Terminal acceptors

In all oxygen-evolving organisms, the PS I reaction centres finally reduce a water-soluble ferredoxin. This small protein of around 10 kDa has a (2Fe-2S) cluster and a rather low midpoint reduction potential of -400 mV. Ferredoxin binds to the PS I centre after reduction it participates both in linear electron flow to NADP, via ferredoxin-NADP reductase, and in cyclic electron flow around the PS I centre. Two membrane-bound iron-sulfur centres, designated Centre A (or F, ) and Centre B (or Fg), appear to be the terminal acceptors in the reaction centre. Their mode of functioning is not clearly established and their structure is not well known, mainly because they cannot be extracted without their complete denaturation. F and Fq can be photoreduced at low temperature in cells or in purified PS I centres. Characteristic EPR spectra are thus obtained with g values of 1.86, 1.94, 2.05 for F, and 1.89, 1.92, 2.05 for F -. 

Detailed studies on and Fb have been hampered by the property that their EPR spectra are not additive. This property has been attributed to a magnetic interaction between reduced F and Fg, indicating that they are very close to each other. The values of F and Fg are -540 and -590 mV respectively in spinach PS I particles. Their values are always in that range, but their relative values vary in different plant species for example, F has a more negative than Fb in barley and in a halophilic alga. The shape and temperature dependence of the EPR spectra of F and Fb are typical of iron-sulfur proteins. They are considered to be 4Fe-4S centers, since after modification by dimethyl sulfoxide their spectrum is characteristic of 4Fe-4S centres and because their Mossbauer spectra are also in agreement with that attribution. The presence of 10-12 Fe-S pairs in each PS I centre is compatible with this assignment (for reviews, see Refs. 25 and 26). 

Flash absorption studies at room temperature have revealed a species named P-430 which behaves as the terminal acceptor of the PS I centre. Ke and coworkers have performed a large number of experiments which fit well with that view the matching kinetic behaviour of P-700 and P-430, the effect of exogenous electron carriers, and redox titrations [9]. The reduction of P-430 induces weak absorption changes with a negative peak at 430 nm Ae = 13000 cm ) compatible with 

Direct electron transfer from P-430 to exogenous acceptors (methyl or benzyl viologen, safranine T, etc.) has been demonstrated the reaction also occurs with ferredoxin [9]. Many other acceptors can accept electrons from PS I but their site of reaction is not known. Recently, methyl purple has been introduced as a specific PS I acceptor with useful spectroscopic properties [33]. 

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The cyclization reactions discussed here either involve the intramolecular reaction of a donor group D with an acceptor group A or a cyclizing dimerization of two molecules with two terminal acceptors and two donors. A polymerization reaction will always compete with cyclization. For macrolides see p. 146 and p. 319 — 329. 

As we have seen, the metabolic energy from oxidation of food materials—sugars, fats, and amino acids—is funneled into formation of reduced coenzymes (NADH) and reduced flavoproteins ([FADHg]). The electron transport chain reoxidizes the coenzymes, and channels the free energy obtained from these reactions into the synthesis of ATP. This reoxidation process involves the removal of both protons and electrons from the coenzymes. Electrons move from NADH and [FADHg] to molecular oxygen, Og, which is the terminal acceptor of electrons in the chain. The reoxidation of NADH,... 

D.R. McMillin, Purdue University In addition to the charge effects discussed by Professor Sykes, I would like to add that structural effects may help determine electron transfer reactions between biological partners. A case in point is the reaction between cytochrome C551 and azurin where, in order to explain the observed kinetics, reactive and unreactive forms of azurin have been proposed to exist in solution (JL). The two forms differ with respect to the state of protonation of histidine-35 and, it is supposed, with respect to conformation as well. In fact, the lH nmr spectra shown in the Figure provide direct evidence that the nickel(II) derivative of azurin does exist in two different conformations, which interconvert slowly on the nmr time-scale, depending on the state of protonation of the His35 residue (.2) As pointed out by Silvestrini et al., such effects could play a role in coordinating the flow of electrons and protons to the terminal acceptor in vivo. 

The manometric methods described (e.g., L7, W3) are based on the uptake of either carbon monoxide or oxygen, the former using MHb as electron acceptor in the presence of toluidine blue, the latter using the dye as the terminal acceptor in a system involving the following reactions ... 

Recent literature contains many examples of the construction of cascades [56], Usually they are made by the covalent linking of monomer dyes, which allows strict control of their stoichiometry. The pyrene-Bodipy molecular dyads and triads are examples [57]. Efficient energy flow was reported in a purpose-built cascade molecule bearing three distinct chromophores attached to the terminal acceptor [58]. A combinatorial approach with the selection of the best hits can be applied using the assembly of fluorescent oligonucleotide analogs [59]. 

Harriman A, Mallon L, Ziessel R (2008) Energy flow in a purpose-built cascade molecule bearing three distinct chromophores attached to the terminal acceptor. Chemistry 14 11461-73... 

Electron-Transport Pathway Linked to Use of Nitrate, DMSO, and Thiosulfate as Terminal Acceptors... 

Reference has been made in Section 62.1.12 to the cytochromes in E. coli which are believed to function in electron transfer to dioxygen as the terminal acceptor, namely cytochromes o, d and a,. In addition, a number of other oxidants may be linked to the E. coli respiratory chain. 

Demonstrating that a redox transformation of a contaminant involves mediated electron transfer requires meeting several criteria (i) the overall reaction must be energetically favorable, (ii) the mediator must have a reduction potential that lies between the bulk donor and the terminal acceptor so that both steps in the electron transfer chain will be energetically favorable, and (iii) both steps in the mediated reaction must be kinetically fast relative to the direct reaction between bulk donor and terminal acceptor. Most evidence for involvement of mediators in reduction of contaminants comes from studies with model systems, because natural reducing media (such as anaerobic sediments) consist of more redox couples than can be characterized readily. Although this is an active area of research, we can identify a variety of likely mediator half-reactions (see Table 16.5). 

Although dioxygen is the terminal acceptor of both electrons and protons (and H+) in the molybdenum and tungsten oxidases, 02 is not directly involved in substrate oxidation. The activation of molecular oxygen is effected by (non-Mo- and non-W-containing) prosthetic groups such as flavin-adenine dinucleo-... 

The vast majority of mitochondria use oxygen as a terminal acceptor of electrons. Along with aerobically respiring mitochondria, versatile mitochondria exist in which both oxygen and other oxidized compounds, e.g. fumarate and nitrate, serve as electron acceptors. Such sophisticated mitochondria were reported in several ciliates, fungi, and even lower animals (Tielens et al. 2002). The yield of ATP is, however, much lower in the cases of anaerobic respiration, as compared with 32-36 mol per mole of glucose produced by aerobic respiration (Saraste 1999). 

In animal fermentations, an organic molecule (e.g., pyruvate) serves as a terminal proton and electron acceptor, forming an organic end product (e.g., lactate). In contrast, 02 is required as a terminal acceptor for the complete oxidation of substrates such as glucose, glycogen, fatty acids, or amino acids. As discussed in chapter 3, 02 was not always available as one of the substrates for oxidative metabolism and organisms in primordial times had to rely on anaerobic metabolic processes. 

In biological photosystems (whole cells, thylakoids or isolated PSI) a simple redox catalyst transfers electrons from the terminal acceptor of PSI to oxygen, the light-transducing system being the natural photosynthetic apparatus itself. In artificial model systems, by contrast, it is a redox photocatalyst (flavin or Ru (II)-tris(2, 2 -bipyridine)) who promotes the light-driven transfer of electrons from appropriate electron donors to molecular oxygen. 

Fig. 1 Schematic drawing of hydrogen peroxide photoproduction by the biological photosynthetic apparatus with electrons either from water or from an exogenous electron donor. A redox catalyst (RC) transfers electrons from the terminal acceptor of photosystem I to molecular oxygen.

When the terminal acceptor FeS-A/B (the PsaC protein) and other small polypeptides are removed from CC I, one obtains the so-called photosystem-I core protein, [P700 Ao A, FeS-X] core Chls 2 polypeptides). This PS-I core protein may be prepared from CC I by a brief incubation in lithium dodecyl sulfate (LDS) followed byultrafiltration, or by treating cyanobacterial CC I with achaotropic agent followed... 

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