Download Mechanism Of Hormonal Action:3

‫ﺑﺴﻢ اﷲ اﻟﺮﺣﻤﻦ اﻟﺮﺣﻴﻢ‬
‫‪Dr. Hani Alrefai - 2014‬‬
Revision
 cAMP pathway
Dr. Hani Alrefai - 2014
cAMP pathway
Dr. Hani Alrefai - 2014
Revision
 cAMP pathway
 Adenylate cyclase
Dr. Hani Alrefai - 2014
Adenylate Cyclase enzyme
 Adenylate cyclase catalyses the formation of cAMP from ATP.
 Stimulation or inhibition of adenylate cyclase depends on
the type of activated G protein.
hormones activate adenylate
cyclase
hormones inhibit adenylate
cyclase
•FSH, LH, TSH, HCG.
•Glucagons.
•β catecholamines.
•PTH and calcitonin.
•Somatostatin.
•Angiotensin II.
Dr. Hani Alrefai - 2014
•α2 catecholamines
Revision
 cAMP pathway
 Adenylate cyclase
 G regulatory proteins
Dr. Hani Alrefai - 2014
G-regulatory protein
 This protein is so named because it binds to guanine
nucleotides: GDP or GTP.
 It consists of 3 polypeptide subunits (heterotrimeric
protein): α subunit, β subunit and γ subunit
 Under basal conditions the α subunit binds to GDP
 Binding of the hormone to its receptor activates G
protein through:
 Exchange of GDP and GTP on the α subunit.
 Dissociation of active α subunit from β- γ
subunits.
Dr. Hani Alrefai - 2014
A. G-regulatory protein
 The active α subunit (α-GTP) affects the activity of
the next reaction according to the type of G protein:
Gs: Stimulates adenylate cyclase.
Gi: Inhibits adenylate cyclase.
Gq: Stimulates phospholipase C.
Other types affect Ca channels and K channels.
 The α subunit has intrinsic GTPase activity.
 After α subunit exerts its effects, GTPase hydrolyses its GTP
into GDP and Pi.
 The inactive α subunit (α –GDP) reassociates with the β- γ
subunits and remains inactive until reactivated again.
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Dr. Hani Alrefai - 2014
Revision
 cAMP pathway
 Adenylate cyclase
 G regulatory proteins
 PKA
Dr. Hani Alrefai - 2014
Protein Kinase A
 cAMP dependent protein kinase A is a tetramer of:
2 regulatory subunits (R).

2 catalytic subunits (C).
The R2C2 tetramer is inactive.
On activation 4 cAMP molecules bind to the 2 regulatory
subunits leaving the 2 catalytic subunits free to act.
Active protein kinase transfers gamma phosphate group
from ATP to serine or threonine amino acid residues of a
specific protein causing its phosphorylation.
The phosphorylated protein e.g. enzyme, mediates the
hormonal effect.
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Dr. Hani Alrefai - 2014
Revision
 cAMP pathway
 Adenylate cyclase
 G regulatory proteins
 PKA
Dr. Hani Alrefai - 2014
Dr. Hani Alrefai
Dr. Hani Alrefai - 2014
ILOs
 Hormones acting through calcium signaling
 Calcium signaling pathway
 Calmidulin
 Mechanism of action of insulin
 Mechanism of action of GH
Dr. Hani Alrefai - 2014
Dr. Hani Alrefai - 2014
Calcium or/and phosphatidyl inositols
 The hormones which use these second messengers
include:
 Gonadotropin releasing hormones (GnRH)
 Thyrotropin releasing hormone (TRH)
 Antidiuretic hormone (ADH)
 α1 adrenergic catecholamines
Dr. Hani Alrefai - 2014
Calcium or/and phosphatidyl inositols
Gq
PLC
PKC
Dr. Hani Alrefai - 2014
Calcium or/and phosphatidyl inositols
 Binding of the hormone to its specific receptor
activates a specific type of G protein (Gq).
 The active G protein activates the enzyme
phospholipase C (PLC).
 The active PLC hydrolyses phosphatidyl inositol
4,5 biphosphate (PIP2) to:
 1,2 Diacyl glycerol (DAG).
 Inositol Triphosphate (IP3).
Dr. Hani Alrefai - 2014
Calcium or/and phosphatidyl inositols
1.
DAG:
 It activates protein kinase C.
 Active protein kinase C phosphorylates specific
substrates e.g. enzymes which mediate the effects of
the hormone.
2.
IP3:
 It releases Ca2+ from intracellular storage sites:
 Mitochondria.
 Endoplasmic reticulum.
Dr. Hani Alrefai - 2014
Calcium or/and phosphatidyl inositols
 Intracellular Ca2+ could be increased by:
1. Intracellular storage sites: IP3.
2. Extracellular calcium: Membrane Calcium channels.
Some hormones when bound to their receptors
stimulate certain type of G protein that leads to
opening of these channels.
Dr. Hani Alrefai - 2014
Calcium or/and phosphatidyl inositols
 4 Ca2+ molecules bind to calmodulin to form Ca2+/
calmodulin complex.
 Ca2+/calmodulin complex can change the activity of
certain enzymes.
 These enzymes mediate the effects of the hormone
N.B. Ca2+ /calmodulin complex regulates some structural
elements in the cell e.g.
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Actin-myosin complex of smooth muscles.
Microfilament-mediated processes
e.g. mitosis and endocytosis.
Dr. Hani Alrefai - 2014
Calcium or/and phosphatidyl inositols
Calmodulin:
 It is a calcium dependent regulatory protein.
 It is a 17 Kilo Dalton protein.
 It is structurally similar to muscle protein troponin C.
 Calmodulin has 4 Ca2+ binding sites.
Dr. Hani Alrefai - 2014
Dr. Hani Alrefai - 2014
Protein Kinase Cascade
Tyrosine kinases: group of enzymes, which
phosphorylate their substrates on tyrosine residues.
 Tyrosine kinase may be:
1. Intrinsic part of the receptor.
2. Intracellular associated with the receptor .
Dr. Hani Alrefai - 2014
1- Receptor with intrinsic tyrosine Kinase
 e.g. insulin receptor.
• Binding of the hormone to its receptor
1
• Activation of tyrosine kinase receptor
2
• Auto-phosphorylation
3
• Phosphorylation of Insulin Receptor Substrates (IRS)
on tyrosine residues.
4
5
• Activate Phosphatidyl inositol 3 kinase (PI3-kinase):
• Activate Mitogen Activated Protein Kinase Cascade (MAP Kinase Cascade).
Dr. Hani Alrefai - 2014
Activate Mitogen Activated Protein Kinase
Cascade (MAP)
IRS
RAS
RAF
MEK
MAP
kinase
 The active MAP kinase affects certain transcription
factors and ribosomal subunits leading to alteration of
protein transcription …. which mediate the effects of
the hormone.
 RAS is a G protein with intrinsic GTPase activity.
 The other are serine/threonine kinases.
Dr. Hani Alrefai - 2014
2- Receptor with intracellular associated intrinsic
tyrosine Kinase
 e.g. Growth hormone, prolactin, interferon and erythropoietin
1
2
3
4
• Binding of the hormone to its receptor
• Activation of the associated tyrosine
kinases (JAK kinases or TYK kinases )
• Phosphorylation of the receptor.
• Phosphorylation of the enzyme itself.
• Activation a number of signaling
pathways
Dr. Hani Alrefai - 2014
IRS
MAP
PLC
Signal Transiducers and
Activators of Transcription
(STAT)
Signal Transiducers and Activators of
Transcription (STAT)
 STAT proteins are cytosolic proteins.
 The phosphorylated STAT:
 Translocate to the nucleus.
 Bind to specific DNA element.
 Activate transcription
Dr. Hani Alrefai - 2014
Dr. Hani Alrefai - 2014