Katharina Devitofranceschi '14 Noah Winters '15
Vision is a crucial element to almost all facets of mammalian
life. For most animals it is required to find food, interact with
conspecifics and locate home. At the most fundamental level, rod and
cone cells in mammalian retina confer the ability of vision. The
process begins with the absorption of light via rod cells and the
G-protein coupled receptor (GPCR), rhodopsin. Detection of the photon
occurs through an extremely fast, highly selective and efficient
reaction mediated by a conformational change in 11-cis-retinal. This
opsin/photon reaction then cues a cascade of signals that excite
neurons involved in vision and allow for the perception of an image. The rhodopsin
GPCR represents a paradigm for the structural functions of these receptor types . Out
of the 2-3% of mammalian genes that code for GPCRs, approximately 90%
of those GPCRs belong to the rhodopsin family. As such, an
understanding for rhodopsin's structural function is paramount.
II. General Structure
All GPCRs contain highly conserved
In rhodopsin, helices I, IV, VI, and
VII are all kinked due to Pro resides, though only significantly
at helices IV and VI.
Helix VII possess a structural irregularity resulting from the
binding of Lys296
to the 11-cis-retinal chromophore.
Rhodopsin contains a cytoplasmic terminal
region, consisting of helix II (
Pro71 and Leu72), C-II (Phe148),
helix V (Leu226, Val230),
and helix VI (Val250, Met253).
Together, this region forms the binding and activation site for a G
an extracellular domain comprised of the NH2
terminal and interhelical loops I, II, and III.
The NH2-terminal tail is composed of 5 strands, the first two being
antiparallel beta sheets
(Gly3-Pro12) which run almost parallel to the
phospholipid membrane. The other three strands run from Phe13 to Pro34.
E-III loops run along the periphery of the molecule,
while the middle of the E-II loop penetrates deep inside the
GPCR with two antiparallel beta sheets. The uppermost
sheet forms part of the chromophore-binding pocket
III. 11-cis-Retinal Binding
As aforementioned, the
11-cis-Retinal chromophore is attached to
Lys296. The residues interact via a Schiff base
linkage, as indicated by the merging of the densities of
chromophore's polyene chain and the side chain of Lys296.
Retinal is located closer to the extracellular region of the
lipid bilayer, rather than the interdiscal region. The portion
of the binding pocket that surrounds the beta-ionone ring of
retinal contains residues that are close to the cytoplasmic side
of the membrane. These residues include
Trp265, as well
as the residues Met207,
Phe212, Tyr268, Ala269 from helix VI.
A kink introduced by
Pro267 causes these residues to cover the beta-ionone
ring within the pocket, binding non-specifically.
Binding of the polyene chain within the binding pocket is
also done through non-specific interactions with the residues Glu113,
Gly120, and Gly121 ,
Leu47, and the
beta sheet from EII.
The unique orientation of Lys296 is directed by the
hydrophobic residues Met44 and Leu47, and the peptide bond
between Phe293 and Phe294.
The entire area is stabilized by the two phenyl rings
interacting with adjacent helices II and VI.
Counterion formation and subsequent Schiff linkage
stabilization are facilitated by Glu113 and Thr94. There is a
distance of 3.3 Å and 3.5 Å between the carboxylate oxygen atoms
of Glu113 and Thr94, and the nitrogen atom of the Schiff base.
IV. Photoactivation of Rhodopsin
When light energy in the form of photons hits the
11-cis-retinal chromophore, the molecule isomerizes into its
all-trans conformation. This isomerization results in
several changes in binding affinity within the receptor. First,
the beta-ionone ring
moves towards helix III, and is accompanied by
displacement of the C9
and C13 methyl groups of retinal.
Movement of the methyl regions results in a transformation
of the salt-bridge between Glu113 and the Schiff base. This action results in
neutralization of the previously charged species and
displacement of helix III. Movement of helix III disrupts the
binding between Glu122
and His211, as well as the C13 methyl of retinal and
Furthermore, photoactivation and trans-isomerization leads
to the splitting of interhelical and hydrophobic constraints,
mediated by Ala299, Asn302, and Tyr306, and Phe294,
respectively. As a result, the receptor
rearrangement which results in subsequent activation of a
cytoplasmic G protein.
After the chromophore is converted to its all-trans
conformation, the molecule is released from the receptor into the
cytoplasm. The four cytoplasmic-facing residues Lys67, Lys66 , Arg69, and His65 mediate this release.
V.Rhodopsin and Other GPCRs
considered to be the paradigm of structural GPCR studies. But
how similar is it really to other GPCRs?
Interestingly, there is a great deal of
extracellular structural divergence. The N terminus of
rhodopsin along with the extracellular loop 2 (ECL2) forms a
four-stranded beta-sheet. This beta-sheet additionally
interacts with the ECL1 and ECL3. These structures serve to
occlude the binding site from other ligands.
In comparison, the
(which catalyzes the crucial epinehrine
signaling cascade) is structurally very open, and is
able to bind several different types of ligands. The primary
feature of the beta-2AR is a short helical segment within
the ECL2. This helix is supported by a few di-sulfide
interactions and contact with ECL1.
The transmembrane region is the
most conserved sequence between GPCRs. They all share a
common structural core of 97 residues. As a result,
the helical bundle orientation is similar across all 4
crystallized GPCRs to date.
It stands to reason that the
ligand-binding pocket is what varies most between GPCRs.
Surprisingly, though, the beta-2-adrenergic receptor has a pocket
that structurally resembles that of rhodopsin. The
position of the pocket is fairly similar and in both cases
the ligand binding extends from TM VII. In beta-2AR the
ligand engages in a strong polar interaction while in
rhodopsin this interaction is a full-fledged covalent
Knowing the structural differences between
GPCRs is crucial to understanding the most ubiquitous receptor type in our body,
and is of particular significance as a pharmacological target for drug therapies.
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