Uncoupler, phosphorylation

It was also observed in earlier studies that mitochondria not only accumulate Ca2+ as an alternative to phosphorylation of ADP (Ca2+ uptake uncouples phosphorylation from electron transport), but could also accumulate much larger amounts of Ca2+ if phosphate was also taken up, resulting in precipitation of Ca2+ within the matrix as insoluble hydroxyapatite, visible as electron-dense granules by EM. An unusual feature of these hydroxyapatite deposits is that they fail to become crystalline and remain amorphous even over protracted periods of time. Their presence in mitochondria in a number of disease conditions underlines the role for mitochondria as a sort of safety device, which can enable the cell to survive, if only for a limited period of time, situations of cytoplasmic Ca2+ overload. 

As a consequence, in the presence of arsenate oxidation of 3-phosphoglyceraldehyde continues but ATP synthesis ceases. Arsenate is said to uncouple phosphorylation from oxidation. Arsenate can also partially replace phosphate in stimulating the respiration of mitochondria and is an uncoupler of oxidative phosphorylation (Chapter 18). Enzymes that normally act on a phosphorylated substrate will usually catalyze a slow reaction of the corresponding unphosphorylated substrate in the presence of arsenate. Apparently, the arsenate ester of the substrate forms transiently on the enzyme surface, permitting the reaction to occur. 

Uncoupler. A substance that uncouples phosphorylation of ADP from electron transfer for example, 2,4-dinitrophenol. 

In vitro,d an uncoupled phosphorylation of fructose-6-phosphate to form the fructose-1,6-bisphosphate has a positive ArG° but in vivo, it is coupled with the ATP-ADP hydrolysis through the enzyme, phosphofructokinase, to yield an overall negative reaction free energy. 

This transfer of electrons from cytochrome a to cytochrome aa would be the step in this pathway that is inhibited by cyanide. It should be kept in mind that Dicumarol not only uncouples phosphorylation and oxidation by acting in the electron transport chain at the site of phosphorylation corresponding to the position of vitamin K in the electron transport chain, but Dicumarol is also active at the site of phosphorylation at the level of cytochrome oxidase. 

The most potent of the triterpene acids were found to be glyc5rrhetinic acid which approached the activity shown by 2,4-dinitrophenol, polyporenic acid A and fusidic acid (LX) which exceeded the activity shown by sahcylic acid. It is considered [6] that all three acids may uncouple phosphorylation by interaction with key lysyl amino-groups which participate in mitochondrial phosphorylation. 

Substances such as 2,4-dinitrophenol, azide, gramicidin, and arsenite, which uncouple phosphorylation from oxidation, inhibit the oxidation of fatty acidsbut not of the primer itself. Lehninger and his co-workers showed that the oxidation of the reduced form of the codehydrogenase, DPNH or reduced DPN, caused oxidative phosphorylation. Thus, the oxidation of a tricarboxylic acid intermediate was not obligatory for fatty acid activation. As might be expected, therefore, fatty acid breakdown was initiated by the oxidation of DPNH in the absence of tricarboxlic acid cycle intermediates. The absence of tricarboxylic acid cycle inter-... 

Evidence consistent with a mechanism of this kind has come from experiments with chloroplasts and mitochondria in which it has been shown that compounds which uncouple phosphorylation from electron transport often also result in the dissipation of the proton gradient (or of gradients derived from the proton gradient which for... 

Sulflutamid or A/-ethylpetfluotoctanesulfonamide [4151 -50-2] CgF yS02NHC2H, is a slow-acting stomach poison used in baits for the control of ants and cockroaches. It acts as an uncoupler of oxidative phosphorylation. 

The modes of action for niclosamide are interference with respiration and blockade of glucose uptake. It uncouples oxidative phosphorylation in both mammalian and taenioid mitochondria (22,23), inhibiting the anaerobic incorporation of inorganic phosphate into adenosine triphosphate (ATP). Tapeworms are very sensitive to niclosamide because they depend on the anaerobic metaboHsm of carbohydrates as their major source of energy. Niclosamide has selective toxicity for the parasites as compared with the host because Httle niclosamide is absorbed from the gastrointestinal tract. Adverse effects are uncommon, except for occasional gastrointestinal upset. 

Phosphorylation is a common method of regulation. As described above, SH2 domains bind to phosphorylated tyrosine residues. Conversely, phosphorylation of serines and threonines proximal to SH3 and PDZ domains uncouples them from their target motifs. Therefore modulation of protein kinase activity in cells regulates interactions between adaptor proteins and their target proteins. 

Many inhibitors of substrate oxidations, substrate transport, electron transport, and ATP synthesis are known including many well-known toxins (see Sherratt, 1981 Harold, 1986 Nicholls and Ferguson, 1992). These are not discussed here except to mention specific uncouplers of oxidative phosphorylation. Classic uncouplers such as 2,4-dinitrophenol have protonated and unprotonated forms, both of which are lipid soluble and cross the inner mitochondrial membrane discharging the proton gradient. This prevents ATP synthesis and stimulates respiration. 

Clofibrate causes a necrotizing myopathy, particularly in patients with renal failure, nephrotic syndrome or hypothyroidism. The myopathy is painful and myokymia of unknown origin is sometimes present. The mechanism of damage is not known, but p-chlorophenol is a major metabolite of clofibrate and p-chlorophe-nol is a particularly potent uncoupler of cellular oxidative phosphorylation and disrupts the fluidity of lipid membranes. Muscle damage is repaired rapidly on the cessation of treatment. 

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