Although ATP has a special role as the energy currency of cells, other nucleoside diphosphates have equally high energies of hydrolysis and are used in some cases to drive endergonic reactions. GTP, for example, is used in protein synthesis.

Because ATP is used so predominantly in cells, however, several mechanisms exist for the exchange of high energy phosphates either to make ATP from ADP or to make NTPs from NDPs.

One such example is the enzyme nucleoside diphosphate kinase, which catalyzes the following reaction:

ATP + CDP (or GDP or UDP) <=> ADP + CTP (or GTP or UTP)

In the forward reaction, a high energy phosphate from ATP is used to make an NTP. In the reverse reaction, a high energy phosphate from an NTP is used to make an ATP.

Adenylate kinase catalyzes the following reaction:

AMP + ATP <=> 2 ADP

In the forward reaction, a high energy phosphate from ATP is used to convert AMP to ADP. The reverse reaction, however, can be used to make ATP from ADP.