Adenosine 5' Triphosphate
ATP is one of the five nucleotides that serve as building blocks of nucleic acids. Structurally, adenine and guanine nucleotides are purines, whereas cytosine, thymine and uracil are pirimidines. ATP is also the main energy source for cells. More recently it has been recognized that ATP, some of its metabolites, as well as some other nucleotides, play a role as extracellular signaling molecules by activating specific cell surface receptors.