Cell Surface Receptors

Question 1

Why is cellular communication important?

Answer 1

• No cell exists in isolation.
• Signals must be received and processed from the environment and from other cells.
• Signals are generally transmitted across the environment in the form of soluble molecules.
• These molecules must be detected and appropriate responses produced.
• The biggest barrier to this process is the cell membrane.

Question 2

How do soluble signalling molecules cross the membrane/ transmit the signal into the cell?

Answer 2

• The membrane is impermeable to large soluble molecules.
• Some are transported across the membrane.
• Others are detected outside of the cell and a signal transmitted into the cell.

This is the role of receptors

Question 3

What is a ligand?

Answer 3

• Ligand” is derived from the latin ligandum meaning “binding”.
• A ligand is a molecule that binds to a receptor.
• For example, the ligand of the insulin receptor is insulin.

Question 4

Describe ligand-receptor relationships

Answer 4

• Some receptors have one ligand whilst others have many.
• Different ligands can produce different responses at the same receptor.
• Studying this is essential for pharmacology.

Question 5

How many receptors exist in humans?

What do they detect?

Answer 5

• Thousands of receptors exist in humans.
• Detect a vast array of signals such as:

Light, smell, taste, hormones, neurotransmitters, sugars, fats, drugs, growth factors and many more….

Question 6

Describe the structure of typical receptors

Answer 6

• Receptors are generally integral (transmembrane) proteins.
• The extracellular region detects ligands.

The intracellular region interacts with effectors

Question 7

Describe the main components of a signalling pathway

Answer 7

1. Ligand binds to ligand binding domain on the receptor
2. The receptor spans the bilayer via the transmembrane region and it has an intracellular region which is responsible for interacting with the effectors
3. Cytoplasmic/ intracellular mediators link the receptos to the effector enzymes to transmit the signal
4. Effector enzymes can act on target proteins like transcription factors and they can generate second messengers to bring about changes in the cell
5. Second messengers alter metabolism, affect effector enzymes (that modulate target proteins) or other cellular processes by diffusing away from the receptor signalling cascade and affecting other components of the cell
6. Target proteins tend to act by altering gene expression but can cause other changes in cell behaviour

Not all signalling pathways will contain all of these components!

Question 8

Give an example of a signalling cascade that contains the main components

Answer 8

Growth factor signalling cascade:

1. A growth factor is seceted by some cells to bind to another cell (or in the case of a cancerous state to bind to the same cell, autocrine signalling)
2. It binds to a receptor tyrosine kinase, an enzyme with a kinase domain on the intracellular side that phosphorylates other proteins
3. Adaptor proteins, docking proteins and GTP-binding proteins are phosphorylated
4. This activates effector enzymes such as protein or lipid kinases, phosphodiesterases and other metabolic enzymes
5. The target proteins in this case is transcription factors which results in changes in gene expression