Electron carriers sequence

We now come to the subject of simplified reaction-center complexes, which have been found to be particularly valuable for the study of various components along the electron-transport chain in photosystem I. The basic strategy used for obtaining simplified PS-I reaction-center complexes was to selectively remove the components one-by-one cumulatively along the electron-carrier sequence, starting from the terminal end of the chain. 

Figure 1. Position of the primary electron acceptor Aq and the electron-carrier sequence of photosystem I.

The early pioneering work ofLichtenthaler andcoworkers established the presence as well as the specific location of phylloquinone in the thylakoid membrane. Subsequent work of Richard Malkin showed that only one ofthe two phylloquinone molecules in the reaction centers of photosystem I participates in the electron-transfer sequence. Before the various spectroscopic characterization studies described in the previous sections were undertaken, much attention was given to the establishing whether or not phylloquinone indeed serves the role of acceptor A in the PS-I electron-transfer chain. In the mid-1980s, several laboratories carried out extraction and reconstitution of phylloquinone as a means of addressing this question. The premise of all such studies may be visualized in terms of the presumed PS-I electron-carrier sequence ... 

Behavior of iron-sulfur proteins in particles after phylloquinone removal. According to the electron-carrier sequence presented above, removal of A] from the chain is expected to block forward electron transfer to succeeding carriers. This area of research, however, has been somewhat controversial. For instance, Itoh et a/initially found that, contrary to expectation, in particles whose phylloquinone was nearly quantitatively extracted, FeS-A/B could still be photochemically reduced at both 10 K and at room temperature. These authors also demonstrated photoaccumulation ofFeS-X . Meanwhile, Mansfield, Hubbard, Nugent and Evans performed ether extraction on PS-I particles prepared from pea chloro-plasts and obtained a similar correlation between phylloquinone extraction and the development of iron-sulfur-protein EPR signals, showing that the iron-sulfur proteins were retained. However, these authors found that the iron sulfur centers could only be reduced chemically, not photochemically. Furthermore, they also reported that addition of synthetic vitamin Kj to the extracted particles did not restore electron transfer to the iron-sulfur proteins. The authors explained that ether not only extracted phylloquinone and pigment molecules but also lipids, whose removal could possibly have affected the structure of the reaction center and consequently electron-transfer behavior. 

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

Although electrons move from more negative to more positive reduction potentials in the electron transport chain, it should be emphasized that the electron carriers do not operate in a simple linear sequence. This will become evident when the individual components of the electron transport chain are discussed in the following paragraphs. 

For nitrate reductase, evidence on the role of molybdenum in the catalytic mechanism of the enzyme from Neurospora was first presented in 1954 by Nicholas and Nason (21) and the position seems to have changed relatively little since then. The original conclusion (23) was that molybdenum functions as an electron carrier in the sequence ... 

The large number of cytochromes identified contain a variety of porphyrin ring systems. The classification of the cytochromes is complicated because they differ from one organism to the next the redox potential of a given cytochrome is tailored to the specific needs of the electron transfer sequences of the particular system. The cytochromes are one-electron carriers and the electron flow passes from one cytochrome type to another. The terminal member of the chain, cytochrome c oxidase, has the property of reacting directly with oxygen such that, on electron capture, water is formed ... 

Iron-sulfur proteins are a group of enzymes and other electron carriers that contain clusters of iron and sulfide linked directly to amino-acyl side chains, usually cysteines. They are widely distributed in nature. Soon after their discovery. Hall et al. (1971) proposed that they could be used to follow the course of evolution. Studies of genome sequences have revealed that iron-sulfur cluster binding motifs are among the most commonly recognized sequences. 

Figure 9.6 Sequence of electron carriers in the electron transfer chain. The positions of entry into the chain from metabolism of glucose, glutamine, fatty acyl-CoA, glycerol 3-phosphate and others that are oxidised by the Krebs cycle are shown. The chain is usually considered to start with NADH and finish with cytochrome oxidase. FMN is flavin mononucleotide FAD is flavin adenine dinucleotide.

FIGURE 19-6 Method for determining the sequence of electron carriers. This method measures the effects of inhibitors of electron transfer on the oxidation state of each carrier. In the presence of an electron donor and 02/ each inhibitor causes a characteristic pattern of oxidized/reduced carriers those before the block become reduced (blue), and those after the block become oxidized (pink). 

Reoxidation of the reduced carriers NADH and FADH2 actually involves a sequence of electron carriers, the electron transport chain, whose function is indicated below the circle near the center of Fig. 10-1. The oxidation of reduced NADH by 02 (Eq. 10-7) is a highly exergonic process and is accompanied by the... 

The development by Chance of a dual wavelength spectrophotometer permitted easy observation of the state of oxidation or reduction of a given carrier within mitochondria.60 This technique, together with the study of specific inhibitors (some of which are indicated in Fig. 18-5 and Table 18-4), allowed some electron transport sequences to be assigned. For example, blockage with rotenone and amytal prevented reduction of the cytochrome system by NADH but allowed reduction by succinate and by other substrates having their own flavoprotein components in the chain. Artificial electron acceptors, some of which are shown in Table 18-5,... 

For the sake of completeness, Figure 4-5 illustrates the more general situation of isothermal, isobaric matter transport in a multiphase system (e.g., Fe/Fe0/Fe304 / 02). A sequence of phases a, (3, y,... is bounded by two reservoirs which contain both neutral components (i) and electronic carriers (el). The boundary conditions imply that the buffered chemical potentials (u,(R)) and the electrochemical potentials (//el(R)) are predetermined in R] and Rr. Depending on the concentrations and mobilities (c/, b), c, 6 ) in the various phases v, metallic conduction, semiconduction, or ionic conduction will prevail. As long as the various phases are thermodynamically stable and no decomposition occurs, the transport equations (including the boundary conditions) are well defined and there is normally a unique solution to the transport problem. 

A Bucket Brigade of Molecules Carries Electrons from the TCA Cycle to 02 The Sequence of Electron Carriers Was Deduced from Kinetic Measurements Redox Potentials Give a Measure of Oxidizing and Reducing Strengths... 

The b cytochromes and cytochrome c, fit into this scheme between reducing substrates and cytochrome c. The idea thus developed that the respiratory apparatus includes a chain of cytochromes that operate in a defined sequence. The next question was whether the cytochromes are all bound together in a giant complex, or whether they diffuse independently in the membrane. Before we address this point, we need to consider three other types of electron carriers that participate in the electron-transport chain flavo-proteins, iron-sulfur proteins, and ubiquinone. 

The Sequence of Electron Carriers Was Deduced from Kinetic Measurements... 

Chance and others also measured the rates at which the different electron carriers became oxidized, following addition of 02 to anaerobic mitochondria, or became reduced after addition of a suitable substrate. When 02 was added, cytochrome a3 became oxidized first, followed by cytochrome a, the c cytochromes, the b cytochromes, and flavins, in that order. Later work showed that UQ became oxidized at about the same rate as the b cytochromes. Observations of this sort led to the proposal that the electron carriers are arranged in the following sequence ... 

Chem. 217 395, 1955. One of a series of papers developing kinetic techniques for elucidating the sequence of electron carriers in the respiratory chain. 

Many of the reactions of the plastocyanins and azurins with other redox proteins follow Marcus behaviour.946 These reactions all show a single mechanism of electron transfer, with no kinetic selectivity and no specific interactions between the proteins. The notable exception to this behaviour is cytochrome / (c552), where a specific interaction occurs,934 appropriate for its natural redox partner. Equation (48) represents a probable sequence of electron carriers, although it is difficult to extrapolate conclusions to the membrane-bound proteins. 

Sometimes electron transfer via the transport of electron carrier and via electron exchange reactions occur simultaneously. Co-existence of both these channels was observed for dark electron transfer across the viologen-containing vesicle membrane [169, 201]. To illustrate this let us turn back to the experiments shown schematically in Fig. 5 a. In accordance with the reaction sequence (34)-(37) and... 

Briefly, the sequence of electron carriers within the PS 1 reaction centre can be summarised as ... 

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