Membranes proton gradient

The basic idea was to monitor the loss of trans-membrane proton gradient, which happens due to Dl protein-herbicide interaction, and to use it for herbicide detection. 

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... 

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 21.31 Structures of several uiicouplers, molecules that dissipate the proton gradient across the inner mitochondrial membrane and thereby destroy the tight coupling between electron transport and the ATP synthase reaction. 

FIGURE 22.21 The mechanism of photophosphorylation. Photosynthetic electron transport establishes a proton gradient that is tapped by the CFiCFo ATP synthase to drive ATP synthesis. Critical to this mechanism is the fact that the membrane-bound components of light-induced electron transport and ATP synthesis are asymmetrical with respect to the thylakoid membrane so that vectorial discharge and uptake of ensue, generating the proton-motive force. 

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. 

In resting muscle the high concentration of ADP does not decrease the proton gradient effectively and the high membrane potential slows electron transport. ADP, formed when ATP is hydrolyzed by myosin ATPase during contraction, may stimulate electron transport. However, the concentration of ATP (largely as its Mg salt) is buffered by its readily reversible formation from creatine phosphate catalyzed in the intermembrane space, and in other cell compartments, by the various isoenzymes of creatine kinase (reviewed by Walliman et al., 1992). 

Aerobic respiration. Many organisms carry out aerobic respiration in which enzymes remove electrons from organic compounds and pass them through a chain of carriers including flavoproteins and cytochromes located in intracellular membranes (Fig. 3-4) until finally they are used to reduce oxygen to produce water. ATP is produced by an enzyme called ATPase, that is located in the cell membrane, and the process is driven by a proton gradient across the membrane. 

Fig. 3-4 Electron transport process schematic, showing coupled series of oxidation-reduction reactions that terminate with the reduction of molecular oxygen to water. The three molecules of ATP shown are generated by an enzyme called ATPase which is located in the cell membrane and forms ATP from a proton gradient created across the membrane.

Uncouplers of oxidative phosphorylation Compounds that uncouple oxidative phosphorylatiou from electron transport in the inner mitochondrial membrane. Most are weak lipophilic acids that can run down the proton gradient across this membrane. 

Figure 12-8. Principles of the chemiosmotic theory of oxidative phosphorylation. The main proton circuit is created by the coupling of oxidation in the respiratory chain to proton translocation from the inside to the outside of the membrane, driven by the respiratory chain complexes I, III, and IV, each of which acts as a protonpump. Q, ubiquinone C, cytochrome c F Fq, protein subunits which utilize energy from the proton gradient to promote phosphorylation. Uncoupling agents such as dinitrophenol allow leakage of H" across the membrane, thus collapsing the electrochemical proton gradient. Oligomycin specifically blocks conduction of H" through Fq.

Spanning the membrane are ATP synthase complexes that use the potential energy of the proton gradient to synthesize ATP from ADP and P,. In this way, oxidation is closely coupled to phosphorylation to meet the energy needs of the cell. 

Reserpine irreversibly inhibits the triphosphatase that maintains the proton gradient and so it depletes neurons of their vesicular store of transmitter. This explains why restoration of normal neuronal function rests on delivery of new vesicles from the cell bodies. Some amphetamine derivatives, including methylenedioxymethamphetamine (MDMA), are also substrates for the transporter and, as a result, competitively inhibit noradrenaline uptake. Another way of inhibiting the transporter is by dissipation of the pH gradient across the vesicular membrane i-chloroamphetamine is thought to act in this way. 

Hsu, CC, Thomas, C, Chen, W, Davis, KM, Foos, T, Chen, JL, Wu, E, Floor, E, Schloss, JV and Wu, JY (1999) Role of synaptic vesicle proton gradient and protein phosphorylation on ATP-mediated activation of membrane-associated brain glutamate decarboxylase. J. Biol. Chem. 274 24366-24371. 

Recent studies by Crompton et al. have shown that oxidant stress may open a Ca-sensitive, non-selective pore in the inner mitochondrial membrane that is blocked by cyclosporin A (Crompton, 1990 Crompton and Costi, 1990). This pore opening results in massive mitochondrial swelling, dissipation of the transmembrane proton gradient and disruption of mitochondrial energy production (Crompton et al., 1992). Since mitochondria may play a role as a slow, high-capacity cytosolic calcium buffer (Isenberg et al., 1993), disruption of mitochondrial function may also contribute to calcium overload and cell injury. 

Alcohol dehydrogenases found in certain microorganisms utilize a pyrroloquino-line quinone (PQQ) or flavin cofactor to pass electrons released upon oxidation of alcohols to the heme electron-acceptor protein, cytochrome c. These membrane-associated alcohol dehydrogenases form part of a respiratory chain, and the energy from fuel oxidation therefore contributes to generation of a proton gradient across... 

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