Oxidative phosphorylation proton-motive force

The thylakoid membrane is asymmetrically organized, or sided, like the mitochondrial membrane. It also shares the property of being a barrier to the passive diffusion of H ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic. 

How is a concentration gradient of protons transformed into ATP We have seen that electron transfer releases, and the proton-motive force conserves, more than enough free energy (about 200 lcJ) per mole of electron pairs to drive the formation of a mole of ATP, which requires about 50 kJ (see Box 13-1). Mitochondrial oxidative phosphorylation therefore poses no thermodynamic problem. But what is the chemical mechanism that couples proton flux with phosphorylation ... 

Although the primary role of the proton gradient in mitochondria is to furnish energy for the synthesis of ATP, the proton-motive force also drives several transport processes essential to oxidative phosphorylation. The inner mitochondrial membrane is generally impermeable to charged species, but two specific systems transport ADP and Pj into the matrix and ATP out to the cytosol (Fig. 19-26). 

For a long time Fe/S clusters in the enzyme complexes of the respiratory chain of oxidative phosphorylation have been suggested to be directly involved in energy transduction, e.g., in the generation of a proton-motive force. A specific example is the putative cubane, center N2, in NADH Q oxidoreductase [6], One could formally write the process as a catalysis of the reaction H+in -> H+out. 

Oxidative phosphorylation is the culmination of a series of energy transformations that are called cellular respiration or simply respiration in their entirety. First, carbon fuels are oxidized in the citric acid cycle to yield electrons with high transfer potential. Then, this electron-motive force is converted into a proton-motive force and, finally, the proton-motive force is converted into phosphoryl transfer potential. The conversion of electron-motive force into proton-motive force is carried out by three electron-driven proton pumps—NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, and... 

Conceptual Insights, Energy Transformations in Oxidative Phosphorylation. View this media module for an animated, interactive summary of how electron transfer potential is converted into proton-motive force and, finally, phosphoryl transfer potential in oxidative phosphorylation. 

This potential, or proton-motive force as it is also called, in turn drives a number of energy-requiring functions which include the synthesis of ATP, the coupling of oxidative processes to phosphorylation, a metabolic sequence called oxidative phosphorylation and the transport and concentration in the cell of metabolites such as sugars and amino acids. This, in a few simple words, is the basis of the chemiosmotic theory linking metabolism to energy-requiring processes. 

Certain chemical substances have been known for many years to uncouple oxidation from phosphorylation and to inhibit active transport, and for this reason they are named uncoupling agents. They are believed to act by rendering the membrane permeable to protons, hence short-circuiting the potential gradient or proton-motive force. 

Inhibition of ATP export. ATP-ADP translocase is specifically inhibited by very low concentrations of atractyloside (a plant glycoside) or bongkrekic acid (an antibiotic from a mold). Atractyloside binds to the translocase when its nucleotide site faces the cytoplasm, whereas bongkrekic acid binds when this site faces the mitochondrial matrix. Oxidative phosphorylation stops soon after either inhibitor is added, showing that ATP-ADP translocase is essential for maintaining adequate amounts of ADP to accept the energy associated with the proton-motive force. 

Cobley JG (1976) Energy-conserving reactions in phosphorylating electron-transport particles from Nitrobacter winogradskyi. Activation of nitrite oxidation by electrical component of the proton motive force. Biochem J 156 481-491... 

The hypothesis that a proton-motive force across the inner mitochondrial membrane is the immediate source of energy for ATP synthesis was proposed in 1961 by Peter Mitchell. Virtually all researchers working in oxidative phosphorylation and photosynthesis Initially opposed this chemlosmotic mechanism. They favored a mechanism similar to the well-elucidated substrate-level phosphorylation in glycolysis, in which oxidation of a substrate molecule is directly coupled to ATP synthesis. By analogy, electron transport through the... 

In addition to powering ATP synthesis, the proton-motive force across the inner mitochondrial membrane also powers the exchange of ATP formed by oxidative phosphorylation inside the mitochondrion for ADP and Pj in the cytosol. This exchange, which is required for oxidative phosphorylation to continue, is mediated by two proteins in the inner membrane a phosphate transporter (HP04 /OH antiporter) and an ATP/ADP antiporter (Figure 8-28). 

The Inner membrane of brown-fat mitochondria contains thermogenin, a protein that functions as a natural uncoupler of oxidative phosphorylation. Like synthetic uncouplers, thermogenin dissipates the proton-motive force across the Inner mitochondrial membrane, converting energy released by NADH oxidation to heat. Thermogenin is a proton transporter, not a proton channel, and shuttles protons across the membrane at a rate that is a millionfold slower than that of typical Ion channels. Its amino acid sequence is similar to that of the mitochondrial ATP/ADP antiporter, and it functions at a rate that Is characteristic of other transporters (see Figure 7-2). 

In brown fat, the inner mitochondrial membrane contains thermogenln, a proton transporter that converts the proton-motive force into heat. Certain chemicals (e.g., DNP) have the same effect, uncoupling oxidative phosphorylation from electron transport. 

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

Describe the chemiosmotic model of oxidative phosphorylation and relate experimental evidence that only the proton-motive force links the respiratory chain and ATP synthesis. 

As the NADH is oxidized, the electrons released are removed by specific carriers, and the protons are transported from cytoplasm to outside the cell. Removal of H+ causes an increase in the nmnber of OH ions inside the membrane. These conditions result in a proton gradient (pH gradient) across the membrane. This gradient of potential energy, termed as proton motive force, can be used to do useful work. This potential energy is captured by the cell by a series of complex membrane-bound enzymes, known as the ATPase in the process called oxidative phosphorylation. In 1961, the concept of proton gradient was first proposed as chemiosmotic theory by Peter Mitchell of England, who won the Nobel Prize for this scientific contribution. 

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