Session 6 - Receptors + Membrane Turnover

Question 1

What is the affinity of a ligand to a receptor in comparison to enzymes and substrates? And why is this necessary?

Answer 1

Much higher

This is required because Ligands are often present in much smaller concentrations than substrates

Question 2

What is an acceptor? How are receptors different?

Answer 2

An acceptor operates in the absence of its ligand

Receptors are silent are rest

Question 3

Define a ligand

Answer 3

Any molecule that binds specifically to a receptor site

Question 4

Differentiate agonists and antagonists?

Answer 4

An agonist causes activation of a receptor

An antagonist binds without causing activation

Question 5

What kind of molecules require signal transduction to exert their actions?

Answer 5

Hydrophilic signalling molecules

Question 6

What three ways can a membrane transduce a signal to the inside of the cell?

Answer 6

• Intergral ion channels
• Integral enzyme activity
• By coupling to effectors through transduce get proteins

Question 7

Describe the classical ligand-gated ion channel and give an example

Answer 7

Petameric subunit structures with 4 transmembranous domains

E.g. Nicotinic Ach receptors

Question 8

How does binding of a ligand to a receptor with integral enzyme activity cause a cellular response? And give an example

Answer 8

• binding causes a conformational change
• this change activates intrinsic enzyme activity

E.g. Tyrosine kinase linked receptors

Question 9

How do tyrosine kinase receptors cause a cellular response upon activation by a ligand?

Answer 9

• dimers auto phosphorylate each other
• phosphorylated receptors are recognised either by transduce get proteins or directly by enzymes
• Effector enzymes become activated allosterically/ or by tyrosine phosphorylation by the receptor

Question 10

Describe the structure of G-protein coupled receptors

Answer 10

7 transmembranous domains coupled to an effector molecule via a transducing molecule

Question 11

Name some examples in the body where G-protein coupled receptors are present

Answer 11

Muscarinic Ach receptors, dopamine receptors, light, smell + taste receptors

Question 12

What is integrated signalling?

Answer 12

Seperate G-protein coupled receptors act simultaneously to both inhibit and activate the effector

Question 13

Name some hydrophobic Ligands

Answer 13

Cortisol, oestrogen, testosterone + T3+4

Question 14

How do hydrophobic Ligands cause a cellular response?

Answer 14

• pass through plasma membrane, into the cell
• activated receptors are usually bound to chaperone proteins which dissociate upon activation
• receptors translocates into nucleus, binds to control regions, regulating gene expression
• usually slower

Question 15

Name the ligand and receptors that can increase and decrease heart rate

Answer 15

Increase = Beta-1 adrenoreceptors - Noradrenaline

Decrease = M-2 Muscarinic receptors - Ach

Question 16

Name the ligands and that can cause glycogen breakdown and synthesis in hepatocytes

Answer 16

Synthesis = Insulin

Breakdown = Glucagon

Question 17

Describe the process of phagocytosis

Answer 17

• binding of specific cell surface receptors
• cell extends pseudopods that permit further receptor interactions + membrane invagination
• phagosomes fuse with lysosomes to form phagolyosomes which permits the degradation of the material

Question 18

Describe the process of pinocytosis

Answer 18

• invagination of plasma membrane to form lipid vesicle
• this permits uptake of impermeable extracellular salutes + retrival of plasma membrane
• RME is a sub type

Question 19

How does a non-functioning receptor effect effect the uptake of LDLs?

Answer 19

• prevents binding and hence uptake of LDL
• will occur if there is a mutation in the LDL binding site
• can cause a Hypercholesterolaemia

Question 20

How can Hypercholesterolaemia still occur even if the receptor functions perfectly well?

Answer 20

• if the is a deletion mutation in C-terminal sequence
• prevents interaction of receptor with clathrin coat
• LDL receptors distributed over entire cell surface
• need to be concentrated to clathrin coated pits for efficient uptake

Question 21

What is the fate of the receptor and ligand in LDL uptake and what’s is the function of the mechanism?

Answer 21

• Receptor is recycled
• Ligand is degraded
• Function is for metabolite uptake

Question 22

What is the fate of the receptor and ligand in transferrin uptake and what’s is the function of the mechanism?

Answer 22

• Receptor is recycled
• Ligand is recycled
• Function is for iron uptake (metabolite uptake)

Question 23

What is the fate of the receptor and ligand in Insulin uptake and what’s is the function of the mechanism?

Answer 23

• Receptor is degraded
• Ligand is degraded
• Function is to down regulate the amount of insulin receptors on the cell surface to stop over absorption of glucose

Question 24

What is the fate of the receptor and ligand in IgA uptake and what’s is the function of the mechanism?

Answer 24

• Receptor is transported
• Ligand is transported
• Function is transfer large molecules across the cell

Question 25

What is mechanism by which some enveloped viruses can take advantage of RME?

Answer 25

• adventitious binding to cell surface receptors allows them to be endocytosed
• once in endosome low pH allows viral membrane fusion with that of enzyme, releasing viral RNA into cell
• uses hosts machinery to form new particles

Question 26

Give two examples of viruses that take advantage of RME

Answer 26

Cholera toxin and Diptheria toxin

Question 27

Define receptor

Answer 27

A molecule that specifically recognises another molecule or family of molecules, and in response to binding brings about the regulation of a cellular process