Full Answer Section

When discussing agonists, especially in a pharmacological or biological context, we typically categorize them by their efficacy and how they interact with receptors. The “four agonists” you’re referring to are likely Full Agonists, Partial Agonists, Inverse Agonists, and Allosteric Agonists. Let’s describe the characteristics of each:

1. Full Agonist

• Definition: A full agonist is a drug or ligand that binds to a receptor and produces the maximum possible biological response for that receptor. It has high efficacy, meaning it can elicit the full activation of the receptor system.
• Characteristics:
  • High Efficacy: When bound, it stabilizes the receptor in its fully active conformation.
  • Maximal Response: Even at concentrations where not all receptors are occupied, a full agonist can still produce a maximal tissue response if there are “spare receptors.”
  • Affinity: It has an affinity for the receptor, meaning it can bind to it.
  • Competitive Binding: It competes with antagonists and other agonists for the same binding site.
• Analogy: Imagine a light switch. A full agonist is like flipping the switch all the way “on” to get the brightest possible light.
• Example: Morphine acting on opioid receptors to produce maximal pain relief and euphoria. Epinephrine (adrenaline) acting on β1​-adrenergic receptors in the heart to increase heart rate and contractility to their maximum potential.

2. Partial Agonist

• Definition: A partial agonist binds to a receptor and produces a submaximal response, even when occupying all available receptors. It has intermediate efficacy, meaning it cannot elicit the full activation of the receptor system, regardless of concentration.