A 2A and A 2B adenosine receptors: The extracellular loop 2 determines high (A 2A ) or low affinity (A 2B ) for adenosine.

A _2A and A _2B adenosine receptors (ARs) are closely related G protein-coupled receptor subtypes, which represent important (potential) drug targets. Despite their almost identical binding sites for adenosine, A _2A ARs are activated by low (nanomolar) adenosine concentrations, while A _2B ARs require micromolar concentrations. In the present study, we exchanged the extracellular loop 2 (ECL2) of the human A _2A AR for that of the A _2B AR. The resulting chimeric A _2A (ECL2-A _2B )AR was investigated in radioligand binding and cAMP accumulation assays in comparison to the wildtype A _2A AR. While the ribose-modified adenosine analog N-ethylcarboxamidoadenosine (NECA) and its 2-substituted derivative CGS-21680 did not exhibit significant changes, adenosine showed dramatically reduced potency and affinity for the A _2A (ECL2-A _2B )AR mutant displaying similarly low potency as for the wt A _2B AR. Supervised molecular dynamics simulation studies predicted a meta-binding site with high affinity for adenosine, but not for NECA, which may contribute to the observed effects.
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