Cytochromes, electron transport coupled

In order to relate structure and function at a more direct level, it is necessary to focus on systems which have better characterized structure than the complex membrane bound proteins like cyt c oxidase. One particularly useful paradigm in this context is the cytochrome c-cytochrome c peroxidase couple [18]. Cep is not involved in electron transport, per se its apparent function [19] is detoxification of hydrogen peroxide via the sequence H2O2 -I- cep Fe(III) -> H2O -I- cep Fe(IV) O (protein) compound ES ... 

A second example of a membrane-bound arsenate reductase was isolated from Sulfurospirillum barnesii and was determined to be a aiPiyi-heterotrimic enzyme complex (Newman et al. 1998). The enzyme has a composite molecular mass of 100kDa, and a-, P-, and y-subunits have masses of 65, 31, and 22, respectively. This enzyme couples the reduction of As(V) to As(III) by oxidation of methyl viologen, with an apparent Kra of 0.2 mM. Preliminary compositional analysis suggests that iron-sulfur and molybdenum prosthetic groups are present. Associated with the membrane of S. barnesii is a h-type cytochrome, and the arsenate reductase is proposed to be linked to the electron-transport system of the plasma membrane. 

Most compounds oxidized by the electron transport chain donate hydrogen to NAD+, and then NADH is reoxidized in a reaction coupled to reduction of a flavoprotein. During this transformation, sufficient energy is released to enable synthesis of ATP from ADP. The reduced flavoprotein is reoxidized via reduction of coenzyme Q subsequent redox reactions then involve cytochromes and electron transfer processes rather than hydrogen transfer. In two of these cytochrome redox reactions, there is sufficient energy release to allow ATP synthesis. In... 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

During the 1940s, when it had become clear that formation of ATP in mitochondria was coupled to electron transport, the first attempts to pick the system apart and understand the molecular mechanism began. This effort led to the identification and at least partial characterization of several flavoproteins, iron-sulfur centers, ubiquinones, and cytochromes, most of which have been described in Chapters 15 and 16. It also led to the picture of mitochondrial electron transport shown in Fig. 10-5 and which has been drawn in a modem form in Fig. 18-5. 

However, if the electron transport between 3-hydroxybutyrate and cytochrome b562 is tightly coupled to the synthesis of one molecule of ATP, the observed potential of the carrier will be determined not only by the imposed potential E of the equilibrating system but also by the phosphorylation state ratio of the adenylate system (Eq. 18-7). Here AG atp is the group transfer potential (-AG of hydrolysis) of ATP at pH 7 (Table 6-6), and n is the number of electrons passing through the chain required to synthesize one ATP. In the upper part of the equation n is the number of electrons required to reduce the carrier, namely one in the case of cytochrome b562. 

ATP is coupled to the electron transport to cytochrome c. Thus, we have experimental evidence that when one-electron carriers such as the cytochromes are involved, the passage of two electrons is required to synthesize one molecule of ATP. Furthermore, from experiments of this type it was concluded that the sites of phosphorylation were localized in or related to complexes I, III, and IV. 

Two cytochromes show exceptional behavior and appear twice in Table 18-7. The midpoint potential E° of cytochrome b566 (bL) changes from -0.030 V in the absence of ATP to +0.245 V in the presence of a high concentration of ATP. On the other hand, E° for cytochrome a3 drops from +0.385 to 0.155 V in the presence of ATP. These shifts in potential must be related to the coupling of electron transport to phosphorylation. 

V). The centers resemble PSII of chloroplasts and have a high midpoint electrode potential E° of 0.46 V. The initial electron acceptor is the Mg2+-free bacteriopheophytin (see Fig. 23-20) whose midpoint potential is -0.7 V. Electrons flow from reduced bacteriopheophytin to menaquinone or ubiquinone or both via a cytochrome bct complex, similar to that of mitochondria, then back to the reaction center P870. This is primarily a cyclic process coupled to ATP synthesis. Needed reducing equivalents can be formed by ATP-driven reverse electron transport involving electrons removed from succinate. Similarly, the purple sulfur bacteria can use electrons from H2S. 

Approximately 2.5 molecules of ADP can be phosphorylated to ATP for each pair of electrons that traverse the electron-transport chain from NADH to 02. About 1.5 molecules of ATP are formed for a pair of electrons that enter the chain via succinate dehydrogenase or other flavoproteins such as glycerol-3-phosphate dehydrogenase. Approximately one molecule of ATP is formed for each pair of electrons that enters via cytochrome c. Electron flow through each of complexes I, III, and IV thus is coupled to phosphorylation. 

Generally, the assimilatory nitrate and nitrite reductases are soluble enzymes that utilize reduced pyridine nucleotides or reduced ferrodoxin. In contrast, the dissimilatory nitrate reductases are membrane-bound terminal electron acceptors that are tightly linked to cytochrome by pigments. Such complexes allow one or more sites of energy conservation (ATP generation) coupled with electron transport. 

Electron transport through oxidases in the plasma membrane contributes to, or controls, part of the proton release from the cell. The details of oxidase function and the mechanism of control remain to be elucidated. The NADPH oxidase of neutrophils is a special case in which proton transport is coupled to the cytochrome >557 electron carrier. This type of proton transport has its precedents in the well-characterized proton pumping through electron carriers in mitochondrial and chloroplast membranes and prokaryotic plasma membranes. 

Figure 2 The dissimilatory denitrification pathway. NO3 is reduced to NO2 by a membrane-bound or periplasmic nitrate reductase(NaR). N02 is reduced to NO by either a cytochrome cdi or copper nitrite reductase (NiR). NO is reduced to N2O by nitric oxide reductase (NOR). N2O is reduced to N2 by nitrous oxide reductase (N2OR). Electron transport from uhiquinol (UQH2) at NaR and the cyt hcj complex is coupled to generation of a proton gradient...

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