Receptor occupancy definition

Intrinsic activity Intrinsic activity is the maximal stimulatory response induced by a compound in relation to that of a criven reference compound. This term has evolved with common use. It was introduced by Ariens as a proportionality factor between tissue response and receptor occupancy. The numerical value of intrinsic activity (alpha) could ran from unity (for full agonists, i.e., agonist inducing the tissue maximal response) to zero (for antacyonists). The fractional values within this ran denoting partial agonists. Arien-8 original definition equates the molecular nature of alpha to maximal response only when response is a linear function of receptor occupancy. This function has... 

AChBP has been crystallized only with HEPES, rather than ACh, bound, and "owing to low occupancy and limited resolution, the precise orientation of the HEPES molecule cannot be definitely resolved" (25). In overall appearance, it is very similar to the extracellular domain of the receptor (Fig. 11.5). The C-loop is particularly prominent both in AChBP (Fig. 11.4) and in the receptor, where it is shown as the projection labeled C in Fig. 

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