Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine.
ADP is the product of ATP dephosphorylation by ATPases. ADP is converted back to ATP by ATP synthases. ATP is an important energy transfer molecule in cells.
ADP is stored in dense bodies inside blood platelets and is released upon platelet activation. ADP interacts with a family of ADP receptors found on platelets (P2Y1, P2Y12 and P2X1), leading to further platelet activation. ADP in the blood is converted to adenosine by the action of ecto-ADPases, inhibiting further platelet activation via adenosine receptors.
ADP is the end-product that results when ATP loses one of its phosphate groups located at the end of the molecule. The conversion of these two molecules plays a critical role in supplying energy for many processes of life. The deletion of one of ATP’s phosphorus bonds generates approximately 31 kilojoules per Mole of ATP (7.3 kcal). ADP can be converted, or powered back to ATP through the process of releasing the chemical energy available in food; in humans this is constantly performed via aerobic respiration in the mitochondria. Plants use photosynthetic pathways to convert and store the energy from sunlight, via conversion of ADP to ATP. Animals use the energy released in the breakdown of glucose and other molecules to convert ADP to ATP, which can then be used to fuel necessary growth and cell maintenance.
Single nucleotides (ADP) have the ability to catalyze organic reactions. This has relevance for prebiotic studies of the RNA world hypothesis for the origin of life on Earth.