L06- Mechanisms of action of peptide hormones

Question 1

Describe how peptide hormones produce intracellular signals?

Answer 1

Peptide = polar
Cannot penetrate plasma membrane
» bind to extracellular domain of specific cell-surface receptor protein
» Cause receptor conformational change at intracellular domain
» produce intracellular signal

Question 2

List some examples of peptide hormones?

Answer 2

• Calcitonin • Glucagon
• Endorphins • Adrenocorticotropic hormone
• Antidiuretic hormone• Oxytoxin

Question 3

Describe the mechanism of GPCR for cAMP production.

Answer 3

α-subunit attach to plasma membrane through lipid anchor
α-subunit binds GDP

1) Hormone-receptor complex cause conformational change in transmembrane domain of GPCR
2) cause GDP dissociation and GTP attachment to activate α-subunit on G-protein
3) βγ-subunits dissociate
4) α- subunit associates with adenyl cyclase (AC)

Question 4

Describe the mechanism of tyrosine kinase receptors?

Answer 4

1) Ligand binding cause dimerization/ oligomerization
2) Mutual self-phosphorylation of tyrosine residues on cytosolic domain
3) Activate protein kinases at intracellular domain&raquo_space; phosphorylate adaptor proteins&raquo_space; in turn phosprylate effector proteins for intracellular signalling

Question 5

List 3 examples of effector proteins for making secondary messengers?

Answer 5

Adenylyl cyclase (AC)

Guanylyl cyclase

Phospholipase C (PLC)

Question 6

Reaction catalyzed by Adenylyl cyclase?

Answer 6

turns ATP into adenosine 3’,5’-cyclic monophosphate (cAMP) + PPi

Question 7

Reaction catalyzed by Guanylyl cyclase?

Answer 7

turns GTP into guanosine 3’,5’-cyclic monophosphate (cGMP) + PPi

Question 8

Reaction catalyzed by PLC?

Answer 8

cleaves phosphatidylinositol 4,5-bisphosphate (= phospholipid) to produce:

1. Inositol 1,4,5-trisphosphate (InsP3/IP3)
2. diacylglycerol (DAG)

Question 9

Compare the functions of IP3 and DAG as intracellular signals?

Answer 9

Inositol 1,4,5-trisphosphate (InsP3) = releases Ca2+ from endoplasmic reticulum

Diacylglycerol (DAG) = activates protein kinase C

Question 10

Define the 2 routes to form IP3 and DAG?

Answer 10

1) Activate GPCR/7-transmembrane receptors
» Activate G-protein&raquo_space; α-subunit (GTP-bound) (and sometimes βγ-subunits) activate PLC
» Produce IP3 and DAG

2) Activate kinase/ single transmembrane region receptors
» receptor activation and cross phosphorylation
» activate PLC
» produce IP3 and DAG

Question 11

Describe how G proteins of GPCR can intrinsically self-limit their activity? On-Off control

Answer 11

• Intrinsic GTPase activity of α-subunit hydrolyzes GTP to GDP&raquo_space; cause adenylyl cyclase to dissociate from GDP/α-subunit&raquo_space; inactivation
• βγ subunits stabilize the association of GDP with Gα

> > ready for another cycle of activity

Question 12

Explain how the hormone/ receptor complex itself can self-limit/regulate its activity?

Answer 12

1) Hormone binding changes receptor conformation
» act as guanine nucleotide exchange factor (GEF)
» increase exchange of GDP for GTP on Gα
» ACTIVATE

2) GTPase accelerating protein (GAP, aka regulator of G-protein signaling (RGS)) promote the intrinsic GTPase activity of Gα&raquo_space; faster hydrolysis of GTP into GDP
» INACTIVATE

Question 13

WHat enzyme degrades cAMP?

Answer 13

𝑝ℎ𝑜𝑠𝑝ℎ𝑜𝑑𝑖𝑒𝑠𝑡𝑒𝑟𝑎𝑠𝑒

turns cAMP into adenosine 5’
monophosphate (AMP)

Question 14

Describe the pattern of activity of phosphodiesterase?

Answer 14

On and off rhythmically to decrease or increase cAMP level within cell&raquo_space; oscillation

Question 15

List all the intrinsic modulations of G protein and adenylyl cyclase activity?

Answer 15

• Intrinsic enzyme activity of a-subunit to turn GTP into GDP
• Stabilization of GDP to a-subunit by βγ-subunits
• Receptor conformational change to act as guanine nucleotide exchange factor (GEF)
• GTP-ase accelerating protein (GAP)

Question 16

List all the extrinsic inhibition on G protein and adenylyl cyclase activity?

Answer 16

• Activate Phosphodiesterase to degrade cAMP

- PGE1, adenosine activate Gai&raquo_space; inhibition on adenylyl cyclase

Question 17

Describe the mechanism of PGE1/ adenosine on adenylyl cyclase?

Answer 17

PGE1/ adenosine = inhibitory ligands

Complex with inhibitory G protein - Gαi

> > α-subunit of Gαi bind to adenylyl cyclase

> > Inactivation

Question 18

List the different levels of intervention to regulate production of peptide hormone?

Answer 18

intervene at different points:

• at gene transcription level
• Translation at RER (preprohormone, prohormone)
• Processing at Golgi complex
• Packaging of mature hormones into secretory vesicle
• Transport of secretory vesicle
• Exocytosis

Question 19

Explain how GnRH causes release of FSH and LH in anterior pituitary? detail intracellular signalling

Answer 19

GnRH binds to GnRH receptor (= GPCR) on gonadotropic cell membrane

phospholipase C activation: turn PIP2/ phospholipids into:

i) DAG (activate PKC)
ii) IP3&raquo_space; increase intracellular Ca&raquo_space; induce exocytosis of FSH and LH

Question 20

Describe the intrinsic negative feedback control of hormone receptors when triggered by high cytosolic cAMP.

Answer 20

High cytosolic cAMP activates cAMP-dependent protein kinase (cAPK)*****

> > activate protein kinase A

> > nonspecifically phosphorylates receptors

> > decrease affinity of hormones to receptors/ desensitization

Question 21

Which type of hormone receptor is down-regulated by GRK-arrestin pathway?

Answer 21

downregulates β adrenergic receptors

Question 22

Mechanism of GRK-arrestin pathway?

Answer 22

1) βγ subunits recruit GRK to hormone-bound receptor
2) GRK phosphorylate specific a.a. on receptor for β-arrestin to bind
3) β-arrestin bind to receptor = promote receptor endocytosis by membrane invagination
4) clathrin-coated vesicles migrate into cytosol and fully internalized
5) receptor inside vesicle either degraded or recycled later

Question 23

What is the function of down-regulation of hormone receptors?

Answer 23

Prevent cells from overstimulation by a particular extracellular hormone

by reducing the abundance of the receptor

Question 24

Which ion level determines the secretion of peptide hormones by exocytosis?

Answer 24

Calcium ion

Question 25

Name the 2 molecules for desensitization of intracellular cAMP mechanisms.

Answer 25

βARK- G-protein coupled receptor kinase - triggered by increased βγ subunits

cAPK - cAMP-dependent protein kinase - triggered by increased cAMP levels

> > both phosphorylate surface receptors for desensitization when intracellular cAMP is high