Terence (Ligand gated ion channel and enzyme linked receptors)

Question 1

Ligand gated ion channels

Answer 1

• Ligand gated ion channels are also referred to as inotropic receptors or receptor operated channels
• They are transmembrane ion channels that allow ions to pass through the membrane in response to the binding of a ligand
• Ligand binding will either cause the opening or closing of an ion channel. This causes an influx or efflux of molecules
• Neurotransmitters are common ligands- released from presynaptic cell e.g. glutamate, Ach, GABA
  Involved in the rapid transmission of a signal
• Important for the communication between neurones and other cells
• Occur at synapses
• Receptors located at post-synaptic cell

Question 2

Receptor structure and classification

Answer 2

Ligand gated ion channels are typically made up of at least 2 different domains- a transmembrane domain which includes the ion pore, and an extracellular domain which includes the ligand binding location.
They are classified into three superfamilies which lack evolutionary relationship
- cycle-loop receptors
- inotropic glutamate receptors
- ATP-gated channels
The prototypic ligand gated ion channel is the nicotinic acetylcholine receptor

Question 3

Ligand gated ion channel structure

Answer 3

Channel is made up of 5 subunits (pentameric)- 2 alpha, beta, gamma, and delta.
ACh binds to the extracellular domain of the alpha subunits.

Question 4

Nicotinic acetylcholine receptor (nAChR)

Answer 4

Cation selective.
Cation selective channels are stimulatory.
Influx of sodium leads to a depolarisation and a change in voltage.
This leads to a nerve impulse or muscle contraction.

Alpha-conotoxin- nAChR antagonist
Nicotine- nAChR agonist

Question 5

GABAa receptors

Answer 5

Anion selective.
Anion selective channels are inhibitory.
Depressive effects of GABA are mediated by an influx of -ve ions e.g. Cl-.
Leads to hyper polarisation and inhibition of neuronal activity.

Question 6

GABA receptor as a target for drug action

Answer 6

Activated to reduce neuronal signalling- used to treat anxiety, hyperactivity, and as an anaesthetic. Most are positive allosteric modulators (PAMs).

Question 7

Different classes of enzyme linked receptors

Answer 7

7 different classes.
Receptor tyrosine kinases (RTKs)- these have intrinsic tyrosine kinase activity
Cytokine receptors- these associate with intracellular proteins that have tyrosine activity.

Question 8

Domains

Answer 8

Domain- unit of a protein that has a function. If the protein is cut the different domains still work.
Extracellular domain- where the ligand binds.
Intracellular domain- has enzymic activity. Upon ligand binding the enzyme is activated- it phosphorylates on tyrosines.
Transcellular domain- to hold it in the membrane.

Question 9

RTK (receptor tyrosine kinases)

Answer 9

• Commonest of all enzyme linked cell surface receptors
• Have intrinsic tyrosine kinase activity
• Phosphorylate their substrate proteins on the amino acid tyrosine
• Reston’s exclusively to peptide/protein ligands e.g. mitogenic growth factors and peptide hormones

Question 10

RTK (receptor tyrosine kinases) structure

Answer 10

RTKs share a common structural organisation
- N-terminal extracellular ligand binding domain
- Single transmembrane alpha helix
- Cytosolic C-terminal domain with protein kinase activity

Most receptors are a single polypeptide although the insulin receptor is a dimer made up of two identical polypeptide chains.

Question 11

RTK (receptor tyrosine kinases) ligands

Answer 11

Many RTK ligands are
- Mitogens- ligands that stimulate cell division
- Growth factors- ligands that stimulate cell growth (because they stimulate cell growth they are ‘turned on’ for more time than the other receptors. This is why they have been linked to cancer and why there is a lot of research done around them)

Notable RTK ligands are epidermal growth factor (EGF), transforming growth factor (TGF), insulin, and nerve growth factor (NGF).
RTKs and their ligands are important targets for anti-cancer drugs.

Question 12

Receptor activation

Answer 12

1. Ligand binding and dimerisation (bonds) of the receptor
2. Activation of tyrosine kinase domains
3. Phosphorylation of the receptor by receptor tyrosine kinase
4. Signalling proteins bind to phosphorylated receptor and are activated
5. Activated signalling protein relay signal downstream leading to a response

Question 13

Growth factor signalling via the MAPK pathway

Answer 13

G-protein has GTPase (intraenzymic activity. Binds GDP. To turn on (bind GTP) it needs GEF (guanine nucleotide exchange factor). To turn off (bind GDP) it need GAP.

Question 14

Ras and the cell proliferation

Answer 14

Activated growth factor receptors can activate the enzyme Ras.
Ras is a monomeric G-protein anchored to the cell membrane.
Ras activates a kinase which activate a protein kinase cascade.
This cascade is called the MAP kinase pathway (Mitogen Activated Protein Kinase pathway).
Activation promotes cell division.

Question 15

Targeting signalling proteins

Answer 15

Overexpression or gain of function mutations in signalling proteins are responsible for many cancers.
A single amino acid mutation of Ras is found in more than 30% of all cancers.
90% pancreatic carcinomas have mutated Ras.
70% melanomas have mutated B-Ras.

Question 16

Small molecule kinase inhibitors

Answer 16

The suffix ‘nib; indicates a small molecule inhibitor.
- The ATP binding cleft is the target for rationally designed small structural analogues of ATP e.g.
- Sunitinib
- Imatinib mesylate
- Vemurafenib

Question 17

Monoclonal antibodies (suffix-mab))

Answer 17

An antibody produced by a single clone of cells or cell line and consisting of identical antibody molecules.
- Target the ligand.
- Inhibit dimerisation (inject enough antibody to overcome the ligand)
- Binds the receptor to inhibit agonist binding (antagonist)

Very expensive to treat someone with monoclonal antibodies as they are expensive to produce.

Question 18

Cytokine receptors

Answer 18

Receptors for cytokines and some hormones including growth hormone and prolactin.
Many are dimers or dimerise upon ligand binding.
These receptors are stably associated with cytoplasmic tyrosine kinases called Janus Kinases (JAKs)
JAKs phosphorylate and activate transcription factors called STATs (Signal Transducers and Activators of Transcription).
Results in their translocation to the nucleus and changes in gene expression.

Question 19

JAK-STAT signalling pathway

Answer 19

Activated JAK phosphorylates STATs.
STATs are transcription factors (i.e. regulate gene expression)
They translocate to the nucleus, bind DNA and regulate gene expression.

Question 20

Target for anti-inflammatory drugs

Answer 20

Receptor blocking antibodies for rheumatoid arthritis.
Cytokine blocking antibody.
JAK inhibitors are effective for rheumatoid arthritis, inflammatory bowel disease and other autoimmune conditions.