ADP is a nucleotide produced as a result of hydrolysis of ATP in the most common energy-yielding reaction of cells. ADP participates in substrate-level phosphorylation, oxidative phosphorylation, and photosynthetic phosphorylation. It is not possible to list here all of the enzymes interacting with ADP. Metabolism of ADP is shown below:

1. ADP <=> ATP + AMP (catalyzed by adenylate kinase).

2. GMP + ATP <=> GDP + ADP (catalyzed by guanylate kinase).

3. NDP + ATP <=> NTP + ADP (catalyzed by nucleoside diphosphokinase).

4. ADP + NADPH <=> dADP + NADP⁺ (catalyzed by ribonucleotide reductase).

ADP is transferred into the mitochondrial matrix by adenine nucleotide translocase and may be a limiting reagent in oxidative phosphorylation.