ATP-ADP antiport

In mitochondria, more than 90 % of the respiratory phosphorylation is catalyzed by the H -ATP-synthase, an enzyme converting the respiratory chain-produced electrochemical potential difference (ApH+) into ATP [1-4]. Very small (but sometimes essential) portion of the respiratory energy is converted to GTP by succinate thiokinase [4]. Both respiratory chain enzymes (Complexes I, III and IV), catalyzing electron transfer from NAD(P)H to O2, and H -ATP-synthase are localized in the inner mitochondrial membrane. The great majority of the formed ATP molecules is exported from mitochondria by the ATP/ADP antiporter in exchange for extramitochondrial ADP (eqs. 1-3). 

ATP/ADP antiporter catalyzes transmembrane exchange of ADP for ATP. This results in import of ADP and export of ATP at the expense of the respiration energy. 

Uncoupling results in dissipation of the respiratory chain-produced ApH+ due to increased conductance of the inner mitochondrial membrane. Thus energy released by respiration is immediately dissipated as heat without formation and hydrolysis of ATP. Non-esterified fatty acids proved to be compounds mediating the thermoregulatory uncoupling. They operate as protonophorous uncouplers with the help of special uncoupling proteins (UCPs) or some mitochondrial antiporters i.e. the ATP/ADP antiporter and aspartate/glutamate antiporter [1-5]. 

The process proved to be inhibited by even a small ApH+ decrease ( mild uncoupling ) [5]. It was suggested that mild uncoupling is carried out by free fatty acids operating as protonophores with the help of UCPs and ATP/ADP-antiporter [5]. 

There is some indications that mitochondria possess a mechanism of self-elimination. This function was ascribed to the so-called permeability transition pore (PTP). The PTP is a rather large nonspecific channel located in the inner mitochondrial membrane. The PTP is permeable for compounds of molecular mass <1.5 kDa. The PTP is usually closed. A current point of view is that PTP opening results from some modification and conformation change of the ATP/ADP antiporter. Oxidation of Cys56 in the antiporter seems to convert it to the PTP in a way that is catalyzed by another mitochondrial protein, cyclophilin. When opened, the PTP makes impossible the performance of the main mitochondrial function, i.e., coupling of respiration with ATP synthesis. This is due to the collapse of the membrane potential and pH gradient across the inner mitochondrial membrane that mediate respiratory phosphorylation. Membrane potential is also a driving force for import of... 

ATP is transported from the mitochondrial matrix to the cytosol in exchange for ADP (the ATP-ADP antiport system). 

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 phosphate transporter catalyzes the import of one HP04 coupled to the export of one OH . Likewise, the ATP/ADP antiporter allows one molecule of ADP to enter only if one molecule of ATP exits simultaneously. The ATP/ADP antiporter, a dimer of two 30,000-Da subunits, makes up 10-15 percent of the protein in the inner membrane, so it is one of the more abundant mitochondrial proteins. Functioning of the two antiporters together produces an Influx of one ADP and one Pj and efflux of one ATP together with one OH . Each OH transported outward combines with a proton, translocated during electron transport to the Intermembrane space, to form H2O. This drives the overall reaction in the direction of ATP export and ADP and Pj Import. 

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). 

Brustovetskii NN, Dedukhova VN, Egorova MV, Mokhova EN, Skulachev VP. Uncoupling of oxidative phosphorylation by fatty acids and detergents suppressed by ATP/ADP antiporter inhibitors. Biochem (Moscow) 1991 56 1042-1048. 

ATP, and the oxidation of one FADH2 produces approximately two ATP. The hydrolysis of ATP provides energy for all cellular activity. ATP is transported from the mitochondrial matrix to the c)dosol in exchange for ADP through the ATP-ADP antiport system. 

The major function of oxidative phosphorylation is to generate ATP from ADP. ATP and ADP do not diffuse freely across the inner mitochondrial membrane. How are these highly charged molecules moved across the inner membrane into the cytoplasm A specific transport protein, ATP-ADP translocdse, enables these molecules to transverse this permeability barrier. Most important, the flows of ATP and ADP are coupled. ADP enters the mitochondrial matrix only if ATP exits, and vice versa. This process is carried out by the translocase, an antiporter ... 

Inhibition of the electron transport chain in coupled mitochondria can occur at any of the three constituent functional processes electron transport per se, formation of ATP, or antiport translocation of ADP/ATP (Table 16-1). The best known inhibitor of the ADP/ATP translocase is atractyloside in the presence of which no ADP for phosphorylation is transported across the inner membrane to the ATP synthase and no ATP is transported out. In the absence of ADP phosphorylation the proton gradient is not reduced allowing other protons to be extruded into the intermembrane space because of the elevated [H+], and thus electron transfer is halted. Likewise the antibiotic oligomycin directly inhibits the ATP synthase, causing a cessation of ATP formation, buildup of protons in the intermembrane space, and a halt in electron transfer. Similarly, a blockade of complex I, III, or IV that inhibits electron flow down the chain to would also stop both ATP formation and ADP/ATP translocation across the inner mitochondrial membrane. 

E. Gluconeogenesis requires ATP, which is in short supply, turning up the catabolism of glucose to lactate in the absence of an intact electron transport chain. ADP cannot be transported into the mitochondrion because ATP, its antiporter partner, isn t made by oxidative phosphorylation as a result of cyanide inhibition of cytochrome oxidase. Metabolism of fatty acids and ketone bodies requires a functional electron transport chain for their metabolism, and these possibilities are also ruled out. 

A FIGURE 8-28 The phosphate and ATP/ADP transport system in the inner mitochondrial membrane. The coordinated action of two antiporters (purple and green) results in the uptake of one ADP - and one HP04 - in exchange for one ATP -, powered by the outward translocation of one proton during electron transport. The outer membrane is not shown here because it is permeable to molecules smaller than 5000 Da. 

The adenine nucleotide translocase, integral to the inner membrane, binds ADP3 - in the intermembrane space and transports it into the matrix in exchange for an ATP4 molecule simultaneously transported outward (see Fig. 13-1 for the ionic forms of ATP and ADP). Because this antiporter moves four negative charges out for every three moved in, its activity is favored by the... 

Figure 6-10. Porters are necessary in the inner mitochondrial membrane to move the substrates for oxidative phosphorylation into the matrix (to replenish ADP and Pj) and to move the ATP formed out to the lumen for export to the cytosol. Nigericin acts as an antiporter in the inner membrane, and valinomycin acts as an ionophore.

Figure 1.4 Compaitmentation of biosynthesis and sequestration. Abbreviations SM, secondary metabolites CS-SM, conjugate of SM with glutathione NPAAs, non-protein amino acids ATP, adenosine triphosphate ADP, adenosine diphosphate mt, mitochondrion cp, chloroplast nc, nucleus 1, passive transport 2, free diffusion 3, H+/SM antiporter 4, ABC transporter for SM conjugated with glutathione 5, ABC transporter for free SM 6, H+-ATPase. (See Plate 3 in colour plate section.)...

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