Hydrolysis of ATP

The key reacfion m bioenergelics is fhe mlerconversion of ATP and ADP usually expressed m terms of fhe hydrolysis of ATP... 

Nevertheless the biosynthesis of glutamine proceeds from glutamic acid The dif ference is that the endergonic process m Equation 1 is coupled with the strongly exer gome hydrolysis of ATP... 

Many formally endergonic biochemical processes become exergomc when they are coupled mechanistically to the hydrolysis of ATP... 

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... 

FIGURE 3.8 The activation energies for phosphoryl group-transfer reactions (200 to 400 kj/mol) are substantially larger than the free energy of hydrolysis of ATP ( — 30.5 kj/mol). 

Similarly, the release of free energy that occurs upon the hydrolysis of ATP and other high-energy phosphates can be treated quantitatively in terms of group transfer. It is common to write for the hydrolysis of ATP... 

For the phosphoric anhydrides, and for most of the high-energy compounds discussed here, there is an additional entropic contribution to the free energy of hydrolysis. Most of the hydrolysis reactions of Table 3.3 result in an increase in the number of molecules in solution. As shown in Figure 3.11, the hydrolysis of ATP (as pH values above 7) creates three species—ADP, inorganic phosphate (Pi), and a hydrogen ion—from only two reactants (ATP and HgO). The entropy of the solution increases because the more particles, the more disordered the system. (This effect is ionization-dependent because, at low pH, the... 

So far, as in Equation (3.33), the hydrolyses of ATP and other high-energy phosphates have been portrayed as simple processes. The situation in a real biological system is far more complex, owing to the operation of several ionic equilibria. First, ATP, ADP, and the other species in Table 3.3 can exist in several different ionization states that must be accounted for in any quantitative analysis. Second, phosphate compounds bind a variety of divalent and monovalent cations with substantial affinity, and the various metal complexes must also be considered in such analyses. Consideration of these special cases makes the quantitative analysis far more realistic. The importance of these multiple equilibria in group transfer reactions is illustrated for the hydrolysis of ATP, but the principles and methods presented are general and can be applied to any similar hydrolysis reaction. 

The Effect of Metal Ions on the Free Energy of Hydrolysis of ATP... 

FIGURE 3.16 The pH dependence of the free energy of hydrolysis of ATP. Because pH varies only slightly in biological environments, the effect on AG is nsnally small. 

FIGURE 3.17 The free energy of hydrolysis of ATP as a function of total ion concen-... 

If the cellular free energy of hydrolysis of ATP is taken as — 50 kj/mol, the free energy available from the hydrolysis of a single ATP molecule is... 

When coupled (by phosphofructokinase) with the hydrolysis of ATP, the overall reaction (Figure 19.7) is strongly exergonic ... 

In the kidney and in muscle tissues, fructose is readily phosphorylated by hexokinase, which, as pointed out above, can utilize several different hexose substrates. The free energy of hydrolysis of ATP drives the reaction forward ... 

FIGURE 24.7 The acyl-CoA synthetase reaction activates fatty acids for /3-oxidation. The reaction is driven by hydrolysis of ATP to AMP and pyrophosphate and by the subsequent hydrolysis of pyrophosphate. 

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