Transcript No Slide Title

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Bi 1 during the next 2 weeks. See also the Bi1 Web Page
Thursday 4/20. Lecture 12. Central dogma, part 2. The G protein pathway
Thursday-Friday 4/20-21. Sections meet as usual.
Monday 4/24 11 AM. Problem Set 4 due, in the Bi 1 Closet.
Monday 4/24. Lecture 13. Recreational drugs.
Tuesday 4/25. Lecture 14.
Central dogma, part 3. Last lecture to be covered on the midterm.
Midterm posted.
Thursday 4/27. Lecture 15. The human genome.
4 PM: Bi 1 Review session. 119 Kerckhoff (Bi1 Lecture room)
Thursday-Friday 4/27-28. Sections meet as usual. No problem set; midterm reviews.
Monday 5/1. No lecture. Work on the midterm.
Monday 5/1. Midterm due, 4 PM. The Bi 1 Closet
Tuesday 5/2. Lecture 16; Problem Set 5 posted.
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receptor
G protein
i q s t
effector
channel enzyme
intracellular
messenger
Ca2+ cAMP
Bi 1 Lecture 12
Thursday, April 20, 2006
The Central Dogma of Drugs and the Brain,
Part 2:
Drugs Act on the G Protein Pathway
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Proof of chemical synaptic transmission, 1921
Vagus nerve
runs from the head to the heart
Spontaneous
heartbeats in both
hearts are
stopped by stimuli
to the “upstream”
vagus
The diffusible
substance:
acetylcholine
acting on
muscarinic
ACh receptors
smoked drum
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Some postsynaptic membranes contain G protein-coupled receptors
rather than ligand-gated channels
cytosol
vesicles containing
serotonin
NH3+
HO
N
H
G protein-coupled
serotonin receptor
vesicles containing
acetylcholine
O
O
N+(CH3)3
G protein-coupled
(muscarinic)
acetylcholine receptor
vesicles containing
dopamine
HO
HO
H2
C
C
H2
NH3+
synaptic
cleft
G protein-coupled
dopamine receptor
cytosol
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On a time scale of seconds,
the language of the nervous system is still electricity;
and the central dogma still describes a set of mechanisms that manipulate
impulse frequencies in individual neurons.
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The central dogma of drugs and the brain, Part 2.
Drugs act on the G protein pathway.
How fast?
100 ms to 10 s
Neurotransmitter or hormone
binds to receptor
How far?
Probably less 1 mm
activates
G protein
Effector:
enzyme or channel
outside
b g
a
GTP
a
GDP + Pi
b g
inside
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G protein-coupled receptors
receptor
G protein
i q s t
1. All have 7 a-helices
effector
channel enzyme
2. There are about 1000 G protein-coupled receptors in the genome.
(Most are still “orphans”; their ligands are unknown)
intracellular
messenger
Ca2+ cAMP
3. Individual receptors respond to:
(a) ) a low-molecular weight neurotransmitter
such as serotonin, dopamine, or acetylcholine
(b) a short protein (8-40 amino acids, a “peptide”) such as an endorphin
(c) an olfactory stimulus;
or
(d) light, in the eye (receptor = rhodopsin)
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receptor
G protein
i q s t
Selective advantage of such a complex pathway?
effector
channel enzyme
The neurotransmitter or hormone does not
directly influence the response--from the viewpoint of
intracellular
messenger
Ca2+ cAMP
(a) Chemistry
(b) Speed
(c) Localization (to some extent)
All this amplification and indirect coupling requires energy!
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receptor
Structure of a heterotrimeric G protein:
a molecular switch
http://www.its.caltech.edu/~lester/Bi1/G protein-alpha-beta-gamma.pdb
G protein
i q s t
effector
Swiss-pdb viewer required
intracellular
messenger
Our first real look at ATP / GTP / GDP:
http://www.its.caltech.edu/~lester/Bi
-1/G protein-GDP.pdb
Note the “propeller” in the b subunit
which caps the a subunit, preventing
either subunit from interacting with the
effector (There is no effector in this
structure):
http://www.its.caltech.edu/~lester/Bi1/G protein-beta-only.pdb
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receptor
G protein
G protein
i q s t
GDP
effector
channel enzyme
Agonist
intracellular
messenger
Ca2+ cAMP
Binding site
on receptor
phosphate
released
to cytosol
Agonist binds succesfully;
GDP is replaced by GTP;
GGTP
protein leaves receptor;

G protein
activates effector
GDP
G protein
awaiting bg
GDP
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G protein  Protein G
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From Lecture 7
How ”tight” is the gigaohm seal?
1. Electrically tight
R = rl/A
R ~ 109 W;
r = resistivity = 22 W-cm;
l = length = 1 mm;
A = area = 10 mm x t (thickness);
Therefore
t ~ 2 x 10-11 m, or less than 1 Å!
1 mm
Little Alberts 12-22A
© Garland
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From Lecture 7
How ”tight” is the gigaohm seal?
acetylcholine in the pipette
opens channels in the pipette
2. Chemically tight
acetylcholine
outside the
pipette opens
channels
outside the
pipette
The seal compartmentalizes
molecules.
Molecules outside the pipette
do not mix with molecules
inside the pipette
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From Lecture 8
How ”tight” is the gigaohm seal?
Strong suction
3a. Mechanically tight
With strong suction, the patch breaks,
but the seal remains intact.
1 mm
Little Alberts 12-22A
© Garland
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K+
Channels are Gi protein effectors
receptor
G protein
i q s t
effector
channel enzyme
intracellular
messenger
Ca2+ cAMP
transmitter
3b. Mechanically tight
Use weak suction.
Excised “inside-out” patch
allows access to the inside
surface of the membrane
+Gbg
Normally:
released from Gi;
Here: added by
experimenter
no GTP
n= 0 (closed)
no channel openings
n = 1 (open)
+Gbg
n = 0 (closed)
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from Lecture 6: Channels
receptor
are miniature conductors
that add in parallel
GNa = SgNa
G protein
i q s t
GK = SgK
effector
channel enzyme
outside
GNa
GK
=
intracellular
messenger
Ca2+ cAMP
gK
gNa
EK
(- 60 mV)
ENa
(+60 mV)
inside
gNa
mostly Na+
mostly K+
gK
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from Lecture 6:
outside
Na+
receptor
K+
Cl-
G protein
i q s t
effector
channel enzyme
G
C
E
inside
resting
potential
E K G K + E Na G Na + E Cl G Cl
DV =
G K + G Na + G Cl
intracellular
messenger
Ca2+ cAMP
+60
mV
additional K+ channels keep the
membrane potential away from
threshold, and therefore decrease
firing rates
-60
1
ms
5
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receptor
Enzymes are Gq, Gs, and Gt protein effectors
Gq
G protein
i q s t
effector
channel enzyme
Enzyme
Ca2+
in vesicles
(not synaptic vesicles)
Ca2+
in cytosol
intracellular
messenger
Ca2+ cAMP
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receptor
Enzymes are Gq, Gs, and Gt protein effectors
G protein
i q s t
effector
channel enzyme
intracellular
messenger
Ca2+ cAMP
ATP
2+
Mg
Mg2+
NH2
N
N
O
O
O
-O P O P O P O CH2 O
H
H
OOOH
OH OH
ATP
cyclic AMP (cAMP)
Gs
NH2
N
N
N
N
N
Enzyme
“cyclase”
O
O
O
P
-O
O
N
H
H
OH
cyclic AMP (cAMP)
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receptor
G protein
i q s t
caffeine prolongs the intracellular messenger cAMP
effector
channel enzyme
intracellular
messenger
Ca2+ cAMP
ATP
cyclic AMP (cAMP)
NH2
N
N
Mg 2+
N
O
O
O
-O P O P O P O CH2 O
H
H
OOOH
OH OH
NH2
N
N
N
N
Enzyme
“cyclase”
O
O
O
P
-O
O
Breakdown enzyme
“phosphodiesterase”
uninteresting
molecule
N
H
H
OH
Inhibited
by caffeine
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receptor
G protein
i q s t
Effector enzyme
“cyclase”
effector
channel enzyme
cAMP
ATP
Inhibited
by caffeine
Breakdown enzyme
“phosphodiesterase”
uninteresting molecule
intracellular
messenger
cAMP
Ca2+ cGMP
Phosphodiesterase inhibitors prolong the life of intracellular messengers
Enzyme
“cyclase”
GTP
cGMP
Breakdown enzyme
“phosphodiesterase”
Inhibited
by Viagra, Cialis, Levitra
uninteresting molecule
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Many drugs are enzyme inhibitors
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receptor
Discussion
G protein
i q s t
Selective advantage of such a complex pathway?
effector
channel enzyme
The neurotransmitter or hormone does not
directly influence the response--from the viewpoint of
(a)
(b)
(c)
Chemistry
Speed
Localization (to some extent)
intracellular
messenger
Ca2+ cAMP
All this amplification and indirect coupling requires energy!
Further advantages? Suggestions in class:
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Diversity of the Central Dogma,
Part 2
receptor
~ 1000 G protein-coupled receptors
All have 7 helices
G proteins all have 3 subunits
There are ~ 18 a subunit genes
in 4 major classes i, q, s, t
~ 5 b subunits
~ 3 g subunits
G protein
i q s t
effector
channel enzyme
intracellular
messenger
Ca2+ cAMP
and many
“accessory
proteins”
There are 2 major types of effectors
Channels affected by G proteins:
~5 known K channel genes
~4 Ca2+ channels
Enzymes
3 major classes, each with 2 to 10 members
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receptor
G protein
i q s t
effector
channel enzyme
intracellular
messenger
Ca2+ cAMP
End of Lecture 12
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