L4: Receptors

State the difference between allosteric and orthosteric drugs ?

Allosteric drugs bing elsewhere on protein surface but orthosteric drugs bind at active site

Define receptor ?

Specific proteins whose job it is to respond to an endogenous ligand which can be a NT/hormone which leads to some signalling within that cell specifically

What are the three cellular locations of receptors ?

1) Nuclear receptors (interact directly with DNA found in cytosol then travel to nucleus)

2) Intracellular membrane (found in structures like ER )

3) Plasma membrane receptors

What is the cellular location of most receptors ?

Plasma membrane receptors

- ligands that can not cross the membrane

- They work on the receptor first for something to happen in the cell

What is the signature of binding to DNA?

Zinc finger

- tells us that the protein interacts indirectly to DNA

What is a zinc finger ?

A particular motif of proteins with a amino acid sequence that will bind to zinc

State two examples of nuclear receptors ?

Oestrogen receptors

Retinoic acid receptors

Describe how nuclear receptors function ?

- Agonist dimerises once ligand binds

- Enters the nucleus

- Binds directly to DNA

- Alters mRNA produced

- Alters protein synthesis of whole cells

(so powerful effect )

How many subgroups of nuclear receptors are there ?

6

What are orphan receptors

Receptors where their structure is known but we are unaware of their endogenous ligand

What are the two families of intracellular membrane receptor ?

1) Ryanodine receptor - activated by calcium & other modulators.

2) IP3 receptor - activated by the second messenger inositol 1,4,5 trisphosphate (IP3).

Where are intracellular membrane receptors commonly found ?

On sarcoplasmic reticulum (muscle) and endoplasmic reticulum (neurones & other cells )

State an organelle which is referred to as a calcium store ?

Endoplasmic reticulum

What are the three subfamilies of plasma membrane receptors and their alternative names?

1)Ligand gated ion channel superfamily (ionotropic, receptor operated channels)

2) G-protein coupled receptor superfamily (metabotropic, seven TMD receptors)

3)Intrinsic enzyme receptor superfamily (pleiotropic, receptor tyrosine kinases)

What are the three subfamilies of ligan-gated ion channels ?

1) Nicotinic R like - nAChR, 5-HT3 , GABAA , Glycine, ZAC

2) ATP R - P2X (7 subtypes)

3) Ionotropic Glutamate R - NMDA, AMPA, Kainate

Which nicotinic receptors have a negative amino acid sequence in pore?

nACHR, 5-HT3, ZAC (so +ve ions enter)

Which nicotinic receptors have a positive amino acid sequence in pore?

GABAa,Glycine

(so -ve ions enter)

What are the similarities in the structure of the three ligand-gated ion channels ?

Similarities

- All have an extracellular LBS

- All TMD

- Most have alpha helixes

What are the differences in the structure of the three ligand-gated ion channels ?

Nicotinic R

- 5 subunit, 4 TMD(alpha helixes), N & C terminus is extracellular

ATP R

- 3 Subunit, 2 TMD, N and C terminus is intracellular

Ionotropic Glut R

- 4 subunits, 3 TMD, N terminus is extracellular and C terminus is intracellular, pore is inverted p loop

What is the relationship between number of subunits and TMD

To find out the number of subunits from TMD add one

State the excitatory receptors which cause depolarisation ?

nAChR, 5-HT3, P2X, NMDA, Kainate, AMPA, ZAC

State the inhibitory receptors which cause hyperpolarisation ?

GABAa, Glycine

Why receptors are only GPCR ?

NA, Cannabinoids, DA, NPY

State three of the GPCR subfamilies and give examples ?

1)Rhodopsin like - (hundreds) e.g. mACh, opioid, noradrenaline, canabinoid, dopamine, ATP, NPY, all 5-HT except 5-HT3, 2)Metabotropic Glutamate Receptor like - GABAB , mGluRs(1..8) 3)Secretin Receptor like- VIP, Parathyroid hormone

What is the biggest family of GPCR

Rhodopsin

What are the similarities and differences between Rhodopsin-like and mGluR-like?

Similarities

- 7TMD

- N terminus extracellular C terminus intracellular

Differences

- LBS for R like is deep within TMD

- mGluR acts as dimer so 2 x 7 TMD

- mGluR LBS is extracellular one LBS binds agonist the other activates G protein

Describe what happens to GPCR when an agonist binds ?

1) Binding leads to activation so R changes shape

2) This shape change allows R to couple to G protein

3) G protein then activates enz or ion channels

What do the different G protein subunits activate ?

Alpha - activates enz which can lead to 2nd messenger which can activate ion channels or other things inside cells

Beta Gamma - activates or inhibits ion channels

What are the two subunits which physiologically stick together ?

Beta and Gamma

Name the intrinsic enzyme families and examples ?

1) Receptor tyrosine kinases - Insulin receptor, Nerve growth factor receptor (TrkA)

2)Receptor guanylate cyclase - Atrial natriuretic peptide (ANP) receptor

Describe the structure of the insulin, NGF and ANP receptor ?

Insulin R

- 2 TMD

- Extracellular binding site

- Tyrosine kinase coupled

NGF R

- LBS

- 1 TMD

- Tyrosine kinase coupled

ANP R

- LBS

- 1 TMD

- Guanylyl cyclase coupled

State the role of tyrosine kinase receptors ?

Phosphorylation of proteins

Which type of receptors have the role to convert GTP to cGMP ?

Intrinsic Enz R

- Guanylate cyclase

- ANP R

(powerful 2nd messenger)

Describe how intrinsic enzyme receptors work ?

1) Agonist binds

2) The monomers dimerise (couple together)

3) Autophosphorylation of each other as they both have tyrosine kinase enz connected to them

4) Catalytic site opens => substrate phosphorylation

Describe protein synthesis simply?

1) DNA is converted to RNA in Nucleus

2) RNA goes through Rough ER to Smooth ER where it becomes protein

3) Then the protein is glycosylated in the golgi

Where are proteins checked ?

the sort system