Protons gradient

A number of substances inhibit oxidative phosphorylation at specific locations. These may be divided into agents that affect electron transport, those that affect complex V, and those that collapse proton gradients (proton ionophores). Such substances have been used as research tools to unravel the complexities of these pathways, as poisons, and as antibiotics. Inhibition of electron transport inhibits phosphorylation, the extent of which depends on the location of the inhibition site. Thus, if complex I is inactivated, electron transport can still take place using FADH2 as an electron donor. The donor P/O ratio is then 2. 

In stage 2, electrons are transported from the reduced quinone via carriers In the thylakoid membrane until they reach the ultimate electron acceptor, usually NADP", reducing It to NADPH. Electron transport is coupled to movement of protons across the membrane from the stroma to the thylakoid lumen, forming a pH gradient (proton-motive force) across the thylakoid membrane. 

The available evidence suggests that chloroplast stromal proteins, like mitochondrial matrix proteins, are imported In the unfolded state. Import into the stroma depends on ATP hydrolysis catalyzed by a stromal Hsc70 chaperone whose function is similar to Hsc70 in the mitochondrial matrix and BiP in the ER lumen. Unlike mitochondria, chloroplasts cannot generate an electrochemical gradient (proton-motive force) across their inner membrane. Thus protein Import Into the chloroplast stroma appears to be powered solely by ATP hydrolysis. 

Realize that coupling of oxidation to phosphorylation by a proton gradient (proton-motive force) forms ATP. 

Uptake and efflux of pyruvate affected by pH of medium and will occur against osmotic gradient. Proton conductive uncouplers inhibit pyruvate uptake. 

As discussed above, cationic peptides can form channels in model bilayers. Thus, it seems likely that their primary amibacterial action is to disrupt the integrity of bacterial cytoplasmic membranes. This would have the effect of permitting leakage of ions arxl small metabolites, and destroying the ability of bacteria to maintain a transmembrane proton gradient (proton-motive force) with consequent loss of ability to generate adenosine triphosphate and transport substrates (see Ref. 119 for review of cytoplasmic membranes). 

The measurements of concentration gradients at surfaces or in multilayer specimens by neutron reflectivity requires contrast in the reflectivity fiDr the neutrons. Under most circumstances this means that one of the components must be labeled. Normally this is done is by isotopic substitution of protons with deuterons. This means that reflectivity studies are usually performed on model systems that are designed to behave identically to systems of more practical interest. In a few cases, however (for organic compounds containing fluorine, for example) sufficient contrast is present without labeling. 

The gradients of H, Na, and other cations and anions established by ATPases and other energy sources can be used for secondary active transport of various substrates. The best-understood systems use Na or gradients to transport amino acids and sugars in certain cells. Many of these systems operate as symports, with the ion and the transported amino acid or sugar moving in the same direction (that is, into the cell). In antiport processes, the ion and the other transported species move in opposite directions. (For example, the anion transporter of erythrocytes is an antiport.) Proton symport proteins are used by E. coU and other bacteria to accumulate lactose, arabinose, ribose, and a variety of amino acids. E. coli also possesses Na -symport systems for melibiose as well as for glutamate and other amino acids. 

A Proton Gradient Drives die Rotation of Bacterial Flagella... 

In 1961, Peter Mitchell, a British biochemist, proposed that the energy stored in a proton gradient across the inner mitochondrial membrane by electron transport drives the synthesis of ATP in cells. The proposal became known as... 

Peter Mitchell s chemiosmotic hypothesis revolutionized our thinking about the energy coupling that drives ATP synthesis by means of an electrochemical gradient. How much energy is stored in this electrochemical gradient For the transmembrane flow of protons across the inner membrane (from inside [matrix] to outside), we could write... 

In 1961, Peter Mitchell proposed a novel coupling mechanism involving a proton gradient across the inner mitochondrial membrane. In Mitchell s chemiosmotic hypothesis, protons are driven across the membrane from the matrix to the intermembrane... 

FIGURE 21.22 The proton and electrochemical gradients existing across the inner mitochondrial membrane. The electrochemical gradient is generated by the transport of protons across the membrane. 

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