Biochemistry Of The Visual System Flashcards
Describe rods
Vision in low light levels
Contain rhodopsin (cannot detect color)
High sensitivity and low spatial resolution
Describe cones
Color detection
Three opsins (red, green and blue)
Low sensitivity and high spatial resolution
What is the convergence of rods?
Rods can respond to a single photon
Many rods converge into a single bipolar cell
Many of these bipolar cells contact a single amacrine cell
This allows for highest levels of sensitivity but sacrifices resolution
What is the convergence of cones?
Cones need ~100 photons to respond
One cone directly contacts only one bipolar cell
This arrangement allows for best resolution but sacrifices sensitivity
When a photoreceptor is stimulated by photons/light what happens in the cell?
The cell becomes hyperpolarized and they release less NT (glutamate)
In conditions of darkness the photoreceptor cells are what?
Consistently depolarized and constantly release glutamate
Which proteins and receptors located on the disc membranes of the outer segments are involved with signal transduction?
Signal transduction pathway originates from a GPCR system
Opsin (7TM receptor)
Transducin (G protein)
cGMP phosphodiesterase which acts as an effector protein and cleaves cGMP to GMP
Peripherin (structural protein)
What receptors are on the surface membrane of the outer segment?
cGMP gated Na channel (Ca leak channel) which is open in dark conditions and contributes to depolarization (closed in light conditions) Na/Ca exchanger (Ca continues out after illumination) Guanylate cyclase (GTP -> cGMP + PPi) which recycles cGMP
Which proteins are involved with desensitization of signal transduction?
Beta-arrestin which performs signal termination by blocking the intersection of rhodopsin with transducin
Rhodopsin kinase which performs the first step in signal termination (phosphorylation)
Describe the structure of rhodopsin
Homologous to beta-adrenergic receptor
Lysine-296 is covalently bound to 11-retinal and is located in the center of the 7TM
Retinal is derived from vitamin A and is a chromophore
Aldehyde of retinal forms a Schiff base with the amine of lysine
What are the absorption wavelengths for the different forms of retinal?
Free retinal = 370nm
Unprotonated Schiff base retinal = 380nm
Protonated Schiff base retinal = >440nm
Thus photoactivated lysine is protonated
Describe photon induced isomerization
Energy of the photon causes geometric isomerization which translates to atomic motion
This conformational change upon light induction mimics the conformational change that occurs upon ligand binding in other 7TMs
11-cis retinal -> 11-trans retinal
Causes 5A conformational shift of Schiff base nitrogen
What is the activated form of rhodopsin known as?
Metarhodopsin II (Rh*)
Describe photoreceptor opsins
Cone receptor opsins are homologous of rod receptor opsins
3 varieties
Blue (absorbs at 420-460nm), green (530nm) and red (>560nm)
Which chromosomes code for each photoreceptor opsin?
Rhodopsin chromosome 3
Blue opsin chromosome 7
Red and green are on the X chromosome
Explain the genetic expression of red/green colorblindness
Expression of the red/green opsins affected in red/green color blind affected individuals is inherited maternally
Assigned male at brith affected with red/green color blindness wont pass this trait on to offspring
Unaffected assigned female at brith carriers can pass R/G color blindness onto their AMAB offspring
What are the three ways signal transduction can be terminated (desensitized)?
- Light activated rhodopsin blocked from activating transducin
- Rapid hydrolysis of GTP to GDP causes dissociation of alpha subunit from PDE and re-association with beta and gamma subunits (innate GTPase activity of transducin)
- Elevated cGMP levels re-open cGMP gated Na channels (ganylate cyclase synthesizes cGMP from GTP)
Describe how light activated rhodopsin can be blocked from activating transducin
Rhodopsin kinase phosphorylates the C terminus of metarhodopsin II (Rh*) at Thr and Ser allowing binding by arrestin
Binding of arrestin prevents the interaction between rhodopsin and transducin
What is the role of Ca in termination of the phototransduction cascade?
In light conditions low cGMP levels close Na and Ca channels reducing their intracellular concentrations
During activation, PDE activity decreases intracellular cGMP by promoting its cleavage —>
cGMP gated Na channels close —> hyperpolarization —> closed Na/Ca channels lead to a decrease in intracellular Ca levels which results in recovery -> guanylate cyclase activity increases -> intracellular cGMP levels increase -> Na/Ca channels open causing depolarization of the cell
What are the three different structures of vitamin A?
Based on functional groups
Hydroxyl (retinol), carboxyl (retinoic acid) and aldehyde (retinal)
What is the function of retinol and retinoic acid?
Used to maintain epithelial cells
Important for maintenance of cornea and conjunctiva
Also helps maintain epithelial tissues in skin, intestines, lungs, bladder and inner ear
Supports T cell function, male and female reproduction and fetal development
What is xeropthalmia?
Abnormal dryness of the conjunctiva and cornea of the eye typically associated with vitamin A deficiency
Second most prevalent nutritional disorder
With a decrease in vitamin A levels the cornea can undergo complete necrosis
Vitamin A deficiency results in what?
Changes in mucosal surfaces throughout the body
Resulting in keratinizing metaplasia in the lining of epithelia of the lungs and intestines which is responsible for the respiratory and GI sx of severely depleted children
What are the dietary sources of vitamin A?
Carrots, leafy greens, sweet potatoes, squash, broccoli and animal products (eggs, dairy, fish and liver)
