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Rhodopsin, a G-Protein Coupled Receptor
Hanson & Stevens, 2008
Signal Transduction & G-Protein Coupled Receptors
Sigma-Aldrich
Physiological Roles
1) Vision (Rhodopsin in Rods); (Photopsins in Cones)
2) Olfaction (Several hundred receptors)
3) Behavior & mood regulation (receptors for neurotransmitters
including dopamine, serotonin, GABA, opiates,glutamate)
4) Immune system & inflammation (e.g. histamine receptors)
5) Autonomic nervous transmission, blood pressure, heart rate,
digestion (e.g. beta adrenergic receptors)
Morphine
diacetylmorphine
(heroin)
adrenaline
Cloning & Early Structural Model of Rhodopsin
(the Light Absorbing Molecule of Vertebrate Rod Cells)
Ovchinnikov 1982, Hargrave 1982
K296
Hargrave 2001
In 1986 it was discovered that the structure and key amino
acids of rhodopsin were shared by the receptor that binds
adrenaline......this was astonishing.
Hamster βAR1
Bovine rhodopsin
Mechanism of Photo-Transduction by Rhodopsin
1) hv discrimination & absorbance
2) Receptor activation
3) G-protein activation
Rhodopsin Coupled to its Heterotrimer
G-Protein, Transducin
Physiology of Photo-Transduction
Rhodopsin & Vision
Portuguese Neurological Society
Dark
Light
Glutamate
Amplification in the Photo-Transduction Pathway
1 photon
1 rhodopsin activated
500 transducin molecules activated
1 cGMP phosphodiesterase activated per transducin
1 cGMP phosphodiesterase activated per transducin
1000 cGMP/s cleaved per phosphodiesterase
Fesenko, 1985
Rhodopsin: Structure & Activity
Palczewski, 2000
Inactive Receptor
(Rhodopsin Model)
1) Inactive
receptor
2) Ligand bound
(activated)
receptor
3)Binding of heterotrimeric G protein
(transducin) to activated receptor
Dark
hv
(fs)
(us)
Retinal
MI
(Metarhodopsin I)
Retinol + opsin
The scale of the 11 cis to all trans retinal structural change is
significant as illustrated here
(but the exact position is unknown)
All
retinal
11trans
cis retinal
H+
TMVII
Dark
hv
(fs)
Retinal
MI
(Metarhodopsin I)
(us)
Deprotonation
(ms)
H+
MII
(Metarhodopsin II)
Activated R*
Hydrolysis
(minutes)
Retinol + opsin
Orienting cis-11-Retinal in the Inactive Receptor
Retinal
TMVII
Palczewski, 2000
Retinal
Dark
K 296 (TMVII)
1)Schiff base at K296
TMVII
Rhodopsin & Photopsins are Unsual Because Ligand is Tethered
& Normally Maintains Inactive Receptor. Another Tethered Ligand:
Thrombin Receptor
Nature Publishing
TMIII
TMVII
Palczewski, 2000
1)Schiff base at K296
2)Counterion at E113
TMIII
TMVI
TMVII
Palczewski, 2000
1)Tryptophan at W265
TMIII
TMVI
TMVII
Palczewski, 2000
1)Phenylalanine at F261
II)Alanine at A269
TMIII
TMVI
TMVII
Palczewski, 2000
Residues of TMIII
Light Absorbance by Rods and the Three Types of Cones Gives
Additional Clues to Which Amino Acids Strongly Interact
with 11-cis-retinal
Blue
Cone
Green Red
Rod Cone Cone
437nm
498nm 533nm 564nm
Dowling, 1987
via U. Florida, Physics
400
450
500
550
Wavelength (nm)
600
650
700
1) Localized effects
1I) Delocalized effects
‘HOOP’
Mathies Friends, 1999
Methanol Control
Protonated 11 cis retinal (PSB) dissolved in
solvent with polar residues
Mathies Friends, 1999
Blue Cone
437nm
KEY POINTS:
1) No strong location-specific
differences relative to methanol
2) No strong delocalized effect
relative to chromophore in methanol.
Prediction: Blue pigment retinal
interacts with polar amino acids in
key positions.
Mathies Friends, 1999
Green Cone
533nm
KEY POINTS:
1) HOOP and ‘fingerprint modes
are virtually identical to blue cone.
2) Dramatic shift in C=C stretch.
3) Vibration at the Schiff base
changes dramatically relative to blue
chromophore.
Mathies Friends, 1999
Molecular explanation for blue to green shift: amino acids proximal
to Schiff base go from polar to non-polar
S90 to A90
S295 to A295
Red Cone
564nm
KEY POINTS:
1) Fingerprints nearly identical to
blue, green pigments.
2) HOOP mode is identical to
blue, green pigments.
3)C=NH unchanged relative to
green.
4) Ethylenic mode shifted further
down in wavelength.
Mathies Friends, 1999
Molecular explanation for green to red shift: amino acids distal from
the chromophore go from apolar to polar
F261Y and
A269T
(Apolar to
polar residues)
Mutations associated with congenital stationary night blindness
TMII
G90
T94
D
I
A292
TMVII
E
Activated Receptor
1) Inactive
receptor
2a) Photoisomerization,
or
ligand binding
to receptor
3)Binding of heterotrimeric G protein
(transducin) to activated receptor
Activated Receptor
1) Inactive
receptor
2b) Conformational
change transmitted
to cytosolic residues
3)Binding of heterotrimeric G protein
(transducin) to activated receptor