U01.05.008 Receptor binding

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Learning Objectives

Master the effects of Competitive and Noncompetitive Antagonists, as well as Partial and Inverse Agonists. Learn to identify their impact on potency ( $EC_{50}$ ) and efficacy ( $E_{max}$ ) using dose-response curves for the USMLE Step 1.

1. Competitive Antagonists

Competitive antagonists bind reversibly to the same active site as the agonist.

Parameter	Change	Clinical Note
Potency ( $EC_{50}$ )	Decreased ( $\uparrow$ $EC_{50}$ )	The curve shifts to the right.
Efficacy ( $E_{max}$ )	No change	The maximal effect can be reached by $\uparrow$ [Agonist].
Example	Diazepam (agonist) + Flumazenil (competitive antagonist) at $GABA_A$ receptors.

2. Noncompetitive Antagonists

These bind irreversibly to the active site or to an allosteric site, preventing the agonist from producing a maximal effect regardless of concentration.

Parameter	Change	Clinical Note
Potency ( $EC_{50}$ )	No change	The remaining receptors have the same affinity.
Efficacy ( $E_{max}$ )	Decreased ( $\downarrow$ $E_{max}$ )	The curve shifts downward; it cannot be overcome by $\uparrow$ [Agonist].
Example	Norepinephrine (agonist) + Phenoxybenzamine (noncompetitive) at $\alpha$ -receptors.

Activity:

3. Partial and Inverse Agonists

These agents modulate receptor activity in unique ways compared to full agonists.

Type	Efficacy ( $E_{max}$ )	Mechanism & Example
Partial Agonist	Decreased	Binds the same site but with lower intrinsic activity. Buprenorphine at $\mu$ -opioid receptors.
Inverse Agonist	Opposite of Agonist	Reduces constitutive (baseline) activity of a receptor. Diphenhydramine ( $H_1$ antihistamine).

Activity:

High-Yield Clinical Pearls:

Competitive vs. Noncompetitive: If a question says the antagonism was reversed by adding more drug, the answer is Competitive. If it says the maximal effect was never reached, the answer is Noncompetitive.
The “Mixed” Role: A partial agonist acts as an antagonist when a full agonist is present (because it displaces the full agonist but provides a weaker signal).
Inverse Agonism: This is distinct from simple antagonism because an antagonist does nothing on its own; an inverse agonist actively lowers baseline activity.