Electron-transfer systems, in biological

We saw in Case studies 4.2 and 4.3 that exergonic electron transfer processes drive the synthesis of ATP in the mitochondrion during oxidative phosphorylation. Electron transfer between protein-bound co-factors or between proteins also plays a role in other biological processes, such as photosynthesis (Section 5.11 and Case study 12.3), nitrogen fixation, the reduction of atmospheric Nj to NH3 by certain microorganisms, and the mechcuiisms of action of oxidoreductcises, which are enzymes that catalyze redox reactions. 

We begin by examining the features of a theory that describes the factors governing the rates of electron transfer. Then we discuss the theory in the light of experimental results on a variety of systems, including protein complexes. We shall see that relatively simple expressions can be used to predict the rates of electron transfer between proteins with reasonable accuracy. 

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Part A. Quantum-Mechanical Theory of Diffusion Independent Electron Transfer in Biological Systems by Ephraim Buhks (University of Delaware)... 

Although electron transfers in biological systems are generally expected to be non-adiabatic, it is possible for some intramolecular transfers to be close to the adiabatic limit, particularly in proteins where several redox centers are held in a very compact arrangement. This situation is found for example in cytochromes C3 of sulfate-reducing bacteria which contain four hemes in a 13 kDa molecule [10, 11], or in Escherichia coli sulfite reductase where the distance between the siroheme iron and the closest iron of a 4Fe-4S cluster is only 4.4 A [12]. It is interesting to note that a very fast intramolecular transfer rate of about 10 s was inferred from resonance Raman experiments performed in Desulfovibrio vulgaris Miyazaki cytochrome Cj [13]. 

In recent years, electrochemical charge transfer processes have received considerable theoretical attention at the quantum mechanical level. These quantal treatments are pivotal in understanding underlying processes of technological importance, such as electrode kinetics, electrocatalysis, corrosion, energy transduction, solar energy conversion, and electron transfer in biological systems. 

Intermolecular electron transfer plays an important role in the operation of biological systems. For example, electron transfer from one biological molecule to another is the primary act of energy conversion in the processes of respiration and photosynthesis. Despite a large number of works dedicated to the study of intermolecular electron transfer in biological systems, the mechanisms of these reactions have been insufficiently elucidated. This is due to great difficulties in the interpretation of experimental results which are in their turn explained by the very intricate structure of biological systems. 

EVOLUTION OF THE IDEAS ABOUT THE MECHANISMS OF ELECTRON TRANSFER IN BIOLOGICAL SYSTEMS... 

Chance and Williams [11] advanced a hypothesis that electron transfer in biological systems should be performed via rotation of electron carriers, cytochromes, between the donor and the acceptor. In this case, the electron transfer is ensured by successive approaches of the active centre of the cytochrome to the active centres of the donor and the acceptor of electrons. 

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). 

Characteristic features of the electron-transfer in biological systems are longdistance and directional electron transfer, and regulation of the rate of electron-transfer. 

Direct evidence for long range electron-transfer in biological systems was first observed by Gray et al.50,51) and Isied et al.481 using [Ru(NH3)5]3+ substituted metallo protein. Histidine-83 of blue copper (azurin) was labeled with Ru(III)(NH3)5 50). Flash photolysis reduction of the His-83 bound Ru(III) followed by electron-transfer from the Ru(II) to Cu2+ was observed with a rate constant of 1.9 s 1. The result shows that intramolecular long distance (approx. 1 nm) electron-transfer from the Ru(II) to the Cu2 + of the azurin takes place rapidly. 

The feasibility of intramolecular electron- and energy-transfer depends on distance and is usually studied in covalently linked systems. However, donor-acceptor dyads can be also arranged by self-assembly what resembles the situation of electron transfer in biological systems. Artificial dyads tethered by a small number of hydrogen bonds immediately dissociate in methanol or water. To improve the binding while keeping the reversibility, a photoinducible electron donor-acceptor dyad linked by a kinetically labile bond was designed. [19]... 

Conduction and Electron Transfer in Biological Systems Retrospect and Prospect... 

Electron transfer in biological systems can be thought of as a two-step process involving formation of a complex between the donor and acceptor molecule followed by an electron transfer event. The overall electron transfer rate will depend on the rate of complex formation and stability of the complex and on the intrinsic electron transfer rate within the complex. Since the flavin electron transfer systems discussed in this chapter are all stable inter- or intramolecular complexes, the discussion will be limited to considerations of a first order intra-complex electron transfer process. 

Zinth, W., Huppmann, P., Arlt, T., and Wachtveitl, J., 1998, Ultrafast spectroscopy of the electron transfer in photosynthetic reaction centres towards a better understanding of electron transfer in biological systems Phil. Trans. Roy. Soc. Land. A, 356 4659476. 

Issues related to the preferred pathways and the distance dependence of electron transfer in biological systems have been addressed by covalently linking electron-transfer donors or acceptors (e.g. a ruthenium complex) to specific sites (e.g. a histidine) of a protein or an enzyme.The distance dependence of the electron-transfer rate constants is generally fitted to equation (44), with most values of for proteins falling in the range of 1.0 to 1.3 A The protein in... 

The distances 18 A and 14 A inside the protein complex are typical of electron transfer in biological systems. The reduced Fe protein with two bound MgATP binds to the MoFe protein and a single electron is transferred from the Fe protein to the MoFe protein. This process is coupled to the hydrolysis of two ATP. 

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]. 

Sutin, N., and B. S. Brunschwig. Some aspects of electron transfer in biological systems. In M. K. Johnson et al., eds., Electron Transfer in Biology and the Solid State (Washington, D.C. American Chemical Society, 1990), pp. 65-88. 

The experiment which triggered much of the theoretical interest in electron transfer in biological systems was that of DeVault and Chance (125), who studied the temperature dependence of the cytochrome c (Cyt c) reduction of the primary donor (BchljJ (bacteriochlorophyll) in the photosynthetic bacterium Chromatium vinosum. The reaction is... 

The electron transfer model presented here recalls the process of charge transport in semiconductors that is, a conduction band is populated by a thermalized electron, which then moves freely through the semiconductor via wavelike k states. While the possibility of semiconductorlike electron transfer in biological systems was first raised many years ago by DeVault and Chance [93], it has never been found experimentally in fact, there was reasonable skepticism that nature would choose such a mechanism in natural biological systems [84]. The density matrix method allows one to construct a model in which the conditions for such a process can be clarified and investigated in a detailed way. 

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