Adenosine triphosphate

Adenosine triphosphate (ATP) has been described as the body’s energy currency—energy-producing metabolic reactions store their energy in the form of ATP, which can then drive energy-requiring syntheses and other reactions anywhere in the cell. The energy for these activities is obtained when a phosphate group is removed from ATP to form adenosine diphosphate (ADP).

Structurally, ATP consists of the purine base designated adenine (a complex, double-ring molecule

containing five nitrogen atoms) attached to the five-carbon sugar ribose; this combination is known as adenosine. Attaching a string of three connected phosphate groups to the ribose produces ATP. Schematically, one may depict the structure of ATP as Ad-Ph-Ph-Ph, where Ad is adenosine and Ph is a phosphate group. If only two phosphate groups are attached, the resulting compound is adenosine diphosphate (ADP).

The final step in almost all the body’s energy-producing mechanisms is attachment of the third phosphate group to ADP. This new phosphate-phosphate bond, known as a high-energy bond, effectively stores the energy that has been produced. The ATP then diffuses throughout the cell, eventually reaching sites where energy is needed for such processes as protein synthesis or muscle cell contraction. At these sites, enzyme mechanisms couple the energy-requiring processes to the breakdown of ATP’s high-energy bond. This regenerates ADP and free phosphate, both of which diffuse back to the cell’s energy-producing sites and serve as raw materials for production of more ATP.

The ATP-ADP couple is analogous to a rechargeable storage battery, with energy production sites representing the battery charger. ATP is the fully charged battery that can supply energy to a flashlight or transistor radio. ADP is the used battery that is returned for charging.

The analogy breaks down somewhat, as ADP is not a fully drained battery, however. It still possesses one high-energy phosphate-phosphate bond. When energy is short and ATP is scarce, the second phosphate can be transferred from one ADP to another. This creates a new ATP molecule, along with one of adenosine monophosphate (AMP). Since the “fully drained” AMP will probably be broken down and disposed of, however, this mechanism represents an emergency response that is inhibited when ATP is plentiful.

ATP is also a building block in DNA synthesis, with the adenosine and one phosphate being incorporated into the growing helix. (The “A” in ATP is the same as in the A-C-G-T “alphabet” of DNA.) This process differs from most other ATP-using reactions, since it releases two phosphate groups—initially still joined, but soon separated. With very little pyrophosphate (Ph-Ph) available in the cell, the chance that it will break the DNA chain and form again—though all enzyme reactions are theoretically reversible—is effectively infinitesimal. Since breaking the DNA chain would probably kill the cell, what at first might appear to be energy wastage turns out to be quite worthwhile. The cell also converts ATP to AMP and pyrophosphate in a few other cases where the reaction must always go only in a single direction.

See also Metabolism.

Adenosine triphosphate

Adenosine triphosphate (ATP) is often described as the body's "energy currency"—energy-producing metabolic reactions store their energy in the form of ATP, which can then drive energy-requiring syntheses and other reactions anywhere in the cell .

Structurally ATP consists of the purine base adenine (a complex, double-ring molecule containing five nitrogen atoms ) attached to the five-carbon sugar ribose; this combination is known as adenosine. Attaching a string of three connected phosphate groups to the ribose produces ATP. Schematically, one may depict the structure of ATP as Ad-Ph-Ph-Ph, where Ad is adenosine and Ph is a phosphate group. If only two phosphate groups are attached, the resulting compound is adenosine diphosphate (ADP).

The final step in almost all the body's energy-producing mechanisms is attachment of the third phosphate group to ADP. This new phosphate-phosphate bond, known as a high-energy bond, effectively stores the energy that has been produced. The ATP then diffuses throughout the cell, eventually reaching sites where energy is needed for such processes as protein synthesis or muscle cell contraction. At these sites, enzyme mechanisms couple the energy-requiring processes to the breakdown of ATP's high-energy bond. This regenerates ADP and free phosphate, both of which diffuse back to the cell's energy-producing sites and serve as raw materials for production of more ATP.

The ATP-ADP couple is thus analogous to a rechargeable storage battery , with energy production sites representing the battery charger. ATP is the fully charged battery that can supply energy to a flashlight or transistor radio. ADP is the used battery that is returned for charging.

ADP is not a fully drained battery, however. It still possesses one high-energy phosphate-phosphate bond. When energy is short and ATP is scarce, the second phosphate can be transferred from one ADP to another. This creates a new ATP molecule, along with one of adenosine monophosphate (AMP). Since the "fully drained" AMP will probably be broken down and disposed of, however, this mechanism represents an emergency response that is inhibited when ATP is plentiful.

ATP is also a building block in DNA synthesis , with the adenosine and one phosphate being incorporated into the growing helix. (The "A" in ATP is the same as in the A-C-G-T "alphabet" of DNA.) This process differs from most other ATP-using reactions, since it releases two phosphate groups—initially still joined, but soon separated. With very little pyrophosphate (Ph-Ph) available in the cell, the chance that it will break the DNA chain and again form—though all enzyme reactions are theoretically reversible—is effectively infinitesimal. Since breaking the DNA chain would probably kill the cell, what at first might appear to be energy wastage turns out to be quite worthwhile. The cell also converts ATP to AMP and pyrophosphate in a few other cases where the reaction must always go only in a single direction.

See also Metabolism.

adenosine triphosphate (ATP): sometimes called ‘the spark of life’. Any bodily movement powered by voluntary or involuntary muscle; other cellular movements such as the migration of white blood cells; the swimming of sperm; and even the contraction of hair cells in the inner ear need a supply of free energy. But so too does the transport of molecules from one body compartment to another, and the synthesis of all biomolecules required for growth, repair, or maintenance of bodily functions. ATP is an energy-rich molecule, which releases free energy when it is broken down to either ADP (adenosine diphosphate) or AMP (adenosine monophosphate). This reaction is usually stimulated by enzymes collectively called ATP-ases. Even a sedentary adult breaks down 40 kg/day of ATP and the rate of consumption rises to 0.5 kg/minute during strenuous exercise. But cells do not store large amounts of ATP, so it must be continually replenished. The energy that must be put back to reconstruct ATP from ADP is supplied by the oxidation (burning) of foods.

Alan W. Cuthbert

adenosine triphosphate (ATP) High-energy phosphoric ester (i.e. nucleotide) of the nucleoside adenosine, which functions as the principal energy-carrying compound in the cells of all living organisms. Its hydrolysis to ADP (adenosine diphosphate) and inorganic phosphate is accompanied by the release of a relatively large amount of free energy (34kJ/mol at pH 7) which is used to drive many metabolic functions.

adenosine triphosphate (ATP) A high-energy phosphoric ester, or nucleotide, of the nucleoside adenosine which functions as the principal energy-carrying compound in the cells of all living organisms. Its hydrolysis to ADP and inorganic phosphate is accompanied by the release of a relatively large amount of free energy (34 kJ/mol at pH 7) which is used to drive many metabolic functions.