Receptor theory SN

Receptor Theories

Overview of Receptor Theories

  • Origin: First postulated by John Langley in 1878 - termed as "receptive substance."
  • Advancement: Further developed by Paul Ehrlich (1854-1915); introduced the receptor theory of drug action.

Key Contributions by Paul Ehrlich

  • Side-Chain Theory:
    • Description: Illustrated through a diagram in his lecture to the Royal Society of London in 1900.
    • Immune cells possess a vast array of receptors (1) specific to particular substances (2).
    • Upon interaction with a toxin (3), the cell activates and produces more receptors, subsequently releasing antibodies into the bloodstream to neutralize the toxin (4).

Classical Theories of Drug-Receptor Binding Interactions

  • Occupation Theory:

    • Description: Drugs act on independent binding sites and activate them.
    • The biological response is proportional to the amount of drug-receptor complex formed.
    • The response ceases when the complex dissociates.
    • Drug effect is directly proportional to the number of receptors occupied.

  • Lock and Key Concept:

    • Originator: Emil Fischer, 1894.
    • Analogy:
    • Key = substrate
    • Lock = enzyme
    • Illustration: A correct fit, akin to a key in a lock, results in a reaction, whereas an incorrect substrate does not react.

  • Quote by Emil Fischer:

    • "To use a picture, I would like to say that enzyme and glucoside have to fit like a lock and key, in order to exert a chemical action on each other."

  • Rate Theory:

    • Description: The response is proportional to the rate of drug-receptor complex formation.
    • Pharmacological activity is directly proportional to the rates of dissociation and association, rather than the number of occupied receptors.
    • Duration of receptor occupation determines whether a molecule is classified as an agonist, partial agonist, or antagonist.
Classification of Drugs Based on Rate Theory
  • Agonist: Drug with fast association and fast dissociation.
  • Partial Agonist: Drug with intermediate association and intermediate dissociation.
  • Antagonist: Drug with fast association and slow dissociation.

Induced-Fit Theory

  • Description: Indicates that the binding site morphology is not necessarily complementary to the ligand’s preferred conformation.

  • Process: Binding leads to mutual plastic molding of both ligand and receptor as a dynamic process.

  • Outcome: The conformational change in the receptor is translated into a biological effect, moving beyond the rigid "key and lock" concept of earlier theories.

  • Mechanism: As the drug approaches the receptor, it alters the conformation of the receptor's binding site, resulting in a drug-receptor complex.

Macromolecular Perturbation Theory

  • Description: Suggests two general types of macromolecular perturbation when a drug-receptor interaction occurs:
    1. A specific conformational perturbation leads to a biological response (agonist).
    2. A non-specific conformational perturbation leads to no biological response (antagonist).

Activation-Aggregation Theory

  • Overview: An extension of the macromolecular perturbation theory.

  • Concept: A drug receptor, in the absence of a drug, exists in equilibrium between an activated state (Bioactive) and an inactivated state (Bio-inactive).

  • Drug Interaction:

    • Agonists bind to the activated state.
    • Antagonists bind to the inactivated state.

  • Receptor Populations and Signalling:

    • Receptor signalling depends on:
    • Activation (conformational change)
    • Aggregation (dimerization/oligomerization/clustering)
    • Ligand binding promotes receptor clustering, crucial for signaling.
    • Particularly relevant for receptor tyrosine kinases (RTKs), immune receptors (e.g., Fc receptors), and cytokine receptors.

Molecular-Level Conceptual Models of Receptors

  • General Insight: Receptors are not simple macromolecules; many are highly dynamic, existing as families of low-energy conformers in equilibrium.
  • Complex Structures:
    • Some receptors comprise multiple proteins with facilitatory and inhibitory interactions among subunits, altering drug-receptor interactions.
    • Receptors may be mobile, akin to an iceberg drifting in the ocean.
Two-State Receptor Model
  • Foundation: Developed based on kinetics of competitive and allosteric inhibition and direct binding experiment interpretations.
  • States of the Receptor:
    • R (resting state) and R* (activated state).
    • These states exist in equilibrium irrespective of ligand presence.
  • Equilibrium Characteristics:
    • Absent ligand: equilibrium favors the R state; few receptors are in R* state.
Agonist, Antagonist, and Inverse Agonist Roles

  • Agonists: Have high affinity for the R* state, shifting equilibrium right (toward R*).

  • Antagonists: Exhibit equal affinity for both R and R*, maintaining the equilibrium.

  • Inverse Agonists: Have affinity for R, stabilizing it, leading to no response at receptor equilibrium.

  • Contrast with Classical Occupation Theory: In the two-state model, agonists do not merely activate but shift equilibrium toward the R* form rather than activating the receptor directly.