Minute-timescale simulations of G Protein Coupled Receptor A2A activation mechanism reveal a receptor pseudo-active state

G protein coupled receptors (GPCRs) are membrane proteins of greatest pharmacological relevance, targeted by over one third of marketed drugs. These receptors are activated by orthosteric ligands and undergo large conformational changes that lead to coupling diverse effector proteins. To achieve a fine regulation of the drug pharmacological response, it is imperative to shed light on the yet poorly understood aspects of GPCRs activation. In this work, we elucidate the entire activation mechanism of the adenosine A2A receptor (A2AR), a class A GPCR, performing minute timescale molecular dynamics and free energy calculations. We have explored the entire conformational landscape of A2AR in its basal apo form and in differently ligated conditions, elucidating the ligand intrinsic activity and the receptors lowest energy functional states. Among these is a novel pseudo-active state (pAs) of the A2AR apo form stabilised by specific "microswitch" residues interactions, including the salt bridge between the class A conserved residues R5.66 and E6.30. In the pAs state, A2AR is able to couple {beta}-arrestin 1 over G proteins, providing unprecedented structural basis for receptor desensitization and G protein-alternative cellular pathways. Our simulation protocol is generalisable and can be applied to study the activation of any GPCR, resulting a precious tool for drug design and biased signaling studies.