Adenosine Diphosphate
Adenosine diphosphate (ADP) is a nucleotide made of the base adenine, the sugar ribose and two phosphate groups. It is the product left when ATP releases energy by losing one phosphate, and the raw material from which ATP is rebuilt.
The ATP-ADP Cycle: How Is Energy Passed Around?
When ATP is hydrolyzed to ADP and inorganic phosphate, about 30 kilojoules per mole of energy are released under standard conditions, and more in the conditions inside a cell. Cells use this to drive reactions such as muscle contraction and active transport. The ADP is then recharged by adding a phosphate group, so each ATP molecule in the body is recycled many times a day, with the body turning over roughly its own weight in ATP daily.
Making ATP from ADP: Where Does the Energy Come From?
In glycolysis, enzymes transfer a phosphate from an intermediate to ADP. Most ATP is made by the enzyme ATP synthase, which uses the proton gradient of chemiosmosis during oxidative phosphorylation in mitochondria. The ratio of ATP to ADP signals the energy state of a cell and regulates many enzymes through allosteric regulation.
Other Roles: Does ADP Do More?
Outside cells, ADP is a signal. When it is released by damaged tissue or activated platelets, it binds receptors on other platelets and promotes clumping during blood clotting, which is why antiplatelet drugs such as clopidogrel block the receptor. A related molecule, ADP-ribose, is used in modifying proteins. ADP is a central part of cellular respiration and the broader topic of bioenergetics.
Adenosine Diphosphate at a Glance
| Property | Value |
|---|---|
| Parts | Adenine, ribose, two phosphates |
| Formed when | ATP loses a phosphate |
| Other role | Platelet activation signal |
See Also
- ATP — the molecule it is recycled into
- ATP Synthase — the enzyme that makes ATP from it
- Cellular Respiration — the process that recharges it