Transport systems adenine nucleotide

Azido-ADP Adenine nucleotide transport system in mitochondria Photoinduced inhibition of nucleotide exchange 55... 

Most ATP-requiring reactions occur in the cytosol and produce ADP and orthophosphate. Since most ATP is formed by mitochondrial oxidative phosphorylation (in appropriate cells) from ADP and orthophosphate, these molecules must traverse the inner membrane. ATP and ADP are translocated by the specific adenine-nucleotide-transport system. This antiport system is widely distributed in the membrane and exchanges one mitochondrial ATP for one cytoplasmic ADP. The carrier selectively binds and transports ADP inwards and ATP outwards. The phosphate enters the mitochondrion via a different antiport system, the phosphate carrier, which exchanges it for a hydroxyl ion. 

Freshly isolated, intact mitochondria contain considerable amounts of adenine nucleotides which are resistant to removal by repeated washings with isotonic sucrose. This indicates that these compounds are in a compartment—presumably within the inner mitochondrial membrane—which is inaccessible to the sucrose solution. When exogenous adenine nucleotide is added to the mitochondria, there is a rapid exchange with endogenous adenine nucleotides with no net increase in the concentration of adenine nucleotides in the mitochondria. ADP exchanges most rapidly, followed by ATP and then by AMP, which is relatively impermeable. It is the inner mitochondrial membrane through which the adenine nucleotides do not permeate and which contains the specific adenine-nucleotide transporting system. The movement of ATP across the inner mitochondrial membrane (and hence out of the mitochondria) depends directly on the translocation of ADP in the presence of adenylate kinase in the outer compartment of the mitochondria. 

A key event in preventing apoptosis is thus the retention of cytochrome c within mitochondria. The permeability transition pore complex is formed between the inner and outer mitochondrial membranes and is reported to control protein release from the intermembrane space. The permeabihty transition pore complex comprises the adenine nucleotide transporter, the voltage-dependent anion channel and possibly other proteins such as the benzodiazepine receptor and cyclophilin D [65]. Thus, cells possess specialised systems and processes for retaining cytochrome c within mitochondria to ensure survival, as well as systems that can rapidly mobilise this molecule when the apoptotic pathway is triggered. 

Figure 9.19 Adenine nucleotide translocase and phosphate transfer into the matrix. Phosphate is transported into the mitochondria with protons in a symport transport system. The adenine nucleotide translocase transports ADP into and ATP out of the mitochondria, i.e. it is electrogenic. The charge is neutralised by H movement into the matrix from the proton motive force which utilises about 25% of the energy in the proton motive force.

The combined effect of exchanging extramitochon-drial ADP-3 and H2P04 for mitochondrial ATP-4 and OH is to move one proton into the mitochondrial matrix for every molecule of ATP that the mitochondrion releases into the cytosol. This proton translocation must be considered, along with the movement of protons through the ATP synthase, to account for the P-to-O ratio of oxidative phosphorylation. If three protons pass through the ATP synthase, and the adenine nucleotide and Pj transport systems move one additional proton, then four protons in total move into the matrix for each ATP molecule provided to the cytosol. 

FoMy Acid Oxidation by Cell-Free Preparations. An important development in the study of fatty acid oxidation was made by Munoz and Leloir, who found that homogenates of liver oxidize fatty acids when supplemented with adenine nucleotides, inorganic phosphate, Mg++, cytochrome c (a compoimd involved in electron transport), and a member of the tricarboxylic acid cycle. Others found that washed mitochondria contain all the necessary enzymes for oxidation of fatty acids to either CO2 or acetoacetate, provided the supplements of Munoz and Leloir were added. Such systems are extremely delicate, and for many years it was considered that maintenance of the intact structure of mitochondria was essential for preservation of fatty acid oxidation. 

The adenine nucleotides (AMP, ADP, and ATP) comprise a family of cofactors which are of prime importance in the transport of phosphate. The importance of phosphate as a means of transforming chemical potential and oxidation energy into metabolically active forms has been amply discussed by Lipmann and by Kalckar and others and needs no elaboration here. The major role of the adenine nucleotide system is the transport and storage of phosphate bond energy. 

This is because a strong feedback (inhibitory) effect of acetyl-CoA (or a related metabolite) on system A (fatty acid activation, transport, and -oxidation) will strongly oppose the increase in the rate of formation of acetyl-CoA via an increase in the extracellular concentration of fatty acids. This is similar to the control of glycolysis in muscle, where the powerful feedback effect of the adenine nucleotides makes it difflcult to increase glycolysis by increasing the supply of glucose. 

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