Catalytic receptors

Classification and General Operating Principle

Catalytic receptors got their name because their intracellular part has the ability to accelerate chemical reactions. Globally, this group is divided into two main subtypes depending on which specific enzyme is activated on the inner side of the membrane:

1. Receptors with tyrosine kinase activity. These make up the overwhelming majority of all catalytic receptors.
2. Guanylyl cyclase-linked receptors. The smallest family of transmembrane receptors.

Regardless of the subtype, the general principle remains the same: a molecule of a substance (ligand) binds to the outer part of the receptor, which instantly "turns on" the enzyme at its inner end.

Receptors with Tyrosine Kinase Activity

This type of structure is characterized by interaction with such important molecules as growth factors, various cytokines, and insulin.

Signal transduction mechanism:

• First, the ligand attaches to the receptor on the cell surface.
• This event immediately activates the tyrosine kinase enzyme located on the intracellular domain.
• The activated tyrosine kinase begins the phosphorylation process—it attaches phosphate groups to specific intracellular proteins.
• As a result of this phosphorylation, target proteins change their spatial configuration and activity, transmitting the signal further into the cell.

Mechanism of Insulin Receptor Activation

The insulin receptor is a classic and vital example of a tyrosine kinase receptor. It has a complex structure and consists of four subunits that are covalently linked together:

• Two $\alpha$-subunits are located entirely outside the cell. They are responsible for recognizing and binding the insulin molecule.
• Two $\beta$-subunits span the cell membrane completely. Their intracellular "tails" possess tyrosine kinase activity.

Stages of receptor function:

1. The insulin molecule docks with the outer $\alpha$-subunits.
2. This interaction leads to a conformational change in the membrane $\beta$-subunits—they physically approach each other.
3. The process of transphosphorylation (or autophosphorylation) is triggered: one $\beta$-subunit enzymatically phosphorylates the neighboring one at specific tyrosine amino acid residues.
4. The resulting phosphorylated tyrosine residues begin to act as "landing pads." Special insulin receptor substrates attach to them, serving as new substrates and carrying the signal deep into the cell.

Guanylyl cyclase-coupled receptors

This family of receptors is the smallest among transmembrane sensors. Their main ligands are natriuretic peptides. A classic physiological example is brain-type (B-type) natriuretic peptide. This hormone is secreted by the heart ventricles when there is an excessive increase in circulating blood volume in the body.

Mechanism of action of guanylyl cyclase receptors:

• Binding of the natriuretic peptide outside the cell stimulates the enzymatic activity of guanylyl cyclase inside.
• The activated enzyme triggers the conversion of GTP (guanosine triphosphate) into cGMP (cyclic guanosine monophosphate).
• The resulting cGMP acts as a powerful intracellular signal regulating the tissue response (e.g., of blood vessels and kidneys) to volume overload.

Class and exam questions

What else to study on this topic

• Functions and Effects of Phosphorylated Target Proteins in the Insulin Cascade
• The Role of B-Type Natriuretic Peptide in the Regulation of Circulating Blood Volume
• Growth Factor and Cytokine Receptors: Similarities and Differences from the Insulin Receptor
• Mechanisms of Downstream Signal Transmission from Cyclic GMP Inside the Cell

Other topics in "General Pharmacology"

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