Electron transport chain uncoupled

Oxidative Phosphorylation. Oxidative phosphorylation, that is the production of ATP during the passage of electrons down the terminal electron transport chain, may be disrupted in two distinct ways. Compounds that divorce the process of electron transport and the phosphorylation of ADP are termed uncoupling agents. They permit NADH and succinate to be oxidised via the electron transport chain without the production of ATP and are lethal. Oxidative phosphorylation may also be inhibited directly, thus preventing the oxidation of NADH and succinate. Several products are available that exploit these modes of action. Characteristically, they have wide activity spectra that span major disciplines of pesticide use. 

Depletion of ATP is caused by many toxic compounds, and this will result in a variety of biochemical changes. Although there are many ways for toxic compounds to cause a depletion of ATP in the cell, interference with mitochondrial oxidative phosphorylation is perhaps the most common. Thus, compounds, such as 2,4-dinitrophenol, which uncouple the production of ATP from the electron transport chain, will cause such an effect, but will also cause inhibition of electron transport or depletion of NADH. Excessive use of ATP or sequestration are other mechanisms, the latter being more fully described in relation to ethionine toxicity in chapter 7. Also, DNA damage, which causes the activation of poly(ADP-ribose) polymerase (PARP), may lead to ATP depletion (see below). A lack of ATP in the cell means that active transport into, out of, and within the cell is compromised or halted, with the result that the concentration of ions such as Na+, K+, and Ca2+ in particular compartments will change. Also, various synthetic biochemical processes such as protein synthesis, gluconeogenesis, and lipid synthesis will tend to be decreased. At the tissue level, this may mean that hepatocytes do not produce bile efficiently and proximal tubules do not actively reabsorb essential amino acids and glucose. 

The basic mechanism underlying the toxicity of salicylate is the uncoupling of oxidative phosphorylation. For oxidative phosphorylation to take place, there is a requirement of a charge difference between the intermembrane space and the matrix of the mitochondria (Fig. 7.60). This is achieved when electrons move down the chain of multienzyme complexes and electron carriers (the electron transport chain), causing protons to move from the mitochondrial matrix to the intermembrane space. Consequently, a pH difference builds up, which is converted into an electrical potential across the membrane of approximately 200 mV over 8 nm. 

Salicylic acid, the major metabolite of aspirin, uncouples the electron transport chain in the mitochondria. This results in (a) increased use of oxygen and production of carbon dioxide, (b) lack of ATP, and (c) excess energy no longer utilized in ATP production. The result is increased respiration and raised temperature. The alterations in respiration lead to alkalosis followed by acidosis. The lack of ATP and loss of respiratory control will cause increased metabolic activity and hypoglycemia after an initial mobilization of glucose from glycogen. 

Corrrect answer = E. When phosphorylation is partially uncoupled from electron flow, one would expect a decrease in the proton gradient across the inner mitochondrial membrane and, hence, impaired ATP synthesis. In an attempt to compensate for this delect in energy capture, metabolism and electron flow to oxygen is increased. This hypermetabolism will be accompanied by elevated body temperature because the energy in fuels is largely wasted, appearing as heat. The electron transport chain will still be inhibited by cyanide. 

Another kind of experiment is to equilibrate the electron transport chain with an external redox pair of known potential using uncoupled mitochondria. 

Uncouplers. Uncouplers dissociate electron transport from photophosphorylation. Both noncyclic and cyclic phosphorylation are inhibited, but electron transport reactions are either unaffected or stimulated. Because uncouplers relieve the inhibition of electron transport imposed by energy transfer inhibitors, they are considered to act at a site closer to the electron transport chain than the site of phosphate uptake. In Figure 2, they are shown (site 2) as dissipating some form of conserved energy represented as on the noncyclic and cyclic ATP-gener-ating pathways. Perfluidone is the only herbicide identified to date that functions as a pure uncoupler at pH 8.0 (2). Compounds that uncouple photophosphorylation also uncouple mitochondrial oxidative phosphorylation. 

Uncoupling of mitochondrial and endoplasmic reticulum electron-transport chains... 

Add the inhibitor with and without an uncoupler, and monitor the rate of O2 consumption. If the O2 consumption increases again in the presence of inhibitor and uncoupler, the inhibitor must be inhibiting ATP synthase. If the uncoupler has no effect on the inhibition, the inhibitor is inhibiting the electron-transport chain. 

D. An uncoupler dissipates the proton gradient across the inner mitochondrial membrane. Therefore, ATP is not produced, and energy is liberated as heat. The low proton gradient causes the electron transport chain to speed up, 02 consumption increases, NADH is rapidly oxidized, the TCA cycle speeds up, and C02 production increases. 

C. Malignant hyperthermia does not involve central brain control of temperature but is caused by metabolic alterations. Regardless of the source of energy—whether fatty acids or the utilization of ATP to support muscle contraction—the electron transport chain is involved. Uncoupling of oxidation from ADP phosphorylation is caused by uncoupling proteins that dissipate energy as heat. This causes the elevation of body temperature seen in this patient. 

ATP is uncoupled from electron transport. Other compounds affect mitochondrial function by inhibiting the electron transport chain at one or more specific sites, such as the toxic metabolite of MPTP which inhibits complex I (see Chapter 7). The toxic metabolite of hexachlorobutadiene is believed to be nephrotoxic due to inhibition of mitochondria function in the proximal tubular cells. 

The mechanism of action of the phenothiazines is still not definitely known. They tend to block important effector substances such as acetylcholine, epinephrine, and histamine. The phenothiazines produce uncoupling of phosphorylation from oxidation. They appear to act at all steps along the electron transport chain. Cytochrome oxidase, succinoxidase, and adenosine triphosphatase are inhibited. Some data indicate that the phenothiazines may decrease the permeability of storage granules for brain amines. 

The electrochemical gradient couples the rate of the electron transport chain to the rate of ATP synthesis. Because electron flow requires proton pumping, electron flow cannot occur faster than protons are used for ATP synthesis (coupled oxidative phosphorylation) or returned to the matrix by a mechanism that short circuits the ATP synthase pore (uncoupling). 

It was demonstrated that the rate of aerobic phosphorylation in rat liver mitochondria is depressed by dicoumarol [274]. Antibacterial activity of dicoumarol has been suggested to be related to its uncoupling characteristic [10]. This compound was reported to be a powerful uncoupler of oxidative phosphoiylation, which inhibits phosphorylation at every step in the electron transport chain [275]. 

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