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Welcome
to the lecture series
Cellular Communication
Thomas Kietzmann
Key points and questions
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What are typical features of a G-protein coupled receptor
What are the effector enzymes and second messengers of Gs, Gi, Gp type of Gproteins?
What is transducin?
How is a receptor associated G-protein activated and what bacterial substances can
interfere with that activation?
How is diacylglycerol generated and what other products with second messenger
function can thereby be generated?
What is protein kinase C and how can it be activated?
What is an EF-hand and for what is it characteristic?
How can calcium activate glycogen breakdown?
What is glucagon, how is it synthesized and what is its major second messenger?
How can protein kinase A be activated?
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Cellular Communication-
TK
Thomas Kietzmann
Mechanism of G-protein activation
GDP 
Pa
H2O


GTP
inactive
s
GTP 
+
R H

GDP

active
+
Effector
ADPR, ADP ribose; GTP/GDP exchange is supported by GEFs, GTP exchange factors
Cellular Communication-
TK
Thomas Kietzmann
G-protein coupled receptors 1
Receptor
Glucagon
GR
 1R
Gs
ACY
Gp(Gq)
PLC
IP3 
 2R
Gi
ACY
 1R,  2R
Gs
ACY
cAMP 
cAMP 
mR-1,-3,-5
mR-2,-4
Gp(Gq)
Gi
ACY
VR-1
Gp(Gq)
PLC
VR-2
Gs
ACY
Noradrenalin
Adrenalin
Acetylcholin
Vasopressin
G-Protein
Effector enzyme
Intracell. Effektor
Stimulus
PLC
cAMP 
IP3 
cAMP 
IP3 
cAMP 
Prostanoide
PGD2
PGE2
DPR
Gs
ACY
EPR-1
EPR-2,-4
Gp(Gq)
Gs
PLC
ACY
EPR-3
FPR
Gi
ACY
PLC
cAMP 
IP3 
ACY
cAMP 
IP3 
cAMP 
IPR
Gp(Gq)
Gs
TXA2
TPR
Gp(Gq)
PLC
cAMP 
IP3 
Light
Rhodopsin
Transducin
cGMP-PDE
cGMP 
PGF2a
PGI2
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Cellular Communication-
TK
Thomas Kietzmann
Glucagon: a hormone signalling via Gs
Glucagon is a 29-amino acid polypeptide
Cellular Communication-
TK
Thomas Kietzmann
The endocrine pancreas
Control (Hematoxylin-Eosin)
Islets of Langerhans (endocrine cell types):
Pancreas
endocrine and exocrine
Duodenum
•Alpha cells producing glucagon (15-20%
of total islet cells)
•Beta cells producing insulin and amylin
(65-80%)
•Delta cells producing somatostatin (310%)
•PP cells producing pancreatic polypeptide
(3-5%)
•Epsilon cells producing ghrelin (<1%)
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Cellular Communication-
TK
Thomas Kietzmann
Localization of glucagon and insulin production in the endocrine pancreas
Control (Hematoxylin-Eosin)
Insulin (  -cells)
Glucagon (-cells)
Cellular Communication-
TK
Thomas Kietzmann
Gene structure
3'
5'
5'
3'
5'-flankiing
region
Promoter
1
A
2
B
3
DNA
3'-flanking
region
Transcription
[Regulation of
transcription]
hnRNA
Processing
5'UTR
3'UTR
mRNA
Translation
S
Exon, expressed sequence 1, 2, 3
Pre-pro-protein
posttranslational
Processing
Intron, intervening sequence A, B
UTR = untranslated region
Protein
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Cellular Communication-
TK
Thomas Kietzmann
Glucagon-Gen
1 A
2
B
3
C
4
D
5
E
6
3'
5'
DNA
hnRNA
3'
5'
K= Lys
R= Arg
5'UTR
Gcg
tGLP1
GLP2
Pre-pro-glucagon
(Polyprotein)
S
-20
Pancreas -cell* :
1
33
61 78 107
33
61
mRNA
3'UTR
KR RR
KR KR
RR
159
126
Glucagon-29
1
30
72
159
GRPP Gcg29
MPGF
Intestine L-cell* :
1
69
Glicentin
78 108 111123 126 159
1
30 33
69
tGLP1
IP2
GLP2
Glicentin
Glucagon-37
GLP1
GLP2
GRPP Gcg37
*Alternative processing of polyproteins
GRPP, glicentin-related pancreatic polypetide; MPGF, major proglucagon fragment; IP, intervening peptide
Cellular Communication-
Thomas Kietzmann
TK
Glucagon-like peptide
GLP-1 physiological functions:
•increases insulin secretion from the pancreas in a glucose-dependent manner.
•decreases glucagon secretion from the pancreas.
•increases beta cells mass and insulin gene expression.
•inhibits acid secretion and gastric emptying in the stomach.
•decreases food intake by increasing satiety.
•promotes insulin sensitivity
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Cellular Communication-
TK
Thomas Kietzmann
Glucagon action
Liver
Muscle
Fat
Transport
Take up of amino acids
+
Metabolism
Glycogen breakdown
+
Glycogen synthesis
--
Gluconeogenesis
+
Glycolysis
--
Protein degradation
+?
(+)
Lipolysis
+
Cellular Communication-
TK
Thomas Kietzmann
Regulation of glucagon secretion
A ( -Zelle
Stimulation
Physiologic
Starvation (low plasma glucose)
Amino acids
Triglycerides via
gi factors
Catecholamines
Sympathicus
Acetylcholine
Cholecystokinin
AA
Glucagon
only
at
Inhibition
Physiologic
Glucose
Free fatty acids
Keton bodies
Somatostatin
Insulin
Glc
+
-
Glucagon
Degradation
Organ specific
Action
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Biochemie IV-III-1-20
TK
Thomas Kietzmann
GPCR: Glucagon receptor
Glucagon-Binding to chimeric Gcg- GLP1 -Receptors
GcgBinding:
+++
+++
G
n
i
-B
cg
G
u
d
g
:
+
n
i
-B
cg
G
u
d
N
n
:
g
e
s
ra
T
n
ä
m
o
D
-B
n
o
g
a
c
u
lG
in
a
g
u
d
P
:G
R
C
rG
e
im
h
c
lu
n
o
g
a
c
1
P
L
-G
g
c
to
p
c
e
R
+
R
rr
to
p
c
e
+++
n
+
+
b
e
n
ra
n
i
-B
o
lca
sG
g
u
d
GcgBinding:
+++
N
GlucagonBindingsite
TransmembraneDomain
 -Helix ~
~25 AA
C
Cellular Communication-
TK
Thomas Kietzmann
Mechanism of G-protein activation
Gi
Gs
CholeraToxin
GDP 
Pa


s
GTP 
+
ADPR
constitutively
active
+
GDP 
GTP
inactive
H2O
CT
PertussisToxin
ADPR
R H


GDP
Pa
H2O


PT
GTP
permanently
inactive
i
GTP 
+
R H

GDP

active
-
Adenylate
Effector
Adenylate
Cyclase
Cyclase
cAMP
Vibrio cholerae
cAMP
Bordetella pertussis
ADPR, ADP ribose; GTP/GDP exchange is supported by GEFs, GTP exchange factors
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Cellular Communication-
TK
Thomas Kietzmann
The adenylate cyclase as a G-protein effector
ATP
+ PPi
Cellular Communication-
Thomas Kietzmann
TK
The adenylate cyclase as a G-protein effector
s
GTP 
active
+
Adenylate
Cyclase
GTPase
activity
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Cellular Communication-
TK
Thomas Kietzmann
G-protein coupled receptors signalling via Gs
AC
GPCR
G G G
cAMP
?
Cellular Communication-
Thomas Kietzmann
TK
Activation of protein kinase A (PKA) by cAMP
PKA
+
C
R
C
R
cAMP
R
C
C
C
C
R
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Cellular Communication-
TK
Thomas Kietzmann
Protein kinase A (PKA) targets (examples)
Glucagon/Adrenalin
GPCR
AC
R
R
cAMP
R
R
C
GPHK
GPH  P
OH
P
Glycogen
C
GPHK 
GPH 
OH
C
C
G G G
+
PKA
G1P
Cellular Communication-
TK
Thomas Kietzmann
Structure of glycogen
CH2OH
H
O
H
CH2OH
O
H
OH
H
H
OH
H
O
O
H
H
OH
H
H
OH
O
 -1,6
CH2OH
CH2OH
H
O
H
OH
H
O
H
H
OH
H
O
H
OH
H
O
H
H
OH
H
O
 -1,4
H
O
H
OH
H
H
OH
O
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Cellular Communication-
TK
Thomas Kietzmann
Glycogen synthesis and breakdown
H 2O
PPi
Glc
Debranching 2
A-1,6-G
UDP
debranched
product
Glycogen
Elongation
Shortening
elongated
primer
GSY
GPH
Branching
1,4-1,6-TG
Debranching 1
1,4-1,4-TG
branched
product
G1P
Glycogenin
"starter"
G1P
GPM
G6P
G6Pase
Glc
Cellular Communication-
UDP -Glc
PPi
branched
primer
UDPG-PPase
ATP
ADP
GK/HK
ATP
Thomas Kietzmann
TK
Activation of protein kinase A (PKA) by cAMP
PKA
+
C
R
C
R
cAMP
R
C
C
C
C
R
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Cellular Communication-
TK
Thomas Kietzmann
The A-kinase anchor proteins (AKAPs)
•The A-kinase anchor proteins (AKAPs) binds to the regulatory subunits (RIα,
RIIα, RIβ, RIIβ) of PKA
•AKAP also binds to either a component of cytoskeleton structure or a
membrane of an organelle, anchoring the enzyme complex to a particular
subcellular compartment
•The AKAPs also bind other components including enzymes like
phosphodiesterases (PDEs) which break down cAMP, phosphatases which
dephosphorylate downstream PKA targets and also other kinases
TK
Thomas Kietzmann
Protein kinase A (PKA) targets (examples)
Glucagon
GPCR
AC
C
C
R
R
R
R
cAMP
C
P
GPHK
C
OH
Glycogen
GPH 
GPH  P
G G G
GPHK 
OH
PKA
+
C
G1P
C
R
OH
Cellular Communication-
CREB
R
AKAP
C
P CREB
CRE
PCK-Gene
C
C
R
R
AKAP
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Cellular Communication-
TK
Thomas Kietzmann
CREB binding to CREs: Coactivation by CBP
CREB is a blzip transcription factor and
binds to CREs
cPKA
CREB subtypes:
CREB1, CREB2 renamed ATF4, CREB3,
CREB5, CREB3L1, CREB3L2, CREB3L3,
CREB3L4
P Ser 133-
-Ser 133 P
CREB binding protein (CBP)
The protein binds to CREB and
coactivates it and stabilizes the
transcription complex
basic leucine zipper
CBP contains a bromodomain, cysteinehistidine-rich region, and histone
acetyltransferase domain
Cellular Communication-
CRE: 5´-TGACGTCA-3´
TK
Thomas Kietzmann
Compartimentalization of glycolysis and gluconeogenesis in hepatocytes
ATP
ATP
ADP
Glucose
ADP
Glc-6-P
Frc-1,6 -P2
P
G-6-Pase
ER
DAP
GAP
Cytosol
NAD+
NADH
1,3-BPG
ADP
ATP
3-BPG
Phosphoenolpyruvate
Carboxykinase (PCK)
CO2
GDP GTP
Phosphoenolpyruvate (PEP)
ADP
ATP
Lactate
NAD+
Oxalacetate
HCO3
-
ATP ADP+P
Pyruvate
Pyruvate
NADH
Oxalacetate
Malate
NADH
NAD+
Malate
Mitochondrium
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Cellular Communication-
TK
Thomas Kietzmann
Gluconeogenesis: Phosphoenolpyruvat Carboxykinase (Cytosol)
COO
C
-
COO
O
CH 2
COO
Oxalacetate
CH 2
PEP
O
O
HN
O~ P
C
GDP
GTP
-
-
O
NH
1
O
C
-
N
1
NH
Biotin
S
Enz
S
Enz
O
O
C
-
Cellular Communication-
Bicarbonate
OH
TK
Thomas Kietzmann
Phosphodiesterases
I
PM
+
PDE4
ACY
PPa
ATP
H 2O
I
O
Adenin
O
- O P O CH
2
I
O
OI
3'
I
I
O
II
II
O P
CH 2
O
OH
3',5'-cyclisches
AMP
(cAMP)
Adenin
I
I
5'
HO - O
I
Gs
HO
OH
AMP
PDE5
+
GCY
PPa
GTP
H 2O
cGMP
GMP
14
Cellular Communication-
Thomas Kietzmann
TK
Phosphodiesterases
The PDE superfamily consists of 11 families: PDE1-PDE11 with different specificities:
•PDE 4, 7 and 8: cAMP
•PDE 5, 6 and 9: cGMP
•PDE 1, 2, 3, 10 and 11: both cAMP and cGMP
Nonselective phosphodiesterase inhibitors: caffeine, aminophylline, theobromine,
theophylline, IBMX (3-isobutyl-1-methylxanthine)
PDE1 selective inhibitors: Vinpocetine
PDE2 selective inhibitors: Anagrelide
PDE3 selective inhibitors: Enoximone, Milrinone,
PDE4 selective inhibitors: Mesembrine, Ibudilast, Piclamilast, Luteolin (peanuts)
PDE4 is the major cAMP-metabolizing enzyme found in inflammatory and immune cells. PDE4
inhibitors are anti-inflammatory drugs (inflammatory pulmonary diseases such as asthma,
COPD, and rhinitis). They suppress the release of cytokines and other inflammatory signals, and
inhibit the production of ROS
PDE5 selective inhibitors: Sildenafil, Tadalafil, Vardenafil, Udenafil, Avanafil, Dipyridamole
Key points and questions
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What is glucagon, how is it synthesized and what is its major second messenger?
What is the difference between glucagon and glucagon-like peptide; how dies the GLP
act and why is it important in terms of glucose metabolism?
How can protein kinase A be activated?
What do you understand under the term cAMP regulatory element binding protein
(CREB) and how is this protein linked to the action of glucagon?
What is major function of biotin in cells?
What reaction is catalyzed by the enzyme phosphoenolpyruvate carboxykinase and in
which cellular compartment does it take place?
Why does coffee increase the intracellular cAMP levels?
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