In all cells, most of the major energy changes and reaction couplings involve nucleotides. Although the cell draws on a number of different nucleotide pools,
ATP → ADP + phosphate
Exchanges are the most common. Other exchanges, such as
ATP → AMP + pyrophosphate
OTP → CDP + phosphate
Occur, for example, during lipid metabolism, as do reactions. Uridine triphosphate (UTP) is utilized in polysacharide synthesis and guanosine triphosphate (GTP) is consumed during protein synthesis. The de- oxy derivatives of the nucleotides, dATP, dGTP, dUTP, and dCTP, are used during DNA synthesis.
Regeneration of the nucleoside triphosphates is catalyzed by a relatively nonspecific nucleoside diphosphate kinase,
CTP + ADP →CTP + ADP
UDP + ATP → UTP + ADP
GTP + ADP → GTP + ADP
dCDP + ATP → dCTP + ADP
dADP + GTP →dATP + GDP
The total concentration of all nucleotides in cells that are metabolizing in a steady state is relatively constant and most of the adenylate is present in the form of ATP. When cellular activity increases, the level of ATP decreases and the combined level of ADP + AMP increases.
The changes that take place in the levels of these compounds cause an increase in reaction sequences such as those of glycolysis. Which generate ATP Conversely, when cellular activity decreases, the transient rise in ATP acts to slow down the ATP-generating systems. The regulation of ATP synthesis is achieved through feedback control of allosteric enzymes by ATP, ADP, and AMP.
Although ATP hydrolysis is a major source of free energy in cells, cells do not form substantial reservoirs of ATP. The amount of ATP present in a cell lasts for only a short time during periods of increased cellular activity. Most cells contain additional reservoirs of compounds that can quickly be consumed to generate additional ATP. For example, striated muscle, smooth muscle, and nerve cells contain reservoirs of phosphocreatine. When the ATP levels in these cells undergo a marked decline, the enzyme creatine kinase is activated and catalyzes the reaction
Phosphocreatine + ADP → creatine + ATP (AG0‘ = – 12.6 kJ/mole)
Phosphoarginine and polymetaphosphate play a similar role in invertebrate and bacterial cells, respectively.