Adenosine Triphosphate
(ATP), molecule found in all living organisms that is the main immediate
source of usable energy for the activities of the cells. ATP is built up by
the metabolism of foodstuffs in the cell in special compartments called
mitochondria. Because the energy-exchanging function of ATP and the catalytic
(work-boosting) function of enzymes are intimately connected, ATP is
characterized as a coenzyme. The adenosine part of the molecule is made up of
adenine, a nitrogen-containing compound (also one of the principal components
of the gene), and ribose, a five-carbon sugar. Three phosphate units (triphosphate),
each made up of one phosphorus atom and four oxygen atoms, are attached to the
ribose. The two bonds between the three phosphate groups are high-energy
bonds, that is, they are relatively weak and yield their energy readily when
split by enzymes. With the release of the end phosphate group, 7 kilocalories
(7 calories, in common usage) of energy become available for work, and the ATP
molecule becomes ADP (adenosine diphosphate). Most of the energy-consuming
reactions in cells are powered by the conversion of ATP to ADP; they include
the transmission of nerve signals, the movement of muscles, the synthesis of
protein, and cell division. Usually, ADP quickly regains the third phosphate
unit through the action of cytochrome, a protein that builds it up by using
food energy. In vertebrate muscle and brain cells, excess ATP can join with
creatine to provide a reserve energy store.
The release of two phosphate
groups from ATP by the enzyme adenyl cyclase forms AMP (adenosine
monophosphate), a nucleotide component of nucleic acids, the material of DNA;
this enzyme is important in many of the body's reactions. The American
biochemist Earl Sutherland, Jr., won the 1971 Nobel Prize in physiology or
medicine for work showing that one form of AMP called cyclic AMP, created by
the action of the enzyme adenyl cyclase, is instrumental in the activities of
many hormones, including epinephrine and ACTH.
Plants produce ATP by direct
utilization of the energy in sunlight. See Photosynthesis.
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