Receptor Theories and Classification of Receptors, Regulation of Receptors

Receptor Theories

Receptors are distributed throughout the body and are responsible for detecting internal and external environment changes.
They pass on this information to the central nervous system to respond.
Two main theories explain how receptors work:

The lock-and-key theory:

This theory states that receptors are specific to a particular molecule type and can only bind with that molecule.

How does lock-and-key theory work?

• The lock-and-key theory proposes that receptors are specialized to bind with a specific kind of molecule.
• This interaction is like an essential fitting into a lock; the key (molecule) can only fit into the lock (receptor) if it is the right shape.
• Once the key is inserted, it triggers a response from the receptor.

The shape theory: This theory states that the receptor's shape determines what kind of molecule it can bind with.

How does shape theory work?

• The shape theory proposes that the three-dimensional shape of the receptor determines what kind of molecule it can bind with.
• This is because the shapes of molecules complement the shapes of receptors.
• For example, a round molecule can fit into a receptor that is shaped like a pocket. Once the molecule is bound, it triggers a response from the receptor.

The induced-fit theory:

This theory suggests that receptors are not specific to a certain molecule, but rather they adapt to fit the molecule that is binding to them.

This allows for greater flexibility in receptor signaling.

How does the induced-fit theory work?

• The induced-fit theory proposes that receptors are not specific to a certain molecule, but they adapt to fit the molecule that is binding to them.
• This allows for greater flexibility in receptor signaling.
• For example, if a receptor is activated by a molecule, it will change its shape to accommodate the molecule.
• This will then allow other molecules of the same type to bind more easily.

Classification of Receptors

Receptors can be classified according to their structure, function, or location in the body.

Structural classification:

Receptors can be classified based on their structure as either integral membrane proteins or G protein-coupled receptors (GPCRs).

Integral membrane proteins

• Located in the cell membrane.
• Responsible for transmission of signals into and out of the cell.

Two categories:

• Ion channels
• Receptor proteins

G protein-coupled receptors (GPCRs)

• Proteins are located on the surface of the cell.
• They are called GPCRs because they bind to a G protein.
• G proteins are located inside the cell.

Functional classification:

Receptors can also be classified based on their function. There are five main types of receptors:
1. Ion channels:
These channels allow ions to enter and exit the cell, which controls the cell's electrical activity.

2. Enzymes:
These receptors bind to hormones and other signaling molecules and activate or inhibit the activity of enzymes.

3. Receptor proteins:
These receptors bind to hormones and other signaling molecules.

4. G protein-coupled receptors (GPCRs):
These receptors bind to a protein named G protein.

Location:

Receptors can also be classified based on their location in the body. There are three main types of receptors:
1. Cell surface receptors: These receptors are located on the surface of the cell.
2. Nuclear receptors: These receptors are located in the nucleus of the cell.
3. Intracellular receptors: These receptors are situated inside the cell.

Regulation of Receptors

Receptors are regulated by a number of mechanisms which may be either positive or negative.

Positive regulation:

Positive regulation is when a molecule increases the activity of a receptor.
1. Activation: When a receptor is activated, it increases its activity.
2. Binding of ligands: When a receptor binds to a molecule, it increases its activity.
3. Phosphorylation: When a receptor is phosphorylated, it increases its activity.

Negative regulation:

Negative regulation is when a molecule causes a downfall in the activity of a receptor.
1. Downregulation: This is when the number of receptors on the surface of the cell decreases.
2. Desensitization: This is when the receptor becomes less sensitive to a hormone or other signaling molecule.
3. Phosphorylation: This was when a protein called a kinase adds a phosphate group to the receptor. This decreases the activity of the receptor.
4. Ubiquitination: The receptor is marked for destruction by a protein called a ubiquitin ligase.

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