ATP-ADP exchange reaction

The existence of the Ei, and E2 states of the phosphorylated protein, i.e., the high-and low-energy phosphoenzyme intermediate, has been demonstrated by the ATP ADP exchange reaction [92,93] and by the exchange between inorganic phosphate and water [94]. 

ADP has little or no stimulatory effect on the rate of dephosphorylation [71,74], which finding is in contrast with the fact that the enzyme catalyses an ATP-ADP exchange reaction [71], Dephosphorylation can also occur with hydroxylamine [74,76], suggesting that the intermediate is an acylphosphate as is also the case for (Na+ +K + )-ATPase [1] and (Ca + + Mg -)-ATPase [2],... 

If these exchange reactions are involved in the coupling mechanism, the oxidation or reduction of the carrier could be expected to influence their process. In the light of this assumption, it is interesting that the ATP- Pj exchange is strongly inhibited when the carriers are maintained in the reduced state. The oxidation or reduction of the carriers does not affect the ATP-ADP exchange reaction of particles obtained by... 

The concepts outlined above are tentatively summarized in the minimal reaction cycles shown in Figure 11. The cycles show not just two conformational states, but multiple states. The E1/E2 notation is used primarily to refer to distinct catalytic specificities for ATP-ADP exchange and Pi-H20 exchange, respectively. There is... 

The ATP- Pj exchange reaction has been studied in digitonin and sonic particles of mitochondria. The reaction requires ADP, and the rate of the reaction is a function of the relative concentration of ATP, ADP, and inorganic phosphorus. The reaction can be inhibited by NAD, pentachlorophenol, gramicidin, and other agents. 

J. E. Snoke Yes. This experiment is, of course, analogous to experiments carried out with the acetate-ATP reaction where there is evidence for the formation of CoA enzyme. We also have tried to inhibit our ATP-ADP exchange by the addition of glutamylcysteine or glycine. Neither of these compounds did inhibit the exchange of ATP and ADP. However, I should say that these experiments were not carried out in the presence of limiting concentrations of ADP, and will have to be repeated. 

Hydroxy- 9-methylglutaryl CoA further yields acetyl CoA and acetoacetic acid, as was shown earlier by Coon et cU. (I48). In biotin deficiency the carboxylation reaction does not occur. It was shown by Lynen et al. that the actual carboxylation is preceded by the enzymic dehydration (rf jS-hydroxyisovaleryl CoA to /8-methylcrotonyl CoA, which is the true substrate for the entry of CO2. TTiis occurs at the expense of the hydrolysis of the terminal P04 of ATP. The unsaturated intermediate is then saturated by the addition of H2O to yield the final product. The critical step of this carboxylation is the conversion of CO2 to a reactive form. The analogy of the biochemical activation of other substances through an acyl adenylate type of compound did not fit CO2 activation. The final mechanism of the activation of CO2 was derived from the discovery that the carboxylase enzyme was a biotin-protein. This observation explains earlier work 149) which indicated that biotin is a cofactor of the fatty acid-synthesizing enzyme system. When the purified carboxylase was incubated with P and ATP an exchange reaction of phosphate occurred, which was inhibited by avidin, a protein which specifically binds biotin. This indicated that the primary reaction in CO2 fixation is the combination of ATP with the biotin-protein enzyme to yield ADP biotin-protein -f P. The active CO2 is then the product of an exchange reaction between ADP and C02 which is finally attached to the biotin-protein complex. 

Monomeric actin binds ATP very tightly with an association constant Ka of 1 O M in low ionic strength buffers in the presence of Ca ions. A polymerization cycle involves addition of the ATP-monomer to the polymer end, hydrolysis of ATP on the incorporated subunit, liberation of Pi in solution, and dissociation of the ADP-monomer. Exchange of ATP for bound ADP occurs on the monomer only, and precedes its involvement in another polymerization cycle. Therefore, monomer-polymer exchange reactions are linked to the expenditure of energy exactly one mol of ATP per mol of actin is incorporated into actin filaments. As a result, up to 40% of the ATP consumed in motile cells is used to maintain the dynamic state of actin. Thus, it is important to understand how the free energy of nucleotide hydrolysis is utilized in cytoskeleton assembly. 

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. 

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. 

Because each half-reaction can behave like an independent chemical process, ping pong enzymes catalyze exchange reactions. For example, yeast NDPK catalyzes an ADP ATP exchange reaction ... 

Figure 2. (A) Effect of simultaneously raising the absolute concentrations of ATP and ADP on the indicated equilibrium exchange reactions catalyzed by yeast hexokinase. (B) Effect of simultaneously raising the absolute concentrations of glucose and glucose 6-phosphate on the indicated equilibrium exchange reactions of yeast hexokinase.

Propionyl-CoA is first carboxylated to form the d stereoisomer of methylmalonyl-CoA (Pig. 17—11) by propionyl-CoA carboxylase, which contains the cofactor biotin. In this enzymatic reaction, as in the pyruvate carboxylase reaction (see Pig. 16-16), C02 (or its hydrated ion, HCO ) is activated by attachment to biotin before its transfer to the substrate, in this case the propionate moiety. Formation of the carboxybiotin intermediate requires energy, which is provided by the cleavage of ATP to ADP and Pi- The D-methylmalonyl-CoA thus formed is enzymatically epimerized to its l stereoisomer by methylmalonyl-CoA epimerase (Pig. 17-11). The L-methylmal onyl -CoA then undergoes an intramolecular rearrangement to form succinyl-CoA, which can enter the citric acid cycle. This rearrangement is catalyzed by methylmalonyl-CoA mutase, which requires as its coenzyme 5 -deoxyadenosyl-cobalamin, or coenzyme Bi2, which is derived from vitamin B12 (cobalamin). Box 17—2 describes the role of coenzyme B12 in this remarkable exchange reaction. 

Most kinases transfer chiral phospho groups with inversion and fail to catalyze partial exchange reactions that would indicate phosphoenzyme intermediates. However, nucleoside diphosphate kinase contains an active site histidine which is phosphorylated to form a phosphoenzyme.869 The enzyme catalyzes phosphorylation of nucleoside diphosphates other than ADP by a nucleotide triphosphate, usually ATP. 

As noted earlier, studies with inhibitors have been of great value. One mole of ouabain binds per enzyme complex and inhibits all enzyme functions. It provides a convenient marker for the extracellular surface of the enzyme. Oligomycin inhibits the (Na+, K+)-ATPase but not the K+-phosphatase reaction. It stimulates the ADP/ATP exchange reaction and this led to the postulate for two phosphoenzymes in the reaction scheme. Anomalous kinetic behaviour for (Na+, K+)-ATPase, over some years, was eventually recognized57 to be due to a vanadate impurity in ATP, which binds with high affinity to the low affinity ATP site and with low affinity to the high affinity ATP site. In accord with this, vanadate effectively inhibits the K+-phosphatase... 

The free energy liberated in the hydrolysis of ATP is harnessed to drive reactions that require an input of free energy, such as muscle contraction. In turn, ATP is formed from ADP and P when fuel molecules are oxidized in chemotrophs or when light is trapped by phototrophs. This ATP—ADP cycle is the fundamental mode of energy exchange in biological systems. 

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