Protein complexes, electron transfer

In particular, the study of SRB ferredoxins enables us to survey the different properties of simple iron-sulfur proteins, including electron transfer, flexibility in coordination chemistry, and ability to undergo cluster interconversions. Most of the observations can be extrapolated to more complex situations. 

Figure 9. Behavior expected if there are two separate functional sites on the protein for electron transfer. One site (Site 1) is blocked directly by redox inactive 3 < 4 < 5 complexes, the other (Site 2) is not blocked.

Figure 8.1. Redox proteins catalyzing electron transfer from hydrogen to sulfate in Desulfovibrio. The cytoplasmic uptake of protons associated with the reduction of sulfate is not shown. Hyd, hydrogenase cyctcs, cytochrome C3 HmcA, HmcB, HmcC, HmcE, HmcF, components of the Hmc complex IM, inner membrane OM, outer membrane.

FIGURE 19-9 IMADH ubiquinone oxidoreductase (Complex I). Complex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the iron-sulfur protein N-2 in the matrix arm of the complex. Electron transfer from N-2 to ubiquinone on the membrane arm forms QH2, which diffuses into the lipid bilayer. This electron transfer also drives the expulsion from the matrix of four protons per pair of electrons. The detailed mechanism that couples electron and proton transfer in Complex I is not yet known, but probably involves a Q cycle similar to that in Complex III in which QH2 participates twice per electron pair (see Fig. 19-12). Proton flux produces an electrochemical potential across the inner mitochondrial membrane (N side negative, P side positive), which conserves some of the energy released by the electron-transfer reactions. This electrochemical potential drives ATP synthesis. 

There are many instances for azurin and plastocyanin where limiting kinetic behaviour is observed, and attributed to the formation of an adduct between the protein and the inorganic complex followed by electron transfer. Values of the association constants and of the electron-transfer rate constants may then be calculated. This situation has not been observed in the case of stellacyanin, which differs from azurin and plastocyanin in that it has an overall positive charge at pH 7 (of +7 in the case of the reduced protein). The electron-transfer rate constants are often associated with fairly large negative values for the entropy of activation (in the range -84 to -210 J K1 mol-1), which are not expected for electron transfer within a compact assembly. 

The basic mechanism of nitrogenase with the use of dithionate as an electron donor for the iron protein involves the following steps (Thomeley and Lowe, 1985 Likhtenshtein, 1988a Burgess and Lowe, 1996 Smith, 1999 Seefeldt and Dean, 1997 Rees and Howard, 2000 Syrtsova and Timofeeva, 2001) 1) reduction of Fe-protein with flavodoxin or dithionate and attachment of two ATP molecules to the protein, 2) formation of a complex between the reduced FeP with two bound ATP molecules and FeMo-protein, 3) electron transfer between the reduced [Fe4S4] cluster of FeP to the P-cluster ofFeMoP coupled to the ATP hydrolysis, 4) electron transfer from P-cluster to... 

Tian, H., White, S., Yu, L., and Yu, C. A., 1999, Evidence for the head domain movement of the rieske iron-sulfur protein in electron transfer reaction of the cytochrome bcl complex, J. Biol. Chem. 274 7146n7152. 

Fe Protein Cycle Electron Transfer from the Fe Protein to the MoFe Protein in the Nitrogenase Complex... 

As mentioned earlier, ESR spectroscopy has made unique contributions to knowledge of iron-sulphur proteins. The areas of greatest relevance to such work are the complex electron transfer chains present in the... 

Quenching of excited-state [Ru(bipy)3] by reduced blue proteins involves electron transfer from the Cu with rate constants close to the diffusion limit for electron-transfer reactions in aqueous solution. It is suggested that the excited Ru complex binds close to the copper-histidine centre, and that outer-sphere electron transfer occurs from Cu through the imidazole groups to Ru. Estimated electron-transfer distances are about 3.3 A for plastocyanin and 3.8 A for azurin, suggesting that the hydrophobic bipy ligands of Ru " penetrate the residues that isolate the Cu-His unit from the solvent. ... 

A method for the study of ET from a protein metal center to a surface ruthenium is given in Scheme IV (103). In this method, [Ru(bpy)3] " acts as an oxidant, selectively removing an electron from a surface a5Ru(II)(histidine). A Ni-RBr scavenger system [Ni(II)hexamethyl-tetraazacyclododecane and an alkyl bromide] oxidizes the [Ru(bpy)3] before it can back react with the a5Ru(III)(histidine) complex. Electron transfer from the reduced protein metal center to the oxidized ruthenium can be monitored spectroscopically. 

In the photosynthesis of green plants, photosystems I and II (PS I, PS II) contain chlorophyll a, a Mg(II)-porphyrin, as an antenna system for light absorption and energy transfer to the reaction centers of PS I and PS II. PS II consists of a dimeric chlorophyll a as reaction center, pheophytin a, a metal-free chlorophyll a as electron transfer system to PS I and - on the other side - a water-oxidizing Mn cluster. The electron connection between PS II and PS I is carried out by a cyth/f complex (heme complexes and an FeS protein). The reaction center of PS I is also a dimeric chlorophyll (perhaps together with other chlorophylls), and chlorophyll and several FeS proteins for electron transfer. 

The Fe-protein is the only known reductant for the MoFe-protein that induces the subsequent formation of ammonia, indicating that the former protein potentially triggers alterations in the latter protein upon ATP-hydrolysis. However, the detailed communication between the Fe-protein and the MoFe-protein is not fully understood. Eight repetitions of this extremely complex electron transfer process are needed per catalytic turnover (Figure 6.2), which is one reason for the slow kinetics of nitrogenase. 

In many biological systems electron transfer occurs over large distances between prosthetic groups located in membranes and proteins. The electron transfer takes place over distances of about 1 nm. Electron transfer reactions and their pathways in a metalloprotein are complex in nature. The reactive centres in the protein are surrounded by polypeptide chains which separate redox sites from each other [17]. In many proteins the prosthetic groups usually contain one or more metal ions (Fe, Cu etc) separated by polypeptide chains [18] and the reactions are usually second order. Electron transfer reactions within polypeptides are surprisingly fast even over long distances, as can be seen fom the results given in Table 6.3. 

For cytochrome c peroxidase, the reduction involves two protein-protein one-electron transfers. There have been extensive studies of the physiologically relevant reaction that provide much support for very rapid formation of a precursory 1 1 electron-transfer complex [217]. Electrostatic interactions are a major factor in this process. Examination of the structure of CcP reveals a ring of aspartate residues that are arranged so as to be complementary with the distribution of highly conserved lysines that surround the exposed heme edge region of the... 

At the protein level, the basic mechanism of nitrogenase involves (1) complex formation between the reduced Fe-protein with two bound ATP and the MoFe-protein (2) electron transfer between the two proteins coupled to the hydrolysis of ATP (3) dissociation of the Fe-protein accompanied by rereduction and exchange of ATP for ADP and (4) repetition of this cycle until sufficient numbers of electrons and protons have been accumulated so that available substrates can be reduced. Key mechanistic questions concern the chemical mechanism of the reduction of dinitrogen and other substrates, and the requirement for ATP hydrolysis in this process. 

Redox processes play an essential role in biological energy conversions. In photosynthesis and in the respiratory chain, for example, electrrMi transfer chains within and between protein complexes couple to proton or ion gradients across the lipid membrane which in turn are used to form chemical energy in the form of adenosinetriphosphate, ATP. Some of these protein complexes simple act as electron shuttles, i.e., they commute between membrane-spanning complexes transferring an electron. Others span the membrane and contain multiple redox centers with complex electron transfer mechanisms and catalytic functions. 

Alvarez-Paggi, D., Martin, D. F., DeBiase, P. M.> Hildebrandt, R, Marti, M. A., Murgida, D. H. (2010). Molecular basis of coupled protein and electron transfer dynamics of cytochrome c in biomimetic complexes. Journal of the American Chemical Society, 132, 5769. 

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