than NO, is the predominant species in neutral solution and indicate that NO cannot be easily converted to NO- by simple outer-sphere electron transfer (Scheme 6), unlike the O2/O2 redox couple. The different potentials and concentrations of NO and O2 in cellular or physiological systems suggest that NO is essentially inert to reduction to NO in mammalian biology. Note that certain processes in bacteria are suggested to have sufficient potentials to reduce NO (165, 166), which may have some importance both to normal bacterial physiology, including nitrification and denitrification, and to antibacterial and pathogenic responses. 

Fig. 14.31. Logarithm of the anodic current density-potential curves of the Q/QH2 redox couple on gold electrode covered by (x) three layers of DPPC + gramicidin,(a) five layers of DPPC + gramicidin. (Reprinted from A. Rejou-Michel, M. A. Habib, and J. O M. Bockris, Electron Transfer at Biological Interfaces, in Electrical Double Layers in Biology, M. Blank, ed., Fig. 9, p. 175, Plenum, 1986.)...

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