Exchange reactions oxidative phosphorylation

Sophisticated isotope experiments were also performed using H2180 (Mildred Cohn) and 32P, and various exchange reactions identified between ATP, ADP, and Pr Analysis of the mode of action of two inhibitors was also relevant. Dinitrophenol (DNP) uncoupled the association between oxidation and ATP generation (Lardy and Elvejhem, 1945 Loomis and Lipmann, 1948). Oligomycin inhibited reaction (ii) above, blocking the terminal phosphorylation to give ATP, but not apparently the formation of A C. 

ATPase activity and exchange reactions, such as P,-ATP exchange, are partial reactions of oxidative and photosynthetic phosphorylation. These reactions have been described in detail and have been considered to consist of a series of reversible chemical reactions forming high energy intermediates (X-Y and X-P) [104-106]. In... 

This large and important class of extractants includes the neutral and acidic esters of ortho phosphoric acid and related compounds, the phosphonates, phosphinates and phosphine oxides. The class divides naturally into neutral and acidic compounds by the differences in extraction mechanism. The neutral compounds extract by solvation of a neutral complex in the organic phase by the phosphoryl oxygen, while the acidic compounds, in general, operate by an ion-exchange reaction to form an extractable species. Further solvation may occur in the organic phase in some extraction systems of this type. 

The phosphorylated histidine could play a role in oxidative phosphorylation. Boyer and his associates found an enzyme in the mitochondria of beef heart that catalyzes the formation of P-labeled histidine at the expense of [ PjATP. Such a reaction could be considered part of the reversal of oxidative phosphorylation. It is interesting that the same enzyme preparation also catalyzes the exchange between inorganic phosphate and oxygen—a reaction discussed in more detail later. 

Many people try to reconstruct the steps of oxidative phosphorylation by studying the exchange reactions outlined above. These reactions were described first in intact mitochondria, and later in mitochondrial fragments obtained by treating the organelle with digi-tonin or ultrasound. The enzymes involved have occasionally been prepared in a soluble form. 

The role played by exchange reactions in oxidative phosphorylation is not known these reactions could be the steps of the coupling mechanism, but in reverse. Some indirect evidence suggests that they are indeed involved in oxidative phosphorylation. First, as already mentioned, these exchange reactions are affected by agents uncoupling oxidative phosphorylation. Their response to the uncouplers, however, does not correlate with the effect of these compounds on the efficiency of oxidative phosphorylation itself. Secondly, these reactions are influenced by the carriers state of oxidation. 

Boyer, P.D., Cross, R.L., Momsen, W. A new concept for energy coupling in oxidative phosphorylation based on a molecular explanation of the oxygen exchange reactions. Proc. nat. Acad. Sci. (Wash.) 70, 2837-2839 (1973)... 

Another approach to the nature of the phosphate esterification reaction has been made with the use of isotope exchange methods. Boyer et al. and Swanson have shown that an exchange reaction between ATP and inorganic P is catalyzed by anaerobic mitochondria, and suggest that the exchange represents a reversible step in the process of oxidative phosphorylation. Similar conclusions were reached by Cohn and Drys-dale, who found in experiments with mitochondria that the oxygen of... 

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. 

Figure 16.8 Catalytic mechanism of glyceraldehyde 3-phosphate dehydrogenase. The reaction proceeds through a thioester intermediate, which allows the oxidation of glyceraldehyde to be coupled to the phosphorylation of 3-phosphogIycerate. (1) Cysteine reacts with the aldehyde group of the substrate, forming a hemithioacetal. (2) An oxidation takes place with the transfer of a hydride ion to NAD. forming a thioester. This reaction is facilitated by the transfer of a proton to histidine. (3) The reduced NADH is exchanged for an NAD" molecule. (4) Orthophosphate attacks the thioester. forming the product 1,3-BPG.

A separate group of metal starchates comprises metal salts of starches carrying carboxyl, xanthate, sulfate, phosphate, and similar groups. Oxidized, carboxyalkylated, sulfated, xanthated, and phosphorylated starches readily form corresponding salts by methatetical reactions between the sodium salts of the aforementioned derivatives and other water-soluble, metal salts. There are several patents on the formation of these salts for ion exchangers and the removal of... 

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