ATP as the Energy Currency of the Cell
Reactions that are endergonic require the input of energy. Cells must perform many endergonic reactions in order to remain alive; protein synthesis and DNA replication are just two examples. While enzymes are required to catalyze these reactions, they cannot provide the energy to drive them. Therefore, all endergonic reactions must be coupled to energy-releasing, or exergonic, reactions in order to proceed. While cells can ingest energy containing molecules in many forms (monosaccharides, fatty acids, amino acids, etc.), the energy contained in these molecules must be harvested and stored in a usable form by the cell. In almost all cases, this usable form of energy is the molecule ATP (adenosine triphosphate), the triphosphate form of the DNA nucleotide adenosine. When the bond linking the terminal phosphate to the rest of the molecule is hydrolyzed (broken), ADP (adenosine diphosphate) and free phosphate are the products, and energy is released. Thus, this is an exergonic reaction. Many enzymes that catalyze endergonic reactions associate with ATP, and often contain ATPase (ATP hydrolyzing) activity in addition to their other catalytic capabilities. Thus, it is ATP that directly provides the energy required by enzymes to catalyze endergonic reactions. This intimate association of exergonic ATP hydrolysis with endergonic reactions is referred to as coupling of reactions. The next chapter addresses the process by which cells transform the chemical energy of food molecules into energy stored as ATP.
Adenosine triphosphate (ATP)