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ATP and Other Nucleoside Triphosphates or: Bonds Rich in Energy


ATP is regarded as a universal source of energy occuring in all cell types. It is produced mainly during the oxidation of energy-rich (reduced) compounds processed in the respiratory chain and in photosynthesis. ATP is needed



as a source of energy for biochemical syntheses
for transport processes (active transport) and
for mechanical work like movements (ciliar movements, plasma currents etc.)


ATP occurs usually as a magnesium or a manganese salt. For its hydrolysis, magnesium ions are necessary. Whenever it is spoken of ATP-degradation, it is always the hydrolysis of the terminal phosphate group(s) that is meant. The reactions are reversible:

ATP + H2O < > ADP + H3PO4 (= Pi)

or

ATP + H2O < > AMP + pyrophosphate (= PP)

ADP + H2O < > AMP + Pi

ADP and AMP are the abbreviations for adenosine diphosphate and adenosine monophosphate.

The phosphates are linked anhydrously, the innermost phosphate residue and the sugar residue are linked by an ester bond. Hydrolysis depends on the pH. The delta G° is -7.3 kcal/mol (ca -30.6 kJ/mol) at pH 7, i.e. at almost physiological conditions. It increases with a rising pH and is -10 kcal/mol (ca -42 kJ/mol) at pH 9.

Since the delta G° for the breakdown of a pyrophosphate and for that of one phosphate residue are roughly the same, ATP, ADP and AMP can rather easily be converted into each other:

ATP + AMP < > 2 ADP

The delta G of the ATP breakdown is not very high compared to other phosphorylated compounds. Under this aspect, the term 'energy-rich linkage' seems irritating, but it has gained acceptance in biochemical literature as its hydrolysis is easily performed (with the help of the respective enzyme) and the energy is actually useable. The reason for the rather easily broken down linkage is in the electron accumulation at the terminal phosphate residues. Identical charges (here they are negative) repel each other and are in this case neutralized by hydrolysis.

In many cases, the terminal phosphate residue that is cleaved off from the ATP is not given away into solution as a free inorganic phosphate, but is transferred onto another molecule that becomes consequently phosphorylated. This process works also the other way round: a phosphorylated compound with a delta G° > -8 kcal/mol (-34 kJ/mol) can transfer its phosphate residue to ADP that as a consequence becomes ATP.



Besides the adenosine nucleotide phosphates, uracil, cytosine and guanine phosphates occur, too:

UMP, UDP, UTP, CMP, CDP, CTP, GMP, GDP, GTP.

The triphosphate nucleosides of these compounds and those of ATP are components of RNA. They are integrated into the polymer by splitting off pyrophosphate ( = PP). The corresponding desoxyribose derivatives (dATP, dGTP, dCTP....) are necessary for DNA synthesis, where dTTP is used instead of dUTP. The terminal phosphate residues of all nucleoside di- and triphosphates are equally rich in energy. The energy set free by their hydrolysis is used for biosyntheses. They share the work equally: UTP is needed for the synthesis of polysaccharides, CTP for that of lipids and GTP for the synthesis of proteins and other molecules. These specificities are the results of the different selectivities of the enzymes, that control each of these metabolic pathways.


© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de