ATP is regarded as a universal source of energy in all kinds of cells. It is produced mainly in the oxidizing of energy-rich (reduced) compounds in the course of the respiratory chain and in photosynthesis. ATP is needed:
as a source of energy for biochemical synthesesfor transport processes (active transport) andfor mechanical work like movements (ciliar movements, plasma
currents etc.)
ATP is usually present as a magnesium or a manganese salt. For
its hydrolysis are magnesium ions necessary. Whenever it is spoken
of ATP-degradation is it always the hydrolysis of the terminal
phosphate group(s) that is meant. The reactions are reversible:
orATP + H2O < > ADP + H3PO4 (= Pi)
ATP + H2O < > AMP + pyrophosphate (= PP)
ADP + H2O < > AMP + Pi
ADP an AMP are the abbreviations for adenosine diphosphate and adenosine monophosphate.
The phosphates are linked together anhydrously, the innermost phosphate residue and the sugar residue are linked by an ester bond. The hydrolysis id dependent on the pH. The delta G° is -7.3 kcal/mol (ca -30.6 kJ/mol) at pH 7 (nearly physiological conditions). It increases with a rising pH and is -10 kcal/mol (ca -42 kJ/mol) at pH 9.
Since the delta Gs for the cleavage of a pyrophosphate
and for that of one phosphate residue are roughly the same are
ATP, ADP and AMP rather easily converted into each other:
ATP + AMP < > 2 ADP
The delta G of the ATP cleavage is not very high compared to other phosphorylated compounds. Under this aspect is the term 'energy-rich linkage' irritating but is has gained acceptance in biochemical literature since the hydrolysis is easily performed (with the help of the respective enzyme) and the energy is actually useable. The reason for the rather easily cleaved linkage is in the electron accumulation at the terminal phosphate residues. Identical charges (negative here) repel each other and are in this case neutralized by the hydrolysis.
In many cases is the terminal phosphate residue cleaved off from
the ATP not given into solution as a free inorganic phosphate
but is transferred onto another molecule that is consequently
phosphorylated. This works also the other way round: a phosphorylated
compound with a delta G° > -8 kcal/mol (-34 kJ/mol)
can transfer its phosphate residue onto ADP and thus change it
to ATP.
Besides the adenosine nucleotide phosphates occur also uracil, cytosine and guanine phosphates:
UMP, UDP, UTP, CMP, CDP, CTP, GMP, GDP, GTP.
The triphosphate nucleosides of the mentioned compounds including
ATP are components of RNA. They are integrated into the polymer
by cleavage of 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
