Adenosine triphosphate (ATP) is often described as the body's "energy currency"—energy-producing metabolic reactions store their energy in the form of ATP, which can then drive energy-requiring syntheses and other reactions anywhere in the cell.
Structurally ATP consists of the purine base adenine (a complex, double-ring molecule containing five nitrogen atoms) attached to the five-carbon sugar ribose; this combination is known as adenosine. Attaching a string of three connected phosphate groups to the ribose produces ATP. Schematically, one may depict the structure of ATP as Ad-Ph-Ph-Ph, where Ad is adenosine and Ph is a phosphate group. If only two phosphate groups are attached, the resulting compound is adenosine diphosphate (ADP).
The final step in almost all the body's energy-producing mechanisms is attachment of the third phosphate group to ADP. This new phosphate-phosphate bond, known as a high-energy bond, effectively stores the energy that has been produced. The ATP then diffuses throughout the cell, eventually reaching sites where energy is needed for such processes as protein synthesis or muscle cell contraction. At these sites, enzyme mechanisms couple the energy-requiring processes to the breakdown of ATP's high-energy bond. This regenerates ADP and free phosphate, both of which diffuse back to the cell's energy-producing sites and serve as raw materials for production of more ATP.
The ATP-ADP couple is thus analogous to a rechargeable storage battery, with energy production sites representing the battery charger. ATP is the fully charged battery that can supply energy to a flashlight or transistor radio. ADP is the used battery that is returned for charging.
ADP is not a fully drained battery, however. It still possesses one high-energy phosphate-phosphate bond. When energy is short and ATP is scarce, the second phosphate can be transferred from one ADP to another. This creates a new ATP molecule, along with one of adenosine monophosphate (AMP). Since the "fully drained" AMP will probably be broken down and disposed of, however, this mechanism represents an emergency response that is inhibited when ATP is plentiful.
ATP is also a building block in DNA synthesis, with the adenosine and one phosphate being incorporated into the growing helix. (The "A" in ATP is the same as in the A-C-G-T "alphabet" of DNA.) This process differs from most other ATP-using reactions, since it releases two phosphate groups—initially still joined, but soon separated. With very little pyrophosphate (Ph-Ph) available in the cell, the chance that it will break the DNA chain and again form—though all enzyme reactions are theoretically reversible—is effectively infinitesimal. Since breaking the DNA chain would probably kill the cell, what at first might appear to be energy wastage turns out to be quite worthwhile. The cell also converts ATP to AMP and pyrophosphate in a few other cases where the reaction must always go only in a single direction.
See also Metabolism.