Chapter 7: 7.2 Receptors and Ligands Flashcards
What are the purpose of ligands? Where do they come from?
To trigger the signalling pathway by binding to a receptor
* Produced by a cell to be transported to the receptor
What are the types of ligands?
- Diffusible ligands
- Surface-bound ligands
Describe characteristics of:
Diffusible ligands
- Enter the cell directly by diffusion
- Small and hydrophobic
- Bind to an intracellular receptor
Describe characteristics of:
Surface-bound ligands
- Bind to a receptor on the cell surface
- Cannot diffuse through the membrane
- Mostly small and hydrophilic
Give an example of a diffusible ligand
- Steroid hormones
- Certain gases (nitrogen oxide etc.)
Give an examples of a surface-bound ligand
- Protein ligands
What is the purpose of receptors? Describe them
Bind to the ligand to activate the signal cascade that leads to the final response
* Can be in the cell (intracellular) or on the surface of the cell
What are the types of receptors?
- Cell-surface receptors
- Intracellular receptors
Describe characteristics of:
Cell-surface receptors
- Bind to small hydrophilic ligands
- Have 3 domains
What are the 3 domains of cell-surface receptors?
- Extracellular domain: Faces the outside and binds to the ligand
- Hydrophobic domain: Spans the membrane
- Intracellular domain: Faces the inside of the cell and transfers the signal inwards
Describe characteristics of:
Intracellular receptors
- Bind to diffusible ligands (small, hydrophobic) such as hormones
- Can often enter the nucleus to regulate gene expression
- Features include: DNA-binding domain, ligand binding domain
How do Ligand-gated Ion Channels work?
Open when a ligand binds to it
Give an example of a Ligand-gated Ion Channels
Neurotransmitters often bind to ligand-gated ion channel on neuron cells to transport the ions Na+ and K+ to create an action potential in order to send signals as electric pulses
Describe characteristics of:
G-coupled Receptors
(5 points)
- One of the largest groups of cell-surface receptors
- 7 transmembrane domains (cross the membrane 7 times)
- Bind to many different types of ligands
- Receptor is bound to a G-protein on the interior of the cell
- G-protein triggers the response, it is also bound to three other subunits labelled alpha, beta, and gamma
What are G-coupled Receptors involved in?
- Cell growth
- Motility
- Cell division
- Cancer
Describe:
The signalling process of G-coupled receptors
(6 points)
- When unbound to ligand, the G-protein (bound to the receptor) has a nucleotide guanosine diphosphate (GDP) attached. This is the OFF state
- The ligand binds to the receptor
- A phosphate group is added to GDP to become GTP.
- This is the ON state
- The activated G-protein breaks off from beta and gamma subunits, goes to activate response (e.x. gene expression changes)
- GTP has one phosphate removed, making GDP, and reattaches to the receptor, resetting the system
What are Receptor Tyrosine Kinases (RTKs)? Describe their characteristics
Enzyme-linked cell-surface receptors
* The kinase enzyme part adds a phosphate group to the amino acid tyrosine
* Two of the proteins are needed to actually work
Describe:
The signalling pathway of RTKs
- Two RTKs are present relatively close to each other on the membrane but not close enough to interact (OFF state)
- When ligands bind to both, they come together until the receptor domains and intracellular domains are in contact
- The intracellular regions which have kinase activity and tyrosine residues - cross modify each other by adding phosphate groups to the tyrosine of each other’s intracellular domain to activate them (ON state)
Triggers activation of the response
What is the Dissociation Constant (Kd)?
Measures the affinity between the ligand (L) and the receptor (R)
True or False:
All ligands and receptors have the same affinity
False, they don’t
Are receptors specific to one certain ligand?
No, many receptors can bind to several ligands with varying affinities and vice-versa
Dissociation Constant formula
Kd = [R] [L] / [L]
What is Kd defined as?
“Ligand concentration needed to bind 50% of receptors”
Lower Kd means…
Higher affinity
* Less ligand is needed to fill 50% of receptors
