1.  BY: FOZIYA KHAN
 PHARMACOLOGY BRANCH
 SEM I

2. Contents
 Introduction
 Second messengers
 Classification
 cAMP Pathway
 cGMP Pathway
 Nitric oxide as 2nd messenger
 IP3/ DAG Pathway
 Calcium as 2nd messenger

3. INTRODUCTION
 Second messengers are molecules that relay signals
from receptors on the cell surface to target molecules
inside the cell.
 They greatly amplify the strength of the signal, cause
some kind of change in the activity of the cell.
 They are a component of cell
signaling pathways.
 Earl Wilbur Sutherland Jr.,
discovered second messengers, for
which he won the 1971 Nobel Prize.

4. SECOND MESSENGERS
 Short lived intracellular signaling molecules
 Elevated concentration of second messenger leads to
rapid alteration in the activity of one or more cellular
enzymes
 Removal or degradation of second messenger
terminate the cellular response
 Four classes of second messengers
• Cyclic nucleotides
• Membrane lipid derivatives
• Ca2+
• Nitric oxide/carbon monoxide

5. TYPES OF SECOND MESSENGERS
Hydrophobic
Diacylglycerol
Phosphatidylinositol
Hydrophilic
cAMP
IP3
cGMP
Ca2+
Gases
NO
H2S
CO

6. • cAMP pathway
• cGMP pathway
• IP3 / DAG pathway
• Calcium as a second messenger
Second Messengers

7. Second Messengers
• General characteristics
– Low amounts in resting state
– Regulated synthesis
– Regulated destruction
– Act through other proteins

8. cAMP PATHWAY
Ligands : Epinephrine Ach
Primary Effector : Adenyl cyclase
Secondary messenger : cAMP

9. CYCLIC AMP
 cAMP is a second messenger that is synthesized
from ATP by the action of the enzyme adenylyl
cyclase.
 Binding of the hormone to its receptor activates a G
protein which, in turn, activates adenylyl cyclase.
 Leads to appropriate response in the cell by either
(or both):
• using Protein Kinase A(PKA) —a cAMP-dependent
protein kinase that phoshorylates target proteins;
• cAMP binds to a protein called CREB(cAMP response
element binding protein), and the resultant complex
controls transcription of genes.
 Ex of cAMP action -adrenaline, glucagon, LH

11. • Cyclic AMP activates Protein Kinase A (PKA)
• In addition to signaling in the cytoplasm, the catalytic
subunit of PKA can enter the nucleus of cells and
phosphorylate and activate the transcription factor
cAMP response element binding (CREB) protein.
• Phospho-CREB protein increases the transcription of
many genes
• Indirect effect of cAMP mediated by PKA

13. CGMP PATHWAY
Ligands : ANP & NO
Primary Effector : Guanylate cyclase
Secondary messenger : cGMP

14. CYCLIC GMP
 cGMP is synthesized from the nucleotide GTP using
the enzyme guanylyl cyclase.
 Nitric oxide stimulates the synthesis of cGMP.
 Many cells contain a cGMP-stimulated protein kinase
that contains both catalytic and regulatory subunits.
 Some of the effects of cGMP are mediated through
Protein Kinase G(PKG)
 cGMP serves as the second messenger for
• nitric oxide (NO)
• the response of the rods of the retina to light.

15. cGMP Pathway
Ligand-receptor
Guanylyl cyclase PDE
GTP cGMP GMP
Protein kinase G Active protein kinase G
Effects

17. Nitric oxide (NO)
NO, a simple gas, is able to diffuse across the
membrane, and alters the activity of intracellular
target enzymes. It’s extremely unstable, so its effects
are local. Ex. It signals the dilation of blood vessels.
Mechanism: Acetylcholine is released from the
terminus of nerve cell in the blood vessel wall. The
endothelial cells are stimulated to produce NO (from
Arginine), which causes an increased synthesis of
cGMP, a second messenger responsible for blood
vessel dilation.

19. PHOSPHATIDYLINOSITOL-DERIVED SECOND
MESSENGERS
 Phosphatidylinositol (PI) is a negatively charged
phospholipid and a minor component in eukaryotic cell
membranes.
 The inositol can be phosphorylated to form
• Phosphatidylinositol-4-phosphate (PIP)
• Phosphatidylinositol-4,5-bis-phosphate (PIP2)
• Phosphatidylinositol-3,4,5-trisphosphate (PIP3)
 Intracellular enzyme Phospho lipase C (PLC),hydrolyzes
PIP2 which is found in the inner layer of the plasma
membrane. Hydrolysis of PIP2 yields two products:
• Diacylglycerol (DAG)
• Inositol-1,4,5-trisphosphate (IP3)

20. IP3 /DAG
LIGAND+RECEPTOR
G protein
Phospholipase C
PIP2 IP3 + DAG
Endoplasmic reticulum opening of Ca channels
Ca++
Protein kinase C Effects

22. IP3/DAG
 Inositol
triphosphate
• Hydrophilic
• Agonist for internal
calcium channel
• [Ca++]i rises
• Multiple effects
through Ca++ binding
protein
 Diacylglycerol
• Hydrophobic
• Target PKC(a kinase)
• PKC requires Ca++
and DAG

23. Calcium as a 2nd Messenger
• Calcium ions - once they enter the cytoplasm exert
allosteric regulatory effects on many enzymes and
proteins.
• Calcium acts as a second messenger by indirect
signal transduction pathways such as via G protein-
coupled receptors.

24. • Low cytoplasmic Ca++ at rest (10–100 nM).
• To maintain this low concentration, Ca2+ is actively
pumped from the cytosol to the extracellular space and
into the endoplasmic reticulum (ER)
• Certain proteins of the cytoplasm and organelles act
as buffers by binding Ca2+.
• Signaling occurs when the cell is stimulated to release
calcium ions (Ca2+) from intracellular stores, and/or
when calcium enters the cell through plasma
membrane ion channels.

25. • sudden increase in the cytoplasmic Ca2+ level up to
500–1,000 nM by opening channels in the
endoplasmic reticulum or the plasma membrane.
• Phospholipase C pathway – IP3 & DAG
• Many of Ca2+-mediated events occur when the
released Ca2+ binds to and activates the regulatory
protein calmodulin.
• Calmodulin may activate calcium-calmodulin
dependent protein kinases, or may act directly on
other effector proteins.
• Besides calmodulin, there are many other Ca2+-
binding proteins such as troponin C that mediate the
biological effects of Ca2+.

27. Calmodulin Targets
• Adenylate cyclase
• Phosphodiesterase
• Myosin light chain kinase
• Calmodulin-dependent kinases
• Calcineurin (a phosphatase)