Electron transfer biological

Biological systems convert enormous amounts of energy-rich products (food) and oxygen to carbon dioxide. The chemical reactions have to take place at a low temperature between 10°C and 40°C without destroying the reaction vessel . Biological processes are controlled in the sense that oxygen is not allowed to perform oxidation until the final step at a metal complex called Cug, which is part of a large membrane protein called 

FIGURE 11.11 (See color insert) FeS centers and their connection to cystein (Cys) peptides in the protein, with one (a), two (b), and four (c) iron atoms. 

Light excitation of the bacterial reaction centre triggers a series of electron transfer reactions that, due to variations in distance between the cofac- 

As in any other chemical reaction, the equilibrium cofiguration of the nuclei before electron transfer is generally not the same as that after electron transfer. Consequently, upon electron transfer the system must convert from a nuclear configuration that is in equilibrium with the reactant elec- 

Applying Fermiis golden rule, the rate of the electron transfer reaction is determined by the product of the probability of the nuclear transition occurring (the Franck-Condon term, FC)) and the probability of the electron tunnelling occurring  

In the classical limit when the thermal energy K T is much higher than the energy ha of the vibrational frequencies that are coupled to the electron transfer reaction, the Franck-Condon factor can be expressed in terms of AG and X and equation 2 converts to the classical Marcus formula for the electron transfer rate  

FIGURE 6. Relationship between the rate of electron transfer, AG and X. The rate is maximal when nAG = X. 

Two factors appear important in establishing sequential selective electron transfer in biological chains of this type. One is the arrangement of free-energy differences AG between chain components, so that the flow of electrons is thermodynamically downhill (Fig. 9.3). The second factor involves the use of distance and the effect of distance on the rate of electron transfer in providing selectivity. This latter factor has attracted much attention, and there have been many studies of the effect of distance on the rate of electron transfer reactions in both biological systems and model systems. We can write a general expression based on the Marcus treatment for the rate of electron transfer  

FIGURE 9.1. Sequence of components of the mitochondrial electron transport chain. NADH-Q reductase spans the mitochondrial membrane with the Q site within the membrane and the NADH site on the matrix side of the membrane. Succinate-Q reductase has a Q site within the membrane and a succinate site on the matrix side of the membrane. Cytochrome c reductase is a membrane-bound enzyme with cytochrome c on the cystolic side of the membrane and cytochrome b in the membrane. Cytochrome c is soluble and found on the cystolic side of the membrane, while cytochrome oxidase translocates protons or electrons across the membrane. (Adapted from Ref. 3.) 

FIGURE 9.2. Example of a bacterial electron transport chain Postulated mechanism for proton translocation in Escherichia coli during aerobic respiration. (Adapted from Ref. 3.) 

Various model systems were used to investigate the dependence of electron transfer rate on distance and except for some minor disagreements in detail, the broad picture that emerges for biological systems shows that Xq falls exponentially as the distance increases and P is of the order of 12-14 nm . Consequently electron transfer is possible only over distances less than about 2 nm. 

When considering long-range electron transfer in proteins, there has been some discussion of the influence of the intervening medium and in particular of whether the electron passes along a particular path or makes significant use of particular protein side chains. For instance it was postulated that aromatic side chains of proteins play an important role. However a recent study by Moser et of electron transfer in the bacterial photoreaction center reveals no such effects in that case. 

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DeRege P J F, Williams S A and Therien M J 1995 Direct evaluation of electronic coupling mediated by hydrogen bonds—implications for biological electron transfer Sc/e/ ce 269 1409-13... 

Moser, C.C., Dutton, P.L. Biological electron transfer measurement, mechanism, engineering requirements. In Quantum mechanical simulation methods for studying biological systems, D. Bicout and M. Field, eds. Springer, Berlin (1996) 201-214. 

Factors that enhance tunnelling are a small particle mass and a narrow potential energy barrier. In biology, electron transfer is known to occur over large distances (up to about 25 X 10 m). Given the mass of protium is 1840 times that of the electron, the same probability for protium... 

Electron transfer from a macrocycle (82), based on cyclen, complex to coordinated riboflavin proceeds via an inner sphere electron transfer pathway. The riboflavin coordinates through the imide and the relevance to the interception of biological electron transfer pathways is discussed.709... 

Towards understanding biological electron transfer processes many researchers have reported the synthesis of dendrimers with electroactive cores (e.g., porphyrin). Dendrimers with organic dendrons attached tetrahedrally around an inorganic, electroactive iron-sulfur core were reported by Gorman and coworkers [109]. These are the first examples of dendrimers with a hybrid... 

De Redge PJF, Williams SA, Therien MJ (1995) Direct evaluation of electronic coupling mediated by hydrogen bonds implications for biological electron transfer. Science 269 1409-1413... 

The second type of biological electron transfer involves a variety of small molecules, both organic and inorganic. Examples of these are (a) nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) as two electron carriers and (b) quinones and flavin mononucleotide (FMN), which may transfer one or two electrons. The structure of NAD and its reduced counterpart NADH are shown in Figure 1.12. 

P. J. F. de Rege, S. A. Williams, M. J. Therien, Direct Evaluation of Electronic Coupling Mediated by Hydrogen Bonds - Implications for Biological Electron Transfer , Science 1995, 269,1409-1413. 

Porphyrins constitute an important class of electron-transfer ligands. In nature, the biological electron-transfer conversion of light into chemical energy, known as photosynthesis , which takes place either in green plants or photosynthetic bacteria, is primarily driven by photosynthetic... 

Blue (or type 1 ) copper proteins (or cupredoxins) are important components of biological electron transfer processes in many organisms ranging from bacteria to animals, from fungi to plants.56 They are characterized by ... 

It is evident that the preceding considerations do not apply to all biological electron transfer systems. Even in the bacterial reaction center, the transfer between the two quinones Qa Qbj which takes place over 18 A [18], is characterized in Rhodobacter sphaeroides by a large entropic contribution, which has been attributed to the high solvent exposure of Qg [126]. By using the activation energy value reported in Ref. [126], two very different X values may be deduced from Eq. (23) = 0.1 eV and Aj = 2.5 eV. The previous considerations... 

It was shown in Sect. 2 that the standard formalism appropriate for non-adiabatic electron transfer processes leads to the definition of an electronic and a nuclear factor in the rate expression. This separation into factors of quite different physical origin is conceptually very useful. As a matter of fact, it is systematically emphasized throughout this presentation to clarify the nature of the different parameters involved in biological electron transfers. It happens also to be very useful when the relation between the kinetics and the biochemical function of these processes is considered. This is illustrated below by a few examples. 

These two lines of investigation illustrate the important advances made in this field since the early theoretical interpretations of biological electron transfer processes. Hence, they allow one to envisage the understanding of these processes at the molecular level, and the synthesis of efficient model systems [86, 194]. 

It is appropriate to begin on a cautionary note with some general comments on biological electron transfer. X-ray crystal structures of a number of electron transport metalloproteins have now been determined [18], from which it is clear... 

While there is at present no full understanding as to why plastocyanin should require two sites for reaction, there is now much evidence detailing this two-site reactivity. Moreover, the recent X-ray crystal structure of ascorbate oxidase (which has 4 Cu atoms per molecule) has indicated a plastocyanin-like domain, with the two type 3 Cu s (in close proximity with the type 2 Cu) located at the remote site. Fig. 2 [5]. Since electrons are transferred, from the type 1 Cu to O2 bound at the type 3 center this structure defines two very similar through-bond routes for biological electron transfer. 

Cytochromes serve as electron donors and electron acceptors in biological electron transfer chains, and with >75,000 members (53) they provide the bulk of natural heme proteins in biology. Cytochromes may be fixed into place within an extended electron transfer chain, such as the membrane-bound 6l and 6h of the cytochrome bci complex, or may be soluble and act as mobile electron carriers between proteins, for example, cytochrome c (54). In either role, the cytochrome may be classified by the peripheral architecture of the porphyrin macrocycle. Figure 1 shows the dominant heme types in biological systems, which are hemes a, b, c, and d, with cytochomes b and c being most prevalent. The self-association of a protein with heme via two axial ligands is a... 

Proteins containing iron-sulfur clusters are ubiquitous in nature, due primarily to their involvement in biological electron transfer reactions. In addition to functioning as simple reagents for electron transfer, protein-bound iron-sulfur clusters also function in catalysis of numerous redox reactions (e.g., H2 oxidation, N2 reduction) and, in some cases, of reactions that involve the addition or elimination of water to or from specific substrates (e.g., aconitase in the tricarboxylic acid cycle) (1). 

Biological electron transfer reactions, NERNST EQUATION ELECTRON TRANSFER KINETICS HALF-REACTIONS... 

Cubane clusters in the specialized subset [S4Fe4(SR)4]z are 10s years old in Nature, as the mediators of biological electron transfer, but are only 13 years old (born 1972) in the laboratory. Comprehensive and detailed knowledge of all facets of the chemistry of these complexes is now available, mainly due to Holm and his collaborators. Complete expositions are contained in recent reviews.12-16... 

The rate of electron tunneling from TZnCCP to the ferriheme of the yeast cytochrome c was found to be roughly 10 times bigger than that observed in the case of the homologous horse cytochrome c. This difference demonstrates the fine degree of species specificity involved in biological electron transfer and must reflect subtle structural differences between horse and yeast cytochromes [70],... 

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