M&R 6.1 Receptors in cell signalling

Question 1

Intercellular signalling molecules can be ________ or ________

Answer 1

Secreted (e.g. hormones, NTs, chemical mediators)

Membrane bound (e.g. in contact inhibiton)

Question 2

What is a ligand?

Answer 2

Any molecule that binds specifically to a receptor site

Question 3

Define ‘receptor’

Answer 3

A molecule that specifically recognises a ligand or a family of ligands, and ligand binding causes regulation of a cellular process.
In unbound state - functionally silent

Question 4

Name the 2 main areas of a cell where receptors are found, and what types of ligands might bind to them

Answer 4

On the cell surface (hydrophilic ligands)

Intracellular - in the nucleus or cytoplasm (small hydrophobic ligands, e.g. steroid hormones, thyroid hormones)

Question 5

Define ‘acceptors’

Answer 5

Molecules whose activities are modified by the binding of ligands
But their basic function can be carried out without the interaction of a ligand
(e.g. VGNCs - operate in absence of ligands but can be modified by them (e.g. by local anaesthetics))

Question 6

What is higher, the affinity of a substrate for an enzyme site (Km) or the affinity of a ligand for a receptor site (Kd)?

Answer 6

The affinity of ligands for receptor sites sends to be higher (Kd is lower than Km - conc required to fill half the sites is lower)

Question 7

How are receptors classified and subclassified?

Answer 7

1. By the agonist they recognise (e.g. ACh receptor)
2. Then by antagonists they recognise (e.g. nicotinic vs muscarinic AChRs
3. Then by affinity of antagonists (e.g. M1, M2, M3 mAChRs each have a more selective affinity for a certain antagonist)

Question 8

What are some similarities between receptor binding sites & enzyme binding sites (both active & regulatory)?

Answer 8

1. Binding is site specific
2. Specificity governed by shape of binding cleft
3. Specificity of binding confers specificity of outcome
4. Binding is often reversible
5. Binding can induce a conformational change (e.g. induced fit) and a change in activity
6. In both receptors and enzyme regulatory sites, the ligand is not chemically changed

Question 9

What are some differences between receptor binding sites and enzyme binding sites?

Answer 9

1. Ligand->receptor has higher affinity than substrate->enzyme
2. Substrates binding to enzyme active sites get chemically changed, ligand binding to receptors do not

Question 10

Name 4 common mechanisms by which extracellular signals are transduced into intracellular events

Answer 10

1. Via membrane-bound receptors with integral ion channels
2. Via membrane bound receptors with integral enzyme activity (e.g. tyrosine kinase)
3. Via membrane bound receptors which couple to effectors via tranducing proteins (e.g. GPCRs)
4. Via intracellular receptors (for hydrophobic ligands)

Question 11

‘Classical’ ligand-gated ion channels have how many subunits and TM domains? Where is the binding domain?

Answer 11

5 subunits (pentameric structures)
Each subunit has 4 TM domains (Domain #2 forms the pore lining) 

The N terminal forms the binding domain.

Question 12

How are ligand-gated ion channels selective for certain ions?

Answer 12

They have a ring of charged amino acid residues in the gating area.
E.g. a nAChR is selective for cations, so has a ring of negatively charged residues

Question 13

Name some classical ligand-gated ion channels, and the ions they allow through

Answer 13

nAChR (gated cation channel - Na+, K+, Ca2+)
GABA receptor (gated Cl- channel)
Glycine receptor (gated Cl- channel)
Glutamate receptors (e.g. NMDA, AMPA - gated Ca2+ channels)

Question 14

Name some non-classical ligand-gated ion channels

Answer 14

ATP-sensitive K+ channel
IP3 receptor
Ryanodine receptor (gated Ca2+ channel)

Question 15

In general terms, how do membrane-bound receptors with integral enzyme activity work?

Answer 15

Agonist binds to the extracellular domain and causes a conformational change, which activates an intrinsic enzyme activity contained within the structure of the receptor.

Question 16

Name 2 enzymes that can be linked with receptors

Answer 16

Tyrosine Kinase
Guanylyl cyclase (GTP-->cGMP)

Question 17

All growth factors act via ________ _________ - linked receptors

Answer 17

Tyrosine kinase

they all effectively want to send the same message so they all act in the same way in order to have the same effect

Question 18

Receptors with integral enzyme activity tend to have ___ parts. Why?

Answer 18

2 parts (they are dimers) 
So they can move relative to each other and reveal an active site that wasn't available before

Question 19

Describe how tyrosine-kinase linked receptors work

Answer 19

Agonist (e.g. hormone) binds to extracellular binding sites
This activates protein kinase activity in the cytoplasmic domain, which autophosphorylates tyrosine residues (Y) on the cytoplasmic domain

Phosphorylated tyrosine residues (P-Y) can be recognised:

• -> directly by enzymes containing phosphotyrosine recognition sites [SH2 domain]
• -> by transducing proteins (e.g. IRS1) which then activate an enzye

The enzyme transduces the message into an intracellular chemical event

Question 20

In general terms, how do GPCRs transduce an extracellular signal into an intracellular event?

Answer 20

Binding of ligand causes a conformational change which activates GDP/GTP exchange in G proteins.
G proteins transduce the message onto an enzyme or channel in the membrane

Question 21

Briefly describe the structure of a GPCR

Answer 21

7 TM domains
Ligand binding site on N terminal
G-protein coupling domain on C-terminal
G protein has 3 subunits (alpha, beta and gamma) - splits into alphas and beta-gamma subunits. Alpha subunit can vary (e.g. s, i, q)

Question 22

What kinds of ligands bind to intracellular receptors? Name some examples.

Answer 22

Hydrophobic ligands e.g. steroid hormones (cortisol, oestrogen, testosterone) or thyroid hormones (T3 & T4)

Question 23

What are intracellular receptors like at rest and what happens when they are activated?

Answer 23

In the resting state they are stabilised by association with heat-shock or chaperone proteins

When activated, the receptor dissociates from the chaperone proteins and translocates to the nucleus. In the nucleus it binds to control regions in DNA to regulate gene expression

Question 24

Are the effects of intracellular receptors fast or slow?

Answer 24

Relatively slow, because transcription and translation are required

Question 25

Name some cells whose processes can be either activated or inhibited depending on which receptor is stimulated

Answer 25

Hepatocytes (insulin stimulates glycogen synthesis, glucagon stimulates glycogen breakdown)

Cardiac pacemaker cells (NA acting at beta1 adrenoceptors increases heart rate, whereas ACh acting at M2 mAChRs decreases heart rate)