| Popis: |
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. |
