ADP-ATP exchange

The control of the rate of ATP synthesis is shared by several processes including the demand for ATP, the rate of ADP/ATP exchange, the activities of the enzymes... 

Researchers studying the stepwise kinetics of nitrogenase electron transfer using stopped-flow kinetic techniques have presented other scenarios. One hypothesis presents kinetic evidence that dissociation of Fe-protein from MoFe-protein is not necessary for re-reduction of Fe-protein by flavodoxins.13 These authors state that the possibility of ADP-ATP exchange while Fe-protein and MoFe-protein are complexed with each other cannot be excluded and that dissociation of the complex during catalysis may not be obligatory when flavodoxin is the Fe-protein reductant. This leads to the hypothesis that MgATP binds to the preformed Fe-protein/... 

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 ... 

For the ATPase activity of the enzyme, Mg2+ is required. Kinetic studies confirm that MgATP is the substrate and that while free ATP binds to the enzyme it inhibits activity. It appears that MgATP binds at high and low affinity sites, with Km values about 3 p,mol dm-3 and 0.2 mmol dm-3 respectively. For phosphorylation of the enzyme, saturation of the high affinity site is sufficient for maximum activity. Binding at the low affinity sites inhibits ADP/ATP exchange and may represent a modulator role for these sites. This low affinity site may be the site for the substrate in the K+-phosphatase reaction. 

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... 

Temkin V, et al. Inhibition of ADP/ATP exchange in receptorinteracting protein-mediated necrosis. Mol. Cell Biol. 2006 26 2215-2225. 

The second step in the reaction mechanism is the phosphorylation of the enzyme by the y-linked phosphate from ATP. The reaction is reversible, which gives rise to a slow ADP-ATP exchange ... 

Another consequence of partial lipid depletion is that the residual K -stimulated phosphatase is not inhibited by Na nor undergoes stimulation by Na" " -I-ATP at low K concentrations [117,124]. This has led to the suggestion that the K" -stimulated phosphatase activity in preparations containing a full lipid complement is a monomeric enzyme function by virtue of a K -induced dissociation of the dimer into monomers. Na would inhibit the phosphatase by inducing dimerization. In lipid-depleted preparations, having only one of the monomers saturated with lipid as a prerequisite for phosphatase activity to occur, Na would be unable to cause dimerization. Hence, it does neither inhibit the phosphatase activity nor stimulate Na-K ATPase activity for which dimerization is essential. As a consequence, the Na -dependent reactions (ATP-phosphorylation and the ADP-ATP exchange) would be expressions of the dimer and the -dependent reactions (K -stimulated dephosphorylation and phosphatase) expressions of the monomer. However, thimerosal, which would inhibit monomer interaction, does not inhibit any of these partial reactions, but increases the affinities for K" " [125,126] and lipid depletion reduces the Na -dependent phosphorylation less than the overall Na-K ATPase activity [122]. It is clear from these studies that an unequivocal answer as to the monomeric and dimeric nature of the partial reactions in the overall Na-K ATPase reaction and their lipid dependence must await determination of their functional molecular weights. 

In a consideration of the mechanism of GSH synthesis from glutamylcysteine, three properties of the enzyme are relevant (a) the formation of GSH, ADP, and P, in equimolar quantities, (6) the exchange of glycine into GSH, and (c) the ADP-ATP exchange. 

A kinetic scheme for photophosphorylation catalyzed by the chloroplast ATP synthetase is described by Equation 1. This sequence of reactions assumes that binding of ADP occurs prior to that of Pi (Selman, Selman-Reimer, 1981). Thus, the Pi-ATP exchange, one of the so-called partial reactions of photophosphorylation, might occur without significant dissociation of the E-ADP complex and without a concomitant ADP-ATP exchange reaction (k- <[Pg.493]

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