In this article we will discuss about the Determination of ΔG°’ for Standard Free Energy Change.
Determination of Standard Free Energy of Hydrolysis of ATP:
Hydrolysis of ATP takes place according to the equation
From which it seems possible to determine the equilibrium constant at pH 7.0 and calculate ΔG° from the equation
ΔG°’ = – 2.303 RT log K’eq
However, the equilibrium constant of such a reaction can hardly be measured directly because the reaction is so much directed in the direction of hydrolysis, it becomes difficult to arrive at the exact equilibrium constants of the compounds. This means that for a reaction like this with a large free energy change (either negative or positive), there is a problem in determining ΔG°’.
It is, however possible to break up the large free energy change for the hydrolysis of ATP into two energetically smaller steps and then add the ΔG°’ values of consecutive reactions. For example, ATP is first allowed to react with glucose to form ADP and glucose-6-phosphate by the enzyme hexokinase
The difference in the energy values of hydrolysis of these phosphorylated compounds is due to the fact that the bonds between the adjacent phosphate groups of ADP and ATP are anhydride linkages having much larger negative standard free energy change of hydrolysis, whereas the bond between phosphoric acid and ribose in AMP is an ester linkage with much less free energy.
The standard free energy of hydrolysis of a number of important cellular phosphate compounds including ATP is shown in Table 9.4.
We see that ATP has a ΔG°’ value of – 7.3 kcal mole-1 which is an intermediate value in this thermodynamic scale (Table 9.4). The significance of ATP placed about midway on the energy scale of phosphate compounds is that ATP is able to serve as a common intermediate in most enzyme- catalysed phosphate group transfer reactions in the cell.
For example, ADP can accept a phosphate from a phosphorylated compound higher on the scale such as phosphoenolpyruvate, and the ATP so formed can then donate it to an acceptor molecule like glucose lower on the scale to produce a phosphate compound.
It is quite apparent from the Table 9.4 that the values of standard free energy of hydrolysis of different phosphorylated compounds differ from each other.
As it happens, the standard free energy of hydrolysis of the terminal phosphate group of ATP is much more negative (ΔG°’ = – 7.3 kcal mole-1) than the standard free energy of hydrolysis of glucose-6-phosphate to yield glucose and phosphate, which is only – 3.3 kcal mole -1.
A few factors may be held responsible for this difference. The phosphate ion in ATP near pH 7.0 is a resonance hybrid, which is stabilized by resonance energy. The difference in resonance energy between the reactants and products is greater for hydrolysis of ATP than for hydrolysis for glucose-6-phosphate which is not a resonance hybrid.
Furthermore, the products of ATP hydrolysis, viz., the anions HPO2-4 and ADP3- are both negatively charged and consequently they will repel each other, and thus will not recombine to form ATP. On the contrary, hydrolysis of glucose-6-phosphate will yield glucose, which is an uncharged molecule and phosphate, a charged ion.
As a result, glucose and phosphate never repel each other, and obviously, there will be a greater chance of their recombination. Moreover, the terminal phosphate atoms of ATP, which are present in anhydride-linkages, are more susceptible to hydrolysis as compared to glucose-6- phosphate, which is a simple ester.
At the same time, ADP and phosphate are more strongly hydrated than ATP itself, which is another factor for providing a pull to ATP hydrolysis farther in the direction of completion.
(a) Hydrolysis of ATP into AMP and Pyrophosphate:
Generally, it is seen that ADP is the product of several ATP-requiring reactions in the cell and it is also the phosphate acceptor in the energy-yielding reactions of glycolysis and oxidative phosphorylation. But in some ATP-utilizing reactions, two terminal phosphate groups of ATP are enzymatically cleaved as pyrophosphate (PPi) with AMP as the other product.
This reaction is termed a pyrophosphate cleavage of ATP, in contrast to the usual orthophosphate cleavage. The standard free energy change, the decrease in free energy ΔG°’ for the reaction
ATP + H2O → AMP + PPi
is – 10.0 kcal mole-1
A typical example is seen in the activation of fatty acid molecule which occurs by the formation of an ester with coenzyme A, i.e., fatty acyl CoA, an energy rich intermediate.
Pyrophosphate cleavage of ATP sometimes provides an additional thermodynamic pull towards completion of a biochemical reaction. This is achieved by two auxiliary enzymes, pyrophosphatase and adenylate kinase. Pyrophosphatase catalyzes the hydrolysis of pyrophosphate (PPi) to yield two molecules of orthophosphate.
This large negative free energy of hydrolysis can be utilized to secure the products of a biochemical reaction. The other enzyme, adenylate kinase catalyzes the re-phosphorylation of AMP to ADP in the reaction
From these two precursors like ADP and Pi, ATP can be regenerated.
(b) Transfer of Cellular Energy through Other Nucleoside 5′-Triphosphates:
Although the ATP-ADP cycle functions as the main carrier of energy in cellular metabolism, the 5′-diphosphates and 5′-triphosphates of other ribonucleosides and deoxyribonucleosides also participate in cellular energy transfer processes. For example, uridine triphosphate (UTP) is the energy donor in polysaccharide synthesis, and GTP is required for protein synthesis.
Similarly, cytidine triphosphate (CTP) acts as the energy donor in lipid biosynthesis. We also see that various ribonucleoside triphosphates (CTP, GTP, UTP, ATP) serve as precursors for RNA synthesis, whereas deoxyribonucleoside triphosphates (dATP. dGTP, dTTP, dCTP) are materials for DNA biosynthesis.