Download Visual Cycle. Learning Objectives. • At the end of lecture, students

Visual Cycle.
Learning Objectives.
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At the end of lecture, students should be able to:
Describe visual cycle.
Give different steps for the formation of visual complex.
Justify the role of retinaldehyde in vision.
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Introduction.
The visual cycle is the process where light impacting on the photosensitive cells of
the retina is converted into electrical polarization that generates nerve stimulation.
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Role of Retinaldehyde in Vision.
In the retina, retinaldehyde functions as the prosthetic group of the light-sensitive
opsin proteins, forming rhodopsin (in rods) and iodopsin (in cones).
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Any one cone cell contains only one type of opsin and is sensitive to only one color.
• In the pigment epithelium of the retina, all-trans-retinol
is isomerized to 11-cis-retinol and oxidized to 11-cisretinaldehyde.
• 11-cis-retinaldehyde  reacts with a lysine residue in
opsin, forming the holoprotein rhodopsin.
• the absorption of light by rhodopsin causes
isomerization of the retinaldehyde from 11-cis to all-trans, and a conformational
change in opsin.
• As a results  release of retinaldehyde from the
protein and the initiation of a nerve impulse.
• The formation of the initial excited form of
rhodopsin, bathorhodopsin, occurs within
picoseconds of illumination.
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There is then a series of conformational changes leading to the formation of
metarhodopsin II, which initiates a guanine nucleotide amplification cascade and then
a nerve impulse.
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The final step is hydrolysis to release all-transretinaldehyde and opsin.
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The key to initiation of the visual cycle is the
availability of 11-cis-retinaldehyde, and hence vitamin
A.
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In deficiency, both the time taken to adapt to darkness
and the ability to see in poor light are impaired.
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Opsins.
Opsins are proteins and the retinal-binding visual
pigments found in the photoreceptor cells in the retinas of eyes.
Similar reactions that are discussed before are responsible for color vision in the
cone cells.
Retinol.
Retinol, is one of the animal forms of vitamin A.
It is convertable to other forms of vitamin A
the retinyl ester derivative of the alcohol serves as the storage form of the vitamin in
animals.
Retinol.
When retinol is converted to the retinal (retinaldehyde) form, then vitamin A is
essential in the vision
when converted to retinoic acid, is essential for skin health and bone growth.
Vitamin A is needed by the retina of the eye in the form of retinal, that is absolutely
necessary for both scotopic and color vision.
Vitamin A also functions, as an irreversibly oxidized form of retinol known as retinoic
acid, which is an important hormone-like growth factor for epithelial and other cells.
Vitamin A.
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Vitamin A is essential for:
Vision
Gene Transcription
Immune Function
Embryonic Development and Reproduction
Bone Metabolism
Haematopoiesis
Skin Health
Reducing Risk of Heart Disease and Cancer
• Antioxidant Activity
Self Assessment.
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Describe visual cycle?
What are the steps involved during visual cycle?
What is the role of retinaldehyde in vision?
THANK YOU ALL.