Energy electron transport linked

This enzyme is commonly found in aerobic sulfur-oxidizing lithotrophs, but its importance as an alternative to a hydration-dehydrogenation electron-transport-linked energy-generating mechanism is still disputed (Kelly 1999) ... 

The two processes are electron transport and oxidative phosphorylation. NADH is reoxidised by the process of electron transport using the electron transport chain and the energy released from this process is harnessed by oxidative phosphorylation to generate ATP. We noted earlier that the two processes are intimately linked or coupled. Normally one cannot proceed without the other. 

Synthesis of ATP by mitochondria is inhibited by oligomycin, which binds to the OSCP subunit of ATP synthase. On the other hand, there are processes that require energy from electron transport and that are not inhibited by oligomycin. These energy-linked processes include the transport of many ions across the mitochondrial membrane (Section E) and reverse electron flow from succinate to NAD+ (Section C,2). Dinitrophenol and many other uncouplers block the reactions, but oligomycin has no effect. This fact can be rationalized by the Mitchell hypothesis if we assume that Ap can drive these processes. 

Figure 5.4 Energy diagram of the two linked photosynthetic systems of green plants. The vertical scale is the energy (in eV) of an electron at the various stages of electron transport. P700 (pigment 700) and P680 (pigment 680) are chlorophylls with absorption maxima at 700 nm and 680 nm, respectively...

In the overall scheme of the photosynthesis of green plants the electron transport cycle starts with the excitation of chlorophyll a in photosystem 2. The excited electron then follows a downward electron acceptor chain which eventually reaches the chlorophyll a of photosystem 1 (P700) in which it can fill the positive hole left by electronic excitation. The energy released in the electron transport chain which links photosystems 2 and 1 is used for other biochemical processes which are thereby related to photosynthesis. One of these is the process of photophosphorylation which is the production of molecules with phosphate chains used as energy transfer agents in many biochemical reactions. 

Mitochondrial phospholipids play a role in electron transport and oxidative phosphorylation, two mechanisms by which the cell accomplishes the final oxidation of the metabolites to produce energy. Phospholipids also are linked in the transport of ions, especially sodium, across membranes. 

Fio. 5.25. Energy generation linked to electron transport process... 

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. 

Oxidative phosphorylation is ATP synthesis linked to the oxidation of NADH and FADH2 by electron transport through the respiratory chain. This occurs via a mechanism originally proposed as the chemiosmotic hypothesis. Energy liberated by electron transport is used to pump H+ ions out of the mitochondrion to create an electrochemical proton (H+) gradient. The protons flow back into the mitochondrion through the ATP synthase located in the inner mitochondrial membrane, and this drives ATP synthesis. Approximately three ATP molecules are synthesized per NADH oxidized and approximately two ATPs are synthesized per FADH2 oxidized. 

Focusing on the mechanisms of action of BOA into the plant cell, Barnes et al.7 suggested that the chlorotic seedlings observed in the presence of BOA and DIBOA could be the consequence of a benzoxazinone effect on the photophosphorylation and electron transport into the plant metabolism. In this way, Niemeyer et al.28 studied the effects of BOA on energy-linked reactions in mitochondria and reported an inhibition of the electron transfer between flavin and ubiquinone in Complex I, with complete inhibition of electron transport from NADH to oxygen in SMP. They could also detect an inhibition of BOA on ATP synthesis by acting directly on the ATPase complex at the F1 moiety. 

Mitochondria from adult H. diminuta exhibit an NADH-coupled fumarate reductase (Table 5.11). This presents a potential dilemma with respect to the utilisation of intramitochondrial reducing equivalents by this worm. As reducing equivalents are generated by the malic enzyme in the form of NADP, a mechanism for the transfer of hydride ions from NADPH to NAD to produce NADH is required so that electron-transport-associated activities can proceed and terminate with the reduction of fumarate to succinate. Such a mechanism does exist in H. diminuta as there is a non-energy-linked, membrane-associated transhydrogenase (214, 217, 221, 476). This transhydrogenase, which also occurs in H. microstoma (216) and Spirometra mansonoides (220) catalyses the reaction ... 

The cytochromes are the electron carrier heme proteins occurring in the mitochondrial respiratory chain.449 There are five cytochromes linking coenzymes Q (ubiquinone) and 02 in this electron transport chain (Scheme 7). Cytochromes are also involved in energy transfer in photosynthesis. The iron atom in cytochromes cycles between the Fe11 and Fe111 states, i.e. they are one-electron carriers, in contrast to CoQ and the NADH flavins they act upon which are two-electron carriers. Thus, one molecule of reduced CoQ transfer its two high potential electrons to two molecules of cytochrome b, the next member of the electron transport chain. 

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