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Receptors: Types and Functions



  • Receptors are like antennas of cells. They are the first people in the cell to receive an external signal sent by other cells in form of a chemical messenger.

  • Upon receiving the signal they transduce it into a response.


Domains on Receptor Molecule

Ligand Binding Domain

  • Recognizes a specific ligand molecule.

Effector Domain

  • Produce downstream effect.


Location of Receptors

On Cell Membrane

  • For hydrophilic messengers.

Hydrophilic messengers cannot cross the cell membrane

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Receptors for them are expressed on the cell membrane

Inside the Cell

  • For hydrophobic messengers.

Hydrophobic messengers can cross the cell membrane

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Receptors for them are inside the cell


Types of Receptors

Ligand Gated Ion Channels

  • Located on the cell surface.

  • Here the same protein molecule works as a channel as well as a receptor.

  • The Channel portion allows the passage of small ions ⟶ receptors are also called ionotropic receptors.

  • Receptor portion binds various ligands and controls the opening and closing of the channel.

  • They are involved in signaling between electrically excitable cells i.e. neurons and muscle cells

  • E.g. acetylcholine receptors on the muscle end plate are sodium channels.

G Protein-Coupled Receptors

  • They are the largest family of cell surface receptors.

  • On the cytoplasmic side, they are associated with G proteins.

  • Regulate functions of various enzymes and ion channels of the cell.

  • E.g. Adrenergic receptors on cardiac muscle cells.

Catalytic Receptors

  • Located on the cell surface.

  • Have an enzymatic activity of their own or are associated with another enzymatic molecule.

  • Mostly the enzymatic activity is kinase or phosphatase.

  • E.g. Insulin receptors (have their own enzymatic activity) and erythropoietin receptors (associated with another enzymatic molecule)

Nuclear Receptors

  • Located inside the cell.

  • Regulate gene expression.

  • As protein synthesis takes time, their effect takes longer to appear.

  • Exact location inside the cell:

    • Cytoplasm: e.g. glucocorticoid and mineralocorticoid receptors.

    • Nucleus: e.g. estrogen and progesterone receptors.

    • Bound to DNA: e.g. thyroid hormone and retinoic acid receptors.

Receptors that Undergo Cleavage

  • Not typical receptors, but work like receptors.

Ligand binding

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Regulated Intramembrane Proteolysis (RIP) of the receptor molecule

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Cytoplasmic fragments

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Go to the nucleus

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Regulate gene expression

  • E.g. Sterol regulatory element-binding protein found on endoplasmic reticulum

 

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