Reverse electron transport

Figure 18-19 The ammonia oxidation system of the bacterium Nitrosomonas. Oxidation of ammonium ion (as free NH3) according to Eq. 18-17 is catalyzed hy two enzymes. The location of ammonia monooxygenase (step a) is uncertain but hydroxylamine oxidoreductase (step b) is periplas-mic. The membrane components resemble complexes I, III, and IV of the mitochondrial respiratory chain (Fig. 18-5) and are assumed to have similar proton pumps. Solid green lines trace the flow of electrons in the energy-producing reactions. This includes flow of electrons to the ammonia monoxygenase. Complexes HI and IV pump protons out but complex I catalyzes reverse electron transport for a fraction of the electrons from hydroxylamine oxidoreductase to NAD+. Modified from Blaut and Gottschalk.315...

Figure 18-20 Electron transport system for oxidation of the nitrite ion to the nitrate ion by Nitrobacter. Only one site of proton pumping for oxidative phosphorylation is available. Generation of NADH for biosynthesis requires two stages of reverse electron transport.

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. 

Figure 23-32 Simplified diagram of cyclic electron flow in purple bacteria. Two protons from the cytoplasm bind to QB2 in the reaction center to form QH2 (ubiquinol), which diffuses into the ubiquinone pool. From there it is dehydrogenated by the cytochrome kq complex with expulsion of two protons into the periplasm. A third and possibly a fourth proton may be pumped (green arrows) across the membrane, e.g., via the Q cycle (Fig. 18-9). The protons are returned to the cytoplasm through ATP synthase with formation of ATP. Some electrons may flow to the reaction centers from such reduced substrates as S2 and some electrons may be removed to generate NADPH using reverse electron transport.345...

The pathways involved in cyclic photophosphorylation in chloroplasts are not yet established. Electrons probably flow from the Fe-S centers Fdx, Fda, or Fdb back to cytochrome b563 or to the PQ pool as is indicated by the dashed line in Fig. 23-18. Cyclic flow around PSII is also possible. The photophosphorylation of inorganic phosphate to pyrophosphate (PP ) occurs in the chromatophores (vesicles derived from fragments of infolded photosynthetic membranes) from Rho-dospirillum rubrum. The PP formed in this way may be used in a variety of energy-requiring reactions in these bacteria.399 An example is formation of NADH by reverse electron transport. 

HAO catalyzes the four-electron oxidation of hydroxylamine to nitrite. " It is present in autotrophic nitrifying bacteria, like Nitrosomonas, which are obligate chemolithotrophs that use the oxidation of ammonia as their sole energy source. For each cycle of hydroxylamine oxidation, two electrons are returned for the initial step of ammonia oxidation and the other two are either transferred to the terminal oxidase via the components of the respiratory chain, or used to generate NADH by reverse electron transport. 

C554 is proposed to bind, and may thus be the electron exit heme. Cytochrome C554 also has two coplanar diheme pairs, which may indicate that it can also accept two electrons simultaneously. This cytochrome then transfers electrons to the membrane-bound tetraheme cytochrome Cm552 (see Section 4), which is a good candidate to reduce the membrane ubiquinone pool, from where electrons are partitioned between the ammonia monooxygenase reaction, the aerobic respiratory chain, and reverse electron transport. ... 

The overall reaction is energy yielding, and allows sufficient ATP production to support reverse electron transport for CO2 fixation. However, the first step, oxidation of NH3 to hydroxylamine, requires the input of reducing power. The second step, hydroxylamine oxidation, yields four electrons. These join the electron transport chain at the level of ubiquinone, from which two are shunted back to AMO for activation of NH3. The N oxidation and electron transport pathways in Nitrosomonas are linked in the cytoplasmic membrane and periplasmic space detailed information from the N. europaea genome (Chain et al., 2003) is consistent with the previous biochemical characterizations of the system (Whittaker et al., 2000). Depending on conditions (and enhanced at low oxygen concentrations), nitric oxide (NO), nitrous oxide (N2O) and even dinitrogen gas (N2) have been reported as secondary products... 

In principle, two mechanisms of coupling can be envisaged (i) activation of CO2 occurs at the level of the substrate at the expense of ATP hydrolysis ( substrate activation ), or (ii) the redox potentials (E° ) of the electron required for CO2 reduction are pushed towards more negative values at the expense of electrochemical potentials of either or Na by the mechanism of reversed electron transport ( redox activation ). Since ATP-consuming synthetases are not involved in CO2 reduction to methylene-Fl4MPT (Reactions 1-4 of Table 2) the latter mechanism is more likely. 

PPj or ATP, when added to coupled chromatophores of R. rubrum, induces oxidation-reduction reactions which can be ascribed to energy-requiring reversed electron transport [2-4,7-12,90] (Fig. 6.3). 

The mobihties /t of the charge carriers in the materials from which OLEDs are fabricated are low in comparison to the mobilities in organic molecular crystals (see Chap. 8). Furthermore, they are very different in the different materials there are some materials in which predominantly electrons are transported (fig ft),), and some in which mainiy holes participate in the transport (/t), /tg). Hole-transport materials, e.g. the naphthyl-phenyldiamine-biphenyl derivative NPB, have a relatively low ionisahon energy and therefore form radical cations preferentially and reversibly. Electron transport materials, e.g. Alqs, have a relatively high electron affinity and thus form radical anions preferentially and reversibly. 

The transient fluorescence rise after switching off the actinic light may be due to ATP-induced back electron transport to Q or by changes in the rates of direct and reverse electron transport between the donors of Photosystem 2. The appearance of this transient in Cl-CCP treated leaves indicates the second possibility (Pig. 2b). The amplitude of transient rise in the dark enhances with the increase of Cl-CCP concentration. Illiunination of Cl-CCP treated leaves by light 1 (after switching off the actinic light) sharply accelerates the dark relaxation of variable... 

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