A coenzyme and one of the most important compounds in the metabolism of all organisms, since it serves as a coupling agent between different enzymatic reactions. Adenosine triphosphate is adenosine diphosphate (ADP) with an additional phosphate group attached through a pyrophosphate linkage to the terminal phosphate group (see illustration). ATP is a powerful donor of phosphate groups to suitable acceptors because of the pyrophosphate nature of the bonds between its three phosphate radicals. For instance, in the phosphorylation of glucose, which is an essential reaction in carbohydrate metabolism, the enzyme hexokinase catalyzes the transfer of the terminal phosphate group.
ATP serves as the immediate source of energy for the mechanical work performed by muscle. In its presence, the muscle protein actomyosin contracts with the formation of adenosine diphosphate and inorganic phosphate. ATP is also involved in the activation of amino acids, a necessary step in the synthesis of protein.
In metabolism, ATP is generated from adenosine diphosphate and inorganic phosphate mainly as a consequence of energy-yielding oxidation-reduction reactions. In respiration, ATP is generated during the transport of electrons from the substrate to oxygen via the cytochrome system. In photosynthetic organisms, ATP is generated as a result of photochemical reactions.
By virtue of its energy-rich pyrophosphate bonds, ATP serves as a link between sources of energy available to a living system and the chemical and mechanical work which is associated with growth, reproduction, and maintenance of living substance. For this reason, it has been referred to as the storehouse of energy of living systems. Because ATP, ADP, and adenylic acid are constantly interconverted through participation in various metabolic processes, they act as coenzymes for the coupled reactions in which they function.