Redox protein chain

Mitochondria are known as the "power plants" of aerobic cells, the primary function of which is fatty acid oxidation to CO2 and H2O, and ATP synthesis. As mentioned in the INTRODUCTION, the chemiosmotic hypothesis of Mitchell suggests that coupled electron and ion movements are crucial to redox protein chain energy transduction into ATP movement. The key questions to be answered are (i) how are electrochemical potential gradients (Ap/Ax) of protons across the biomembranes generated, (ii) how are electrons, ions and chemical species transported across the membrane, and (iii) how are such potential gradients (Ap/Ax) used to drive the synthesis of ATP ... 

Figure 2.11 Beta sheets are usuaiiy represented simply by arrows in topology diagrams that show both the direction of each (3 strand and the way the strands are connected to each other along the polypeptide chain. Such topology diagrams are here compared with more elaborate schematic diagrams for different types of (3 sheets, (a) Four strands. Antiparallel (3 sheet in one domain of the enzyme aspartate transcarbamoylase. The structure of this enzyme has been determined to 2.8 A resolution in the laboratory of William Lipscomb, Harvard University, (b) Five strands. Parallel (3 sheet in the redox protein flavodoxin, the structure of which has been determined to 1.8 A resolution in the laboratory of Martha Ludwig, University of Michigan, (c) Eight strands. Antiparallel barrel in the electron carrier plastocyanln. This Is a closed barrel where the sheet is folded such that (3 strands 2 and 8 are adjacent. The structure has been determined to 1.6 A resolution in the laboratory of Hans Freeman in Sydney, Australia. (Adapted from J. Richardson.)...

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... 

Electron transfer (ET) is a key reaction in biological processes such as photosynthesis and respiration [1], Photosynthetic and respiratory chain redox proteins contain one or more redox-active prosthetic groups, which may be metal complexes or organic species. Since it is known from crystal structure analyses that the prosthetic groups often are located in the protein interior, it is likely that ET in protein-protein complexes will occur over large molecular distances ( > 10 A) [2-4],... 

Besides the enzyme, the superoxide ion can also be an electron donor. The ion arises as a result of detoxication of xenobiotics (xenobiotics are outsiders, which are involved in the chain of metabolism). Xenobiotics yield anion-radicals by the neutralizing influence of redox proteins. Oxygen (inhaled with air) takes an unpaired electron off from a part of these anion radicals and forms the superoxide ion. The superoxide ion plays its own active role in biochemical reactions. 

The majority of the enzyme-catalyzed reactions discussed so far are oxidative ones. However, reductive electron transfer reactions take place as well. Diaphorase, xanteneoxidase, and other enzymes as well as intestinal flora, aquatic, and skin bacteria—all of them can act as electron donors. Another source of an electron is the superoxide ion. It arises after detoxification of xenobiotics, which are involved in the metabolic chain. Under the neutralizing influence of redox proteins, xenobiotics yield anion-radicals. Oxygen, which is inhaled with air, strips unpaired electrons from these anion-radicals and gives the superoxide ions (Mason and Chignell 1982). 

In principle, glucose oxidase could be oxidized directly at the electrode, which would be the ultimate electron acceptor. However, direct electron transfer between redox enzymes and electrodes is not possible because the FADH2/FAD redox centers are buried inside insulating protein chains (Heller, 1990). If it were not the case, various membrane redox enzymes with different standard potentials would equalize their potentials on contact, thus effectively shorting out the biological redox chains. The electron transfer rate is strongly dependent on the distance x between the electron donor and the electron acceptor. 

The structure and enzyme kinetics of bovine erythrocyte superoxide dismutase are reviewed. The protein has a novel imidazolate-bridged copper(II)-zinc(II) catalytic center in each of two identical subunits. Since a C /Cu1 redox couple is responsible for the dismutase activity of the enzyme, the role of zinc is of interest. Both 220-MHz NMR measurements of the exchangeable histidine protons and chemical modifications using diethylpyrocarbonate demonstrate that zinc alone can fold the protein chain in the region of the active site into a conformation resembling that of the native enzyme. Other possible roles for zinc are discussed. Synthetic, magnetic, and structural studies of soluble, imidazolate-bridged copper complexes of relevance to the 4 Cu(II) form of the enzyme have been made. 

This structural unit occurs in the one-iron proteins, rubredoxins (Rd), obtained from bacteria, in which the active site is [Fe(S-Cys)4], where S-Cys is a cysteinyl residue of the protein chain [structure (38) of Figure 6]. These proteins have molecular weights typically about 6000. The variety, of physical investigations (magnetic susceptibility,210 ESR,211 Mossbauer,212 optical absorption,207,213 MCD214) have demonstrated that the two redox states, Rdox and Rered, coupled by one-electron... 

An average of one mediator was boimd per f 2-75 kDa of enzyme. The mediators used have redox potentials 0.07 V to 0.55 V positive of the FAD/FADH enzyme s redox potential (—0.05 V relative to the NHE at pH 7), but those which have a redox potential more negative than glucose oxidase enzyme do not mediate electron transfer. Electrons can be relayed both by tunneling and by motion of the mediator in and out of the protein chains. For distances >8 A, tunneling rates decrease ex-... 

Without biological electron transfer reactions (also called reduction/oxidation or redox reactions) life would not exist. Well-organized electron transfer reactions in a series of membrane-bound redox proteins form the basis for energy conservation in photosynthesis and respiration. The basic reaction is simply the transfer of electrons from the donor to the final electron acceptor. Perhaps the best example of these redox reactions, their importance for living organisms, and the nature of the different type of biocatalysts that are involved is the respiration chain present in the membranes of mitochondria. The membrane-bound nature of this electron transport chain, supporting electron transfer from NADH to O2 as... 

The name ferredoxin was first proposed for a non haem iron-redox-protein (hence its name) isolated from Clostridium pasteurianum and presumably involved in the hydrogen gas evolution from pyruvate by this bacterium (254). The smallest of the known iron-sulfur proteins, 6,000 dalton, the clostridial ferredoxins are, however, the most complex in terms of the iron and inorganic sulfur content 8Fe 8S. They are single chain polypeptides of about 55 residues of which eight are cysteines (Fig. 18). 

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