Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbone attached to adenine and two phosphate groups bonded to the 5-carbon atom of ribose. The diphosphate group of ADP is attached to the 5' carbon of the sugar backbone, while the adenine attaches to the 1' carbon.

ADP can be interconverted to adenosine triphosphate (ATP) and adenosine monophosphate (AMP). ATP contains one more phosphate group than ADP, while AMP contains one fewer phosphate group. Energy transfer used by all living things is a result of dephosphorylation of ATP by enzymes known as ATPases. The cleavage of a phosphate group from ATP results in the coupling of energy to metabolic reactions and a by-product of ADP.

Breaking one of ATP's phosphorus bonds generates approximately 30.5 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. During the payoff phase of glycolysis, the enzymes phosphoglycerate kinase and pyruvate kinase facilitate the addition of a phosphate group to ADP by way of substrate-level phosphorylation.

thumb|class=skin-invert-image|Glycolysis overview

Glycolysis

Glycolysis is performed by all living organisms and consists of 10 steps. The net reaction for the overall process of glycolysis is:

:Glucose + 2 NAD+ + 2 P<sub>i</sub> + 2 ADP → 2 pyruvate + 2 ATP + 2 NADH + 2 H<sub>2</sub>O

Steps 1 and 3 require the input of energy derived from the hydrolysis of ATP to ADP and P<sub>i</sub> (inorganic phosphate), whereas steps 7 and 10 require the input of ADP, each yielding ATP. The enzymes necessary to break down glucose are found in the cytoplasm, the viscous fluid that fills living cells, where the glycolytic reactions take place. It is only in step 5, where GTP is generated, by succinyl-CoA synthetase, and then converted to ATP, that ADP is used (GTP + ADP → GDP + ATP).

<!-- Deleted image removed: thumbnail|chemical conversion of GTP to ATP -->

Oxidative phosphorylation

Oxidative phosphorylation produces 26 of the 30 equivalents of ATP generated in cellular respiration by transferring electrons from NADH or FADH2 to O<sub>2</sub> through electron carriers. The energy released when electrons are passed from higher-energy NADH or FADH2 to the lower-energy O<sub>2</sub> is required to phosphorylate ADP and once again generate ATP. It is this energy coupling and phosphorylation of ADP to ATP that gives the electron transport chain the name oxidative phosphorylation. that aid in the electron transport chain's ability to produce a proton gradient across the inner mitochondrial membrane. The ATP synthase complex exists within the mitochondrial membrane (F<sub>O</sub> portion) and protrudes into the matrix (F<sub>1</sub> portion). The energy derived as a result of the chemical gradient is then used to synthesize ATP by coupling the reaction of inorganic phosphate to ADP in the active site of the ATP synthase enzyme; the equation for this can be written as ADP + P<sub>i</sub> → ATP.

Blood platelet activation

Under normal conditions, small disk-shape platelets circulate in the blood freely and without interaction with one another. 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), which leads to platelet activation.

  • P2Y1 receptors initiate platelet aggregation and shape change as a result of interactions with ADP.
  • P2Y12 receptors further amplify the response to ADP and draw forth the completion of aggregation.

ADP in the blood is converted to adenosine by the action of ecto-ADPases, inhibiting further platelet activation via adenosine receptors.

See also

  • Nucleoside
  • Nucleotide
  • DNA
  • RNA
  • Oligonucleotide
  • Apyrase
  • Phosphate
  • Adenosine diphosphate ribose

References