2- And 8- alkynyladenosines: conformational studies and docking to human adenosine A(3) receptor can explain their different biological behavior

Adenosine (Ado) derivatives substituted at the C2 position with an alkynyl chain are endowed with high affinity for A1, A2A and A3 human adenosine receptors, while being less active at the low affinity A2B subtype. On the other hand, the introduction of an alkynyl chain at the C8 position of adenosine is detrimental for the affinity and potency at A1, A2A, and A2B receptors, while is more tolerated by the A3 receptor. The evaluation of the stimulation of [35S]GTPγS binding revealed that 2-alkynyladenosines behave as adenosine receptors agonists while, on the contrary, 8-alkynyladenosines behave as antagonists. With this work we demonstrated, by means of an NMR-based and a computational conformational analysis, that 8-alkynyladenosines, differently from 2-alkynyladenosines, cannot adopt the sugar-base anti conformation required for adenosine receptor activation. Furthermore, using the recently reported X-ray crystal structure of bovine rhodopsin as template, we built a 3D model of the seven transmembrane domains of the human adenosine A3 receptor with the homology modeling. After identification of the binding site we carried out docking experiments, demonstrating that the two class of molecules have different binding modes that explain their different degree of affinity and the shift of their activity from agonism to antagonism.