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. Author manuscript; available in PMC: 2010 Nov 1.
Published in final edited form as: Trends Endocrinol Metab. 2009 Oct 1;20(9):460–468. doi: 10.1016/j.tem.2009.05.005

Figure 2.

Figure 2

Purinergic signaling pathways, (a) and (b) The nucleotide-hydrolyzing pathways. The extracellularly released ATP is hydrolyzed to AMP by two enzyme families, ecto-nucleotide pyrophosphate/phosphodiesterase (E-NPP) and ecto-nucleoside triphosphate diphosphohydrolase (E-NTDPase), whereas AMP was efficiently hydrolyzed by ecto-5′nucleotidase (E-5′-N), also known as CD73 family of enzymes. Adenosine is further deaminated via inosine into hypoxanthine by adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP), respectively, (c) ATP is an agonist for two transmembrane domain P2XRs and several seven transmembrane domain P2YRs, whereas ADP activates a few P2YRs but not P2XRs. Adenosine is also acts as an agonist of four G-protein coupled ARs. Three subunits in homomeric or heteromeric organization are required for functional P2XRs, whereas dimerization is possible for P2YRs and ARs.