The dynamic behavior of activation microswitches in glycoprotein hormone receptors (GpHRs) is central to understanding signaling bias; a phenomenon in which GPCRs preferentially activate specific intracellular pathways over others in response to distinct ligands or regulatory conditions. While biased signaling in GPCRs has traditionally been attributed to ligand-induced stabilization of unique active-state conformations, emerging evidence suggests that bias may instead arise from subtle shifts in the probability distribution of pre-existing conformational states, closely linked to the coordinated action of these microswitches. We performed MD simulations of both FSHR and LHCGR on the ∼1.5 μs time scale. Our aim was to determine how differences in microswitch dynamics correlate with overall receptor flexibility, basal activity, and the efficiency of allosteric communication. These findings are anticipated to highlight the critical role of microswitch dynamics as a central mechanism by which GpHRs encode their signaling specificity and adaptability. We rely on the A100 index to differentiate among active, partially active, and inactive states based on critical interhelical distances within the receptor’s transmembrane domain. This parameter is well suited for identifying active (A100 > 55), inactive (A100 < 0), or intermediate states (0 < A100 < 55). Importantly, the A100 index is independent of specific structural switches that may or may not be involved in the activation mechanism. Nevertheless, the distances included in the calculation are often close to those involved in structural switches. Our goal is to establish whether the A100 index can be related to specific microswitches proposed to be relevant for the activation of other GPCRs, such as the β2-AR, which could, in principle, reveal the key conformational changes involved in the transition from the active to the inactive states in prototypical members of the GpHR subfamily.
Singh et al. (Sun,) studied this question.