Introducing Kappa: An Integrated, ‘Catch-And-Hold’ Reaction Co-Ordinate that Triggers AChR Gating

Abstract

For many years drug action on membrane receptors has been conceptualized as occurring in two distinct steps, initially termed ‘affinity’ and ‘intrinsic activity’. In ligand-gated ion channels these stages of activation are called ‘binding’ and ‘gating’. Single-channel kinetic analysis of adult-type mouse neuromuscular acetylcholine receptor-channels (AChRs) indicates that a conformational change in the protein is an essential part of the ligand association process, and that this structural rearrangement shares a common mechanism with the ensuing low-to-high affinity transformation of the binding site that triggers channel-gating. The energy source for gating is the change in affinity for the agonist. We find that in AChRs there is a close correlation between the low- and high-affinity equilibrium association constants. This indicates that agonist association and the low-to-high affinity-change that powers gating are linked processes that reflect a single, integrated ‘catch-and-hold’ structural adaptation of the protein to the ligand. The slope of the correlation, kappa, is an index of the relative position in this reaction co-ordinate, which occurs in the initial stages of the gating isomerization. Residue GlyB2 changes energy (‘moves’) early in the process (k=0.85), before the agonist and four a-subunit aromatic amino acids (k≈0.5). In 1957 del Castillo and Katz separated binding and gating. The results suggest that they are just different stages of a single reaction co-ordinate.