ADP/ATP translocase

FIGURE 31-7 Mitochondrial carriers. Ions and small molecules enter the intermembrane space, since the outer mitochondrial membrane is not a significant permeability barrier. However, the inner mitochondrial membrane is impermeable to ions except those for which there are specific carriers. Most of the carriers are reversible, as indicated by two-headed arrows. Compounds transported in one direction are indicated in red. The ATP/ADP translocase and the aspartate-glutamate carrier are both electrophoretic their transport is driven in the direction of the mitochondrial membrane potential, as indicated by red arrows. Glutamine is carried into the matrix by an electroneutral carrier. The unimpaired functioning of mitochondrial carriers is essential for normal metabolism. (Adapted with permission from reference [70].)... 

ATP is made by the FiFo ATPase. This enzyme allows the protons back into the mitochondria. Since the interior is alkaline, the reaction is favorable—favorable enough to drive the synthesis of ATP by letting protons back into the mitochondria. Exactly how the FiFq ATPase couples the flow of protons down their concentration gradient to the formation of ATP is not known in molecular detail. The proton flow through the FiFo ATPase is required to release ATP from the active site where it was synthesized from ADP and Pj. The ATP is made in the interior of the mitochondria and must be exchanged for ADP outside the mitochondria to keep the cytosol supplied with ATP. The exchange of mitochondrial ATP for cytoplasmic ADP is catalyzed by the ATP/ADP translocase. 

The drug indirectly alters the regulation of intracellular calcium levels via protein kinase C and can alter the transcription of gene- (downregulating) encoding enzymes (e.g., ATP/ADP translocase) and those involved in energy production and specific for cardiac tissue. 

The major function of oxidative phosphorylation is to generate ATP from ADP. However, 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 cytosol A specific transport ATP-ADP translocase (also called adenine nucleotide... 

Figure 18.39. Mechanism of Mitochondrial ATP-ADP Translocase. The translocase catalyzes the coupled entry of ADP and exit of ATP into and from the matrix. The reaction cycle is driven by membrane potential. The actual conformational change corresponding to eversion of the binding site could be quite small.

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 cytosol, 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. 

Mitochondria employ a host of carriers, or transporters, to move molecules across the inner mitochondrial membrane. The electrons of cytoplasmic NADH are transferred into the mitochondria by the glycerol phosphate shuttle to form FADH2 from FAD. The entry of ADP into the mitochondrial matrix is coupled to the exit of ATP by ATP-ADP translocase, a transporter driven by membrane potential. 

Since it was shown by Cannon et al. [23] that ADP acted from the outside of the mitochondria to induce respiratory control, a specific site of interaction could be envisaged. Such a site was characterized by Nicholls [33] by binding of [ H]GDP. Further, by labelling brown fat mitochondria with [ P]azido-ATP, Heaton et al. [34] demonstrated that — besides the ATP/ADP-translocase at 30 kDa — a specific band with a molecular weight of 32000 was labelled, and this was identical with the GDP-binding site. This protein (i.e., thermogenin) had already been observed by Ricquier and Kader as the only protein the concentration of which was markedly altered in brown fat mitochondria isolated from cold-acclimated animals [35] (Fig. 10.8). 

As not more than 16 nmol of GDP can be bound per mg isolated thermogenin, it would seem that only one binding site is exposed per dimer. This is similar to what is known about the ATP/ADP translocase, but it is perhaps difficult to visualize (Fig. 10.12). 

The inner membrane is studded with spheres, each 8-10 nm in diameter, that are attached via stalks 4-5 nm in length. These inner membrane spheres are present on the matrix side (M-side) but absent from the cytoplasmic side (C-side). The components of the inner membrane include respiratory chain proteins, a variety of transport molecules, and a part of the ATP-synthesizing apparatus (the base piece of ATP synthase). The ATP-ADP translocase and... 

Figure 18.38 Structure of mitochondrial transporters. The structure of the ATP-ADP translocase is shown. Notice that this structure comprises three similar units (shown in red, biue, and yellow) that come together to form a binding site, here occupied by an inhibitor of this transporter. Other members of the mitochondrial transporter family adopt similar tripartite structures. [Drawn from lOKC.pdb.]...

Examination of the amino acid sequence of the ATP-ADP translocase revealed that this protein consists of three tandem repeats of a 100-amino-add module, each of which appears to have two transmembrane segments. This tripartite structure has recently been confirmed by the determination of the three-dimensional structure of this transporter (Figure 18.38). The transmembrane helices form a teepeelike structure with the nucleotide-binding site (marked by a bound inhibitor) lying in the center. Each of the three repeats adopts a similar structure. 

ATP-ADP translocase is but one of many mitochondrial transporters for ions and charged metabolites (Figure 18.39). The phosphate carrier, which works in concert with AT F-ADP translocase mediates the elec-troneutral exchange of H7P04 for OH . The combined action of these two transporters leads to the exchange of cytoplasmic ADP and Pj for ma trix ATP at the cost of the influx of one H" " (owing to the transport of one OH out of the matrix). These two transporters, which provide ATP synthase with its substrates, are associated with the synthase to form a large complex called the ATP synthasome. 

JO. Exaggerating the difference. Why must the ATP-ADP translocase (also called adenine nucleotide translocase or ANT)... 

I he Entry of ADP into Mitochondria Is Cloupled to the Exit of ATP by ATP-ADP Translocase... 

Figure 16-1. Schematic diagram of electron transport chain ATP synthase and ATP/ADP translocase.

Investigators reported finding antibodies against cardiac ATP-ADP translocase in an individual who died of a viral cardiomyopathy. How could these antibodies result in death ... 

As ATP is hydrolyzed during muscular contraction, ADP is formed. This ADP must 1 exchange into the mitochondria on ATP-ADP translocase to be converted back to ATP. Inhibition of ATP-ADP translocase results in rapid depletion of the cytosol ATP levels and loss of cardiac contractility. 

Describe the ATP-ADP translocase mechanism and appreciate its energy cost. 

Explain why the rate of eversion of the binding site from the matrix to the cytosolic side is more rapid for ATP than for ADP when the ATP-ADP translocase functions in the presence of a proton gradient. 

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