A Closer Look into G Protein Coupled Receptor Activation: X-Ray Crystallography and Long-Scale Molecular Dynamics Simulations

G protein coupled receptors (GPCRs) are a large eukaryotic protein family of transmembrane receptors that react to a signal coming from the extracellular environment to generate an intracellular response through the activation of a signal transduction pathway mediated by a heterotrimeric G protein. Their diversity, dictated by the multiplicity of stimuli to which they respond and by the variety of intracellular signalling pathways they activate, make them one of the most prominent families of validated pharmacological targets in biomedicine. In recent years, major breakthroughs in structure determination of GPCRs have given new stimuli to the exploration of the biology of these proteins, providing a structural basis to understand the molecular origin of GPCR mechanisms of action. Based on the information coming from these structural studies, a number of recent in silico investigations used molecular dynamics (MD) simulations to contribute to our knowledge of GPCRs. In this review, we will focus on investigations that, taking advantage of the tremendous progress in both hardware and software, made testable hypotheses that have been validated by subsequent structural studies. These stateof- the-art molecular simulations highlight the potential of microsecond MD simulations as a valuable tool in GPCR structural biology and biophysics.