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Protein Geometry Changes Following Retinal Isomerization
As we shall see below, the isomerization of retinal has an
important effect on special proteins in the rod cell: the isomerization
event actually causes the proteins to change their shape. This
shape change ultimately leads to the generation of a nerve impulse.
Hence, the next step in understanding the vision process for
monochrome vision is to describe these proteins, and how they
change their shape after retinal isomerizes.
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Opsin consists of 348 amino acids, covalently linked
together to form a single chain. This chain has seven hydrophobic,
or water-repelling, alpha-helical regions that pass through the
lipid membrane of the pigment-containing discs. This region consists
primarily of nonpolar amino acids, which do not attract the polar
water molecule. The cis-retinal is situated among these alpha
helixes in the hydrophobic region. It is covalently linked to
Lysine 296, one of the amino acids in the opsin peptide chain.
The linkage is as a Schiff base reaction.
Rhodopsin + cis-retinal - Chime
in new window
When the cis-retinal absorbs a photon it isomerizes to the
all-trans configuration without (at first) any accompanying change
in the structure of the protein. Rhodopsin containing the all-trans
isomer of retinal is known as bathorhodopsin. However, the trans
isomer does not fit well into the protein, due to its rigid,
elongated shape. While it is contained in the protein, the all-trans-retinal
adopts a twisted conformation, which is energetically unfavorable.
The molecule undergoes a series of shape changes to try and better
fit the binding site. Therefore, a series of changes in the protein
occurs to expel the trans-retinal from the protein.
Bathorhodopsin + trans-retinal - Chime
in new window
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These rapid movements of the retinal are transferred to the
protein, and from there into the lipid membrane and nerve cells
to which it is attached. This generates nerve impulses which
travel along the optic nerve to the brain, and we perceive them
as visual signals - sight. The free all-trans-retinal is then
converted back into the cis form by a series of enzyme-catalyzed
reactions, whereupon is reattaches to another opsin ready for
the next photon to begin the process again.
Link to Animation of rhodopsin
changes - NEUROBIOLOGY, Molecules, Cells and Systems, Gary
G. Matthews
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Activated rhodopsin causes electrical impulses in the following
way:
1.The cell membrane (outer layer) of a rod cell has an electric
charge. When light activates rhodopsin, it causes a reduction
in cyclic GMP, which causes this electric charge to increase.
This produces an electric current along the cell. When more light
is detected, more rhodopsin is activated and more electric current
is produced.
2.This electric impulse eventually reaches a ganglion cell,
and then the optic nerve.
3.The nerves reach the optic chasm, where the nerve fibers
from the inside half of each retina cross to the other side of
the brain, but the nerve fibers from the outside half of the
retina stay on the same side of the brain.
4.These fibers eventually reach the back of the brain (occipital
lobe). This is where vision s interpreted and is called the primary
visual cortex. Some of the visual fibers go to other parts of
the brain to help to control eye movements, response of the pupils
and iris, and behavior.
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