adenosine triphosphate(redirected from ATP/ADP cycle)
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adenosine triphosphate (ATP) (ədĕnˈəsēn trīˌfŏsˈfāt), organic compound composed of adenine, the sugar ribose, and three phosphate groups. ATP serves as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of proteins. It is broken down by hydrolysis to yield adenosine diphosphate (ADP), inorganic phosphorus, and energy. ADP can be further broken down to yield adenosine monophosphate (AMP), additional phosphorus, and more energy. When the phosphorus and energy are immediately used to drive other reactions, such as the synthesis of uridine diphosphate (UDP), an RNA precursor, from uridine monophosphate (UMP), the pair of reactions are said to be coupled. New ATP is produced from AMP using the energy released from the breakdown of fuel molecules, such as fats and sugars.
Extracellularly, ATP has been found to act as a neurotransmitter. ATP receptors are widespread through the body. On its own it is known to have effects in the arteries, intestines, lungs, and bladder. It is also often released in tandem with other neurotransmitters, perhaps to add chemical stability. See phosphorylation.
Adenosine triphosphate (ATP)
A coenzyme and one of the most important compounds in the metabolism of all organisms, since it serves as a coupling agent between different enzymatic reactions. Adenosine triphosphate is adenosine diphosphate (ADP) with an additional phosphate group attached through a pyrophosphate linkage to the terminal phosphate group (see illustration). ATP is a powerful donor of phosphate groups to suitable acceptors because of the pyrophosphate nature of the bonds between its three phosphate radicals. For instance, in the phosphorylation of glucose, which is an essential reaction in carbohydrate metabolism, the enzyme hexokinase catalyzes the transfer of the terminal phosphate group.
ATP serves as the immediate source of energy for the mechanical work performed by muscle. In its presence, the muscle protein actomyosin contracts with the formation of adenosine diphosphate and inorganic phosphate. ATP is also involved in the activation of amino acids, a necessary step in the synthesis of protein. See Muscle
In metabolism, ATP is generated from adenosine diphosphate and inorganic phosphate mainly as a consequence of energy-yielding oxidation-reduction reactions. In respiration, ATP is generated during the transport of electrons from the substrate to oxygen via the cytochrome system. In photosynthetic organisms, ATP is generated as a result of photochemical reactions. See Carbohydrate metabolism, Cytochrome
By virtue of its energy-rich pyrophosphate bonds, ATP serves as a link between sources of energy available to a living system and the chemical and mechanical work which is associated with growth, reproduction, and maintenance of living substance. For this reason, it has been referred to as the storehouse of energy of living systems. Because ATP, ADP, and adenylic acid are constantly interconverted through participation in various metabolic processes, they act as coenzymes for the coupled reactions in which they function. See Biochemistry, Coenzyme, Metabolism