7.2Free Energy Change and ATP as the Currency of Biological Energy
In cellular activities, thermal, mechanical, electrical and other types of energy are used, and these are generally exchanged using a material called ATP (short for adenosine 5'-triphosphate) consisting of a nucleobase called adenine, ribose and three phosphates (see Fig. 2-5 in Chapter 2). These three phosphates form a high-energy compound through dehydration condensation. High energy in this context means that more free energy can be obtained from the hydrolysis of the terminal phosphate group in ATP than from the hydrolysis of other phosphate compounds. The hydrolysis reaction of ATP is expressed as follows:
ATP + H2O → ADP + Phosphate
∆G˚’ = - 30.5 kJ/mol
This large change in free energy is believed to stem from the repulsion of the four negative charges in ATP being reduced to three negative charges.
In cells, energy obtained through the oxidative degradation of nutrients (mainly glucose) is stored as ATP molecules, and when energy is required for activity such as movement or the synthesis of high molecular compounds, ATP is hydrolyzed. In addition to chemical actions (material synthesis), electrical and mechanical actions within cells are also mediated by ATP, which can therefore be thought of as the “currency” of biological energy.
In cells, reducing power is also stored in the form of molecules such as NADH and NADPH*1. While NADH generates ATP via oxidative phosphorylation (see Chapter 8), these two reducing substances are also used to mediate the oxidization and reduction of various materials through metabolic pathways.