Adenosine diphosphate (ADP) is an organic molecule involved in the central part of the metabolism of the cell that generates energy. It is interconvertible with the cell’s energy currency, adenosine triphosphate (ATP). The conversion back and forth between these compounds is critical for maintaining energy levels in the cell.
ATP has three phosphate groups that all have high-energy bonds. ATP is formed by the energy-intensive process of the addition of an inorganic phosphate group to ADP, which has two high-energy phosphate bonds. This creates an energy store for the cell. The cleavage of the phosphate bond from ATP, to produce ADP, releases energy that can be used in the cell’s metabolism.
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The interconversion of ATP and ADP takes place in various aspects of cellular metabolism. For instance, in aerobic respiration, carbohydrates are broken down, in the presence of oxygen, into carbon dioxide and water. The first stage of this process takes place in the cell’s furnaces — the mitochondria. As glucose is degraded, the process generates ADP molecules. They end up being converted to ATP to supply the major source of energy for the cells.
Another process in which ATP and ADP act together to supply energy is in the fast twitch of muscle cells. ATP supplies the initial energy that muscles use to contract. A high-energy phosphate bond is donated to ADP, to make ATP, to power the muscle fibers in the few seconds it takes before the normal energy sources are activated.
In mammals, ADP is involved in blood platelet activation, and is stored within platelets. There is a series of adenosine diphosphate receptors found on the platelets, and when ADP interacts with them, there is further activation of the platelets. When the platelets are activated, they disrupt the process of bleeding by forming plugs. This activation is stopped by the degradation of ADP in the blood.
There are pharmaceutical adenosine diphosphate receptor inhibitors that slow the activation of the platelets. This inhibits the formation of blood clots in several vascular diseases, and in coronary artery disease. One such inhibitor is clopidogrel, better known as Plavix®. There is research towards developing new receptor inhibitors.
Adenosine diphosphate is a nucleotide composed of adenosine, which is comprised of the base adenine and the sugar ribose. ADP also has the two phosphate groups. The basic structure of this molecule is similar to a base that is part of DNA or RNA.
A derivative of ADP is adenosine diphosphate ribose, in which the sugar ribose is linked to the final phosphate of ADP by an ester linkage. This compound is linked into chains by an enzyme known as poly(ADP ribose)polymerase (PARP) that has been traditionally thought to be involved in programmed cell death and DNA repair, but is turning out to have a much broader range of biological activities. ADP ribose can be transferred to proteins to regulate them.
To fully discuss the role of ADP in biology, one should also mention photosynthesis in plants. They absorb the energy from sunlight and transfer it to ADP and inorganic phosphate, generating a molecule of ATP. This ATP is used in the process of generating glucose from carbon dioxide, starting the process of producing the plant food we consume.