Sequence of electron acceptors

In the photosynthetic reactions, the primary electron donor P-700 becomes excited to its lowest excited singlet state and reacts by transferring an electron to the primary electron acceptor. The electron is then further transferred among a set of electron carriers arranged in order of increasing redox potentials (Fig. 2). This set of molecules is often viewed as a linear chain, a view which may not be the case in PS 1. A photochemical description of these events would follow the electron path from the first (more primary) acceptor to more remote (secondary) acceptors. This is not possible because of the uncertainties concerning the early acceptors. We shall thus describe the more remote acceptors first and then move closer to the primary photoreaction. 

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Organisms will utilize compounds which provide the most energy under any set of conditions. This prnciple establishes the sequence of electron acceptors in redox processes... 

The sequence of electron-acceptor utilization can be spatially observed in horizontal layers of increasing depth in aquatic water columns and sediments. In a typical littoral marine sediment, only the first few millimeters of the sediment are oxygenated and nitrate serves as the electron acceptor. Below this, for several meters, sulfate is the principal electron acceptor. Methanogenesis is usually confined to the sulfate-depleted deeper sediment layers methane... 

Figure 1 Vertical biogeochemical zones in sediments. The top is the sediment-water interface. Processes on the left represent the use of various electron acceptors (respirations) during the degradation of organic matter. Plots on the right represent the chemical profiles most widely used to delineate the vertical extent of each zone. Rotating the figure 90° to the left shows the sequence of electron acceptors used over time (x-axis) if a sample of oxic sediment were enclosed and allowed to become anaerobic over time.

Fig. 1. Location of the intermediary electron acceptor FeS-X (a [4Fe 4S] cluster) in the reaction center of photosystem I (A) and in the sequence of electron acceptors (with the years of their discovery shown) (B).

The depth sequence of electron acceptors in marine sediments, from oxygen to sulfate, is accompanied by a decrease in the degradability of the organic material remaining at that depth. This... 

Other examples of the use of electron acceptors whose ion radicals are unstable with respect to fragmentation to an anion and a radical capable of initiation of polymerization were provided by Eaton (63,101,102). It was shown that -nitrobenzyl halides could be used in dye-sensitized compositions of semiconductor pigments such as Ti02 and CdS to induce polymerization of vinyl monomers using visible light. The sequence of events is outlined in eqs. 46-49 and Scheme 6 ... 

As discussed above, the photosynthetic reaction center solves the problem of rapid charge recombination by spatially separating the electron and hole across the lipid bilayer. In order to achieve photoinitiated electron transfer across this large distance, the reaction center uses a multistep sequence of electron transfers through an ensemble of donor and acceptor moieties. The same strategy may be successfully employed in photosynthesis models, and has been since 1983 [42-45]. The basic idea may be illustrated by reference to a triad Dj-D2-A, where D2 represents a pigment whose excited state will act as an electron donor, Di is a secondary donor, and A is an electron acceptor. Excitation of D2 will lead to the following potential electron transfer events. 

As summarized earlier, there is consensus with regard to the sequence of electron transfer in cytochrome oxidase. The Cua center is the initial acceptor of electrons from cytochrome c (k 3 x 10 M s ). This electron transfer depends cmcially on a conserved tryptophan residne in snbnnit n ca. 5 A away from the Coa center. Then follows fast electron eqinlibration between CnA and the low-spin heme (kf 10 s Iq 5 x 10 s , kf and kr denoting the... 

While the small R-[2Fe 2S]-cluster binding domains ofthe chloroplast and mitochondrial R-ISPs are identical in 47-61 % ofthe sequences, the larger subdomain are the same in only 5-18% ofthe sequences, despite a common folding topology. The sequence ofthe large subdomain is apparently correlated with the type of electron acceptor for the Rieske protein. Since an extensive, specific contact between the Rieske protein and its electron acceptor is needed to ensure a productive docking between them, some... 

Reaction centers in photosynthetic bacteria typically contain three membrane-bound subunits (L, M, and H), and the following cofactors four bacteriochlo-rophyll (Bchl or B), two bacteriopheophytin (Bphe or 4>), two quinones (Q), and one Fe atom 28, 178). The sequence of electron transfer steps along the various cofactors has been established largely by spectroscopic methods. The primary donor, D, which initially absorbs light (creating the excited state D ) is a dimer of Bchl molecules [also designated (Bchl)2 or P]. Electron transfer proceeds from D to an intermediate acceptor (a Bphe molecule), to a primary acceptor, Qa, and finally to the secondary acceptor Qb. After these initial events, the RC... 

The theoretical concept is based on the following sequence of electron transfer steps, where C and P stand for the reaction centres of photosystem II and I, respectively, Q for plastoquinone, R for the Rieske-FeS-centre, Y for plastocyanin, and A for the terminal electron acceptor. 

Electron transport chain (1) A series of compounds that pass electrons to oxygen (the final electron acceptor). (2) A sequence of electron carriers of progressively higher reduction potential in a cell that is linked so that electrons can pass from one carrier to the next. The chain captures some of the energy released by the flow of electrons and uses it to drive the synthesis of ATP. 

Nitrate reductase enzymes which catalyse the reduction of nitrate to nitrite. All N.r. studied so far contain iron and molybdenum. In the sequence of electron transfer, molybdenum appears to be the ultimate acceptor, which then transfers electrons to nitrate during this process the molybdenum alternates between Mo(V) and Mo(VI). Dissimilatory N.r. from E. coii (also called respiratory N.r.) (EC 1.7.99.4) is a transmembrane protein, containing Mo, inorganic sulfur and nonheme iron, approximate M,... 

The enzyme is simpler than the other, iron-sulfiir cluster-containing hydro-genases discussed below. It shows no sequence homology with any other hydrogenase, or any other protein. It does not catalyze the reduction of electron acceptors, or the exchange of D2 with H2O, except in the presence of its cofactor, methyleneH4MPT. This is an extraordinary and vmprecedented reaction, the direct transfer of hydride from H2 to a carbon atom. The enzyme is extremely active, but very sensitive to O2 and light. 

The strategy devised in order to obtain the photoinduced shuttling movement of R between the two stations Ai + and A2 + is based on a four stroke synchronized sequence of electron transfer and molecular rearrangement processes, as illustrated in Figure 27(b).Light excitation of the photoactive unit P + (process 1) is followed by the transfer of an electron from this unit to Ai + (process 2) which competes with the intrinsic decay of the P + excited state (process 3). After the reduction of Ai +, with the consequent deactivation of this station, the ring moves (process 4) by 1.3 nm to encircle A2 +, a step that is in competition with the back electron transfer from Ai+ (still encircled by R) to the oxidized unit P + (process 5). Eventually, a back electron transfer from the free reduced station Ai + to the oxidized unit P + (process 6) restores the electron-acceptor power to this radical cationic station. As... 

Light-induced electron transport in bacterial photosynthetic reaction centers leads to the creation of a charge-separated state stable for milliseconds to seconds. The structures provided by X-ray crystallography (Michel et aL, 1986 Allen et al., 1988 Deisenhofer Michel, 1989 El-Kabbani et al., 1991) constitute a unique guideline to address questions on how the function may be related to the arrangement of the cofactors and of specific amino acid residues in their vicinity. The sequence of electron transfer reactions, the identity of the reaction partners, and the reaction mechanisms have been characterized from static and time-resolved absorbance measurements (for a review, see Parson Ke, 1982). Transfer of the first electron to the primary (Q ) and secondary (Qg) quinone electron acceptors has received considerable attention, since it is associated with intraprotein protolytic reactions (for a recent review, see Okamura Feher, 1992), which have a potential role in electrostatic charge stabilization. 

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