Receptors are macromolecules with which endogenous or exogenous compounds interact to produce a characteristic biologic effect.
Law Mass Action provides that [L] X [R]/[LR] = Koff/Kon = Kd. Kd is the concentration of ligand at equilibrium that causes 1/2 maximal saturation of receptors since when half the receptors are occupied by ligand, # free receptors = # bound receptors or [R] = [LR] and Kd = [L].
The law mass action (occupancy theory) provides that E = ? Emax[L] / [L] + Kd where E is the effect produced, ? is the intrinsic activity of the ligand (from 0 to 1), and Emax is the maximal attainable effect.
Agonist are ligands (small molecules that bind receptors) that activate a receptor upon binding and are capable of eliciting the maximal attainable effect Emax and have intrinsic activity of 1.0. Partial agonists are ligands that produce only weak activation upon binding to a receptor and have intrinsic activity between 0 and 1. Antagonists are ligands that bind to a receptor but do not activate it and have an intrinsic activity of 0. Although antogonists have no intrinsic activity, they can block receptors from occupancy by either full or partial agonists.
Agonists differ in efficacy which denotes how large the maximum effect of a drug is relative to other drugs. More efficacious drugs are capable of producing greater effects at their maximal effective doses. Agonists also differ in potency which is the ability of a drug to cause a measured biology or functional change. The greater the potency, the less of the drug one needs to use. A L N
For example, in A, drugs L and N are more efficacious than M
drug M which is a partial agonist. But drug L is more potent than log [Agonist]
any of the drugs where the Y axis is % of maximal effect.
A linear plat displays a rectangular hyperbola as in B. B C
A semi log plot reveals a sigmoidal relationship
between occupancy and response such that in the absence of (+) or EC50 EC50
[Agonist] Log [Agonist]
(-) cooperativity, 10-90% of response occurs over a 100 fold range of agonist concentration center around the EC50 which is the concentration of drug that produces 1/2 of the drug’s maximal response. Often there is amplification between occupancy and response such that the EC50 lies
to the left of the Kd for receptor occupancy. This often occurs when the receptor can exist in more than one state which can be altered by the presence of an allosteric effector. EC50 Kd
The affinity of a ligand for its receptor is defined as 1/Kd. A high affinity for reversible ligands usually refers to Kds in the nM range or lower.
Experimental Protocols:
The following approaches are commonly used:
Saturation binding experiments measure the extent of binding in presence of different concentrations of radioligand and are used to determine Bmax and Kd.
The data is analyzed by Rosenthal-Scatchard Plot. Specific binding is simply the total radiolagand bound minus non-specific binding to things like the tube.
Bmax is where the line crosses the X axis.
Slope = -1/KD Specific Binding
All of the terms can be determined from the formula: B=FBmax/Kd+F where “B” = the concentration of receptor that has radioligand bound to it, “F” or “free” is the concentration of ligand that is not bound to the receptor and Bmax is the total amount of receptor present (maximal # binding sites).
Kinetic experiments are used to determine the rate constants Kon and Koff which permit a calculation of the Kd.
Competitive binding experiments are used to determine the affinity of the receptor for the competitor.