The Cell's Energy Currency

The hydrolysis of ATP to yield ADP and phosphate is highly exergonic. This loss of free energy is due to the structure of the phosphoanhydride, which involves two negatively charged groups being brought into close proximity. Additionally, the phosphate group is stabilized by resonance not available to the anhydride (see Figure 1).


    

                                       Figure 1

Because the free energy of hydrolysis of ATP's first two phosphates is so highly negative, biochemists often use the shorthand term high energy phosphate to describe the role of ATP in the cell. In general, the reactions of catabolism lead to the synthesis of ATP from ADP and phosphate. Anabolic reactions, as well as the other reactions involved in cellular maintenance, use the coupled hydrolysis of ATP to drive the reactions. For example, a muscle fiber will metabolize glucose to synthesize ATP. The ATP can be used to drive muscle contraction, to synthesize proteins, or to pump Ca 2+ ions out of the intracellular space. ATP serves as cellular energy currency because it is a common component of many reactions. It serves this role so well because it is metastable: In the cell, it does not break down extensively by itself over time (kinetic stability), but at the same time, it releases large amounts of free energy when it is hydrolyzed to release inorganic phosphate (thermodynamic instability).

All the free energy calculations shown in the previous examples have been done in the standard state, with all the products and reactants present at 1M concentration. However, very few compounds, except perhaps water, are present in the standard state. Because the free energy change of a reaction under nonstandard concentration is dependent on the concentrations of products and reactants, the actual ΔG of the reaction of glucose and ATP will be given by the equation:




The ratio of ATP to ADP is kept very high, greater than 10 to 1, so the actual ΔG of ATP hydrolysis is probably greater than 10 kcal/mole. This means that the reaction of ATP and glucose is even more favored than it would be under the standard state.

LeChatelier's Principle is fundamental to understanding these relationships. A reaction is favored if the concentration of reactants is high and the concentration of products is low. The free energy relationships shown in this section are a quantitative way of expressing this qualitative observation.