Agonist has affinity for -
Ans. is 'a' i.e., Active receptor Drug-receptor binding and agonism o A receptor can exist in two conformational states: (i) Active state (Ra) (ii) Inactive state (Ri) o Usually, inactive state predominates in the absence of drug and the basal signal output will be low. o The extent to which the equilibrium is shifted toward the active state is determined by the relative affinity of the drug for the two conformations. o Agonist has higher affinity for the active confirmation and drives the equilibrium to the active state and thereby activate the receptor. A full agonist is sufficiently selective for the active confirmation that at a saturating concentration it will drive the receptor essentially completely to active state -3 maximum activity can be produced by agonist. o Paial agonist has only moderately higher affinity for the active confirmation (Ra) than inactive confirmation (Ri). Its effect will be less, even at saturating concentrations, so paial agonist cannot produce a full biological response at any concentration --> paial agonist has submaximal intrinsic activity. o Antagonist has equal affinity for both active (Ra) and inactive (Ri) confirmation. So, antagonist will not alter the equilibrium between Ra & Ri, and will not produce an effect --> antagonist has no intrinsic activity. o Inverse agonist has greater affinity for inactive confirmation (Ri) and drives the equilibrium to the inactive state. So, inverse agonist has an effect opposite to that of an agonist --> inverse agonist has intrinsic activity with a minus sign that means opposite intrinsic activity. Examples of such inverse agonists at G-protein coupled receptors are Famotidine, losaan, metoprolol and risperidone. 1. Agonist has affinity for active configuration (Ra) 2. Paial agonist has affinity for bath active (Ra) as well as inactive (Ri) confirmation, but has more affinity for active configuration (Ra > Ri) 3. Antagonist has equal affinity for active and inactive confirmation (Ra = Ri) 4. Inverse agonist has affinity for inactive confirmation (Ri)
Active receptor
Inactive receptor
Both
None