Adenosine A2A receptor
{{DISPLAYTITLE:Adenosine A2A receptor}} The adenosine A2A receptor, also known as ADORA2A, is an adenosine receptor, and also denotes the human gene encoding it. Structure This protein is a member of the G protein-coupled receptor (GPCR) family which possess seven transmembrane alpha helices, as well as an extracellular N-terminus and an intracellular C-terminus. Furthermore, located in the intracellular side close to the membrane is a small alpha helix, often referred to as helix 8 (H8). The crystallographic structure of the adenosine A2A receptor reveals a ligand binding pocket distinct from that of other structurally determined GPCRs (i.e., the beta-2 adrenergic receptor and rhodopsin). Below this primary (orthosteric) binding pocket lies a secondary (allosteric) binding pocket. The crystal-structure of A2A bound to the antagonist ZM241385 (PDB code: 4EIY) showed that a sodium-ion can be found in this location of the protein, thus giving it the name 'sodium-ion binding pocket'. Heteromers The actions of the A2A receptor are complicated by the fact that a variety of functional heteromers composed of a mixture of A2A subunits with subunits from other unrelated G-protein coupled receptors have been found in the brain, adding a further degree of complexity to the role of adenosine in modulation of neuronal activity. Heteromers consisting of adenosine A1/A2A, dopamine D2/A2A and D3/A2A, glutamate mGluR5/A2A and cannabinoid CB1/A2A have all been observed, as well as CB1/A2A/D2 heterotrimers, and the functional significance and endogenous role of these hybrid receptors is still only starting to be unravelled. The receptor's role in immunomodulation in the context of cancer has suggested that it is an important immune checkpoint molecule. Function The gene encodes a protein which is one of several receptor subtypes for adenosine. The activity of the encoded protein, a G protein-coupled receptor family member, is mediated by G proteins which activate adenylyl cyclase, which induce synthesis of intracellular cAMP. The A2A receptor binds with the Gs protein at the intracellular site of the receptor. The Gs protein consists of three subunits; Gsα, Gsβ and Gsγ. A crystal structure of the A2A receptor bound with the agonist NECA and a G protein-mimic has been published in 2016 (PDB code: 5g53). The encoded protein (the A2A receptor) is abundant in basal ganglia, vasculature, T lymphocytes, and platelets and it is a major target of caffeine, which is a competitive antagonist of this protein. Physiological role A1 and A2A receptors are believed to regulate myocardial oxygen demand and to increase coronary circulation by vasodilation. In addition, A2A receptor can suppress immune cells, thereby protecting tissue from inflammation. The A2A receptor is also expressed in the brain, where it has important roles in the regulation of glutamate and dopamine release, making it a potential therapeutic target for the treatment of conditions such as insomnia, pain, depression, and Parkinson's disease. Ligands A number of selective A2A ligands have been developed, with several possible therapeutic applications. Older research on adenosine receptor function, and non-selective adenosine receptor antagonists such as aminophylline, focused mainly on the role of adenosine receptors in the