Hydrolysis of ATP to ADP

Within each sarcomere the relative sliding of thick and thin filaments is brought about by "cross-bridges," parts of the myosin molecules that stick out from the myosin filaments and interact cyclically with the thin actin filaments, transporting them hy a kind of rowing action. During this process, the hydrolysis of ATP to ADP and phosphate couples the conformational... 

Formation of glutamine is catalyzed by mitochondrial glutamine synthase (Figure 29-7). Since amide bond synthesis is coupled to the hydrolysis of ATP to ADP and P , the reaction strongly favors glutamine synthesis. One function of glutamine is to sequester ammonia in a nontoxic form. 

The phosphorylation reaction is also localized in the cell mitochondria. The enzyme ATP synthetase present in the mitochondrial membranes is involved in this reaction. The back reaction, which is the hydrolysis of ATP to ADP, occurs at other points of the ecu and involves another enzyme, ATPase. The concentrations of the main reaction components, ADP and ATP, in cytoplasm are about 1 mM. 

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

As Christian de Duve has put it Ah biological expenditures are, in the last analysis, supported by the hydrolysis of ATP to ADP and inorganic phosphate, one of the most noteworthy singularities offered by hfe today. ... 

In the 1940s Carl and Gertrude Cori isolated and purified an active form (phosphorylase a) and an inactive form (phosphorylase b) of an enzyme from muscle. Phosphorylase b is activated by AMP (see page 64). In 1955, Fischer Krebs found an enzyme that catalysed the conversion of phosphorylase b to phosphorylase a, together with hydrolysis of ATP to ADP. Thus it appeared to bring about phosphorylation of the enzyme. The enzyme was termed phosphorylase b kinase, was partially purified and the interconversion was established as... 

Hydrolysis of ATP to ADP is rationalized as nucleophilic attack of water on to the terminal P=0 double bond, followed by cleavage of the anhydride bond and expulsion of ADP as the leaving group. 

In both cases, the mixed anhydride is used to synthesize ATP from ADP. Hydrolysis of the anhydride liberates more energy than the hydrolysis of ATP to ADP and, therefore, can be linked to the enzymic synthesis of ATP from ADP. This may be shown mechanistically as a hydroxyl group on ADP acting as nucleophile towards the mixed anhydride, and in each case a new phosphoric anhydride is formed. In the case of succinyl phosphate, it turns out that GDP rather than ADP attacks the acyl phosphate, and ATP production is a later step (see Section 15.3). These are enzymic reactions therefore, the reaction and the nature of the product are closely controlled. We need not concern ourselves why attack should be on the P=0 rather than on the C=0. 

ATP — ADP + P both represent hydrolysis of anhydride bond hydrolysis of ATP to ADP... 

There are two anhydride linkages in ATP, but nucleophilic attack in the enzyme-controlled reaction usually occurs on the terminal P=0 (hydrolysis of ATP to ADP), and only occasionally do we encounter attack on the central P=0 (hydrolysis of ATP to adenosine monophosphate, AMP). Both reactions yield the same amount of energy, AG—34 kJmoD This is not surprising, since in each case the same type of bond is being hydrolysed. The further hydrolysis of AMP to adenosine breaks an ester linkage and would liberate only a fraction of the energy, AG — 9 kJmol and this reaction is not biochemically important. 

The ATPase enzyme activity of actomyosin has been assigned the classification number EC 3.6.1.32. Myosin catalyzes the hydrolysis of ATP to ADP and orthophosphate. In the absence of actin, myosin is a more feeble ATPase. See also Myosin... 

This class of enzymes [EC 3.6.1.36] (also known as the hydrogen/potassium-exchanging ATPase, the potassium-transporting ATPase, proton pump, and the gastric H+/K+ ATPase) catalyzes the hydrolysis of ATP to ADP and orthophosphate, coupled with the exchange of and ions. The gastric mucosal enzyme has been the best characterized. 

At first step ATP and three Na" ions bind to the inside of the membrane and the enzyme is phosphorylated. The product of the reaction undergoes a conformational change called eversion and brings the Na ions to the outside of the cell membrane. There three Na" ions are replaced by two K ions. The attachment of the K+ ions induces dephosphorylation and hydrolysis of ATP to ADP produces a conformational change that carries two K+ ions to the interior of the cell where they are released. The process builds up a charge gradient across the membrane because three Na" ions... 

The positive value of AG" predicts that under standard conditions the reaction will tend not to proceed spontaneously in the direction written. Another cellular reaction, the hydrolysis of ATP to ADP and Pi is very exergonic ... 

Effects of elevated ADP Energy consumption as a result of mus cular contraction, biosynthetic reactions, or other processes results in the hydrolysis of ATP to ADP and Pj. The resulting increase in the concentration of ADP accelerates the rate of reac tions that use ADP to generate ATP, most important of which is oxidative phosphorylation (see p. 77). Production of ATP increases until it matches the rate of ATP consumption by energy-requiring reactions. 

There it is cleaved by ATP-citrate lyase. To ensure that the reaction goes to completion, cleavage is coupled to the hydrolysis of ATP to ADP and inorganic phosphate (Eq. 13-39). The value of G given here is extremely dependent upon the concentration of Mg2+ as a result of strong chelation of Mg2+ by citrate.220 The reaction sequence is complex but can be understood in terms of an initial ATP-dependent synthesis of... 

Met-tRNAfMct first binds to the small subunit as a ternary complex with the eIF-2 and GTP. This ternary complex then binds to the 5 end of mRNA with the aid of several factors, one of which, eIF-4F, contains a subunit that specifically binds to the terminal cap structure of the mRNA. Binding to mRNA is followed by a scanning reaction that moves the small subunit along the mRNA, usually to the first AUG, in a reaction driven by the hydrolysis of ATP to ADP and Pj. 

Figure 1.9 Plot of the base 10 logarithm of the apparent equilibrium constant for the hydrolysis of ATP to ADP and Pf at 298.15 K and 0.25 M ionic strength (see Problem 1.7).

The functions of pH and ionic strength that yield ArGj°, ArH °, and Ar/VH can also be used to plot these properties in terms of pH at a chosen ionic strength and in terms of ionic strength at a chosen pH. Figure 4.2 shows the dependence of the standard transformed Gibbs energy of the hydrolysis of ATP to ADP on pH. 

Coupling does not necessarily involve constraints in addition to element balances. For example, glucokinase (EC 2.7.1.2) couples the hydrolysis of ATP to ADP with the phosphorylation of glucose to G6P. The reaction catalyzed is... 

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