ATP (Adenosine triphosphate)

ATP is a nucleotide that performs many essential roles in the cell.

• It is the major energy currency of the cell, providing the energy for most of the energy-consuming activities of the cell.
• It is one of the monomers used in the synthesis of RNA and, after conversion to deoxyATP (dATP), DNA.
• It regulates many biochemical pathways.

Energy

When the third phosphate group of ATP is removed by hydrolysis, a substantial amount of free energy is released. The exact amount depends on the conditions, but we shall use a value of 7.3 kcal per mole.
ATP + H₂O → ADP + P_i
ADP is adenosine diphosphate. P_i is inorganic phosphate. [structure]
Because of the substantial amount of energy liberated when it is broken, the bond between the second and third phosphates is commonly described as a "high-energy" bond and is depicted in the figure by a wavy red line. (The bond between the first and second phosphates is also "high-energy".) (But please note that the term is not being used in the same sense as the term "bond energy". In fact, these bonds are actually weak bonds with low bond energies.)

Discussion of bond energy.

Cells contain a wide variety of enzymes — called ATPases — that catalyze the hydrolysis of ATP and couple the energy released to particular energy-consuming reactions in the cell (see examples below).

Synthesis of ATP

• ADP + P_i → ATP + H₂O
• requires energy: 7.3 kcal/mole
• occurs in the cytosol by glycolysis
• occurs in mitochondria by cellular respiration
• occurs in chloroplasts by photosynthesis

Consumption of ATP

ATP powers most of the energy-consuming activities of cells, such as:

• Most anabolic reactions. Examples:
  • joining transfer RNAs to amino acids for assembly into proteins [Link]
  • synthesis of nucleoside triphosphates for assembly into DNA and RNA
  • synthesis of polysaccharides
  • synthesis of fats
• active transport of molecules and ions
• nerve impulses
• maintenance of cell volume by osmosis
• adding phosphate groups (phosphorylation) to many different proteins, e.g., to alter their activity in cell signaling.
• muscle contraction
• beating of cilia and flagella (including sperm)
• bioluminescence

Extracellular ATP

In mammals, ATP also functions outside of cells. Its release

• from damaged cells can elicit inflammation and pain;
• from the carotid body signals a shortage of oxygen in the blood;
• from taste receptor cells triggers action potentials in the sensory nerves leading back to the brain;
• from the stretched wall of the urinary bladder signals when the bladder needs emptying.

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