Receptor for ADP and ATP coupled to G-proteins that inhibit the adenylyl cyclase second messenger system. Required for normal platelet aggregation and blood coagulation (By similarity). The transmembrane domain is composed of seven transmembrane helices; most of these are not strictly perpendicular to the plane of the membrane, but are tilted and/or kinked. Agonist binding promotes a conformation change in the extracellular loops that leads to an inward movement of the transmembrane helices. Antagonists can bind to an overlapping site, but block the inward movement of the transmembrane helices (By similarity). Belongs to the G-protein coupled receptor 1 family.