Biochemistry of the Visual System Flashcards
What are the types of photoreceptor cells?
Rods and cones
What do rods respond to?
Light detection
What do cones respond to?
Color detection
What receptor protein are rods associated with?
Rhodopsin
What receptor protein are cones associated with?
3 different opsins - red, green, blue
Between rods and cones, which has a high sensitivity but low resolution?
Rods
Between rods and cones, which has a low sensitivity but high resolution?
Cones
Why do cones have a low sensitivity but high resolution?
A single cone associates with a single bipolar cell
Why do rods have a high sensitivity but low resolution?
Many rods converge upon a single bipolar cell
Describe the organization or rods and cones
Synaptic ending, axon, nucleus, inner segment, outer segment that includes discs
Disc membranes in the outer segment include a GPCR system that initiates the pathway, what is in that system?
Opsin receptor protein, Transducin G protein, and cGMP Phosphodiesterase
Describe what happens when light enters the disc membrane of the outer segment of a rod
- Light activates the rhodopsin receptor protein which activates the Transducin G protein
- Transducin binds a GTP and the alpha subunit and GTP dissociate
- Activated alpha subunit and GTP activate the phosphodiesterase
- Phosphodiesterase takes cGMP to GMP, thus reducing the concentration of cGMP
- This causes the Na/Ca channels to close and the cell is hyperpolarized and releases less glutamate
What is necessary to open the na/ca channel on the outer segment membrane?
cGMP, which is why the channels are closed when light is present and actively leading to its decrease
What will recreate cGMP leading to the cells depolarization and thus more release of glutamate?
Guanylate cyclase (GC)
Light leads to?
Hyperpolarization and thus less release of glutamate
Darkness leads to?
Depolarization and more release of glutamate
What forms a covalent link (schiff base) with 11-Retinal?
Lysine 296
Photo-activated lysine in the 7TM is ____
Pronated! It can now absorb light’s wavelength
What creates Retinal?
Vitamin A
When light is added to 11-cis-retinal what results?
11-trans-retinal conformational change
What opsin receptor proteins on cones are X chromosome inherited?
Red and green
What wavelength is blue light?
420 nm
What wavelength is red light?
560 nm
What wavelength is green light?
530 nm
Red/Green color blindness is inherited in what way and primarily affects what gender?
X-linked recessive, so carrier females often produce affected male sons
What are 3 ways to terminate the signal that light started?
Rhodopsin Kinase, Innate GTPase activity of Transducin, and Guanylate cyclase
Rhodopsin kinase phosphorylates C-terminus and results in what?
Arrestin binds and does not allow Metarhodopsin to interact with Transducin
GTPase activity of transducin causes what?
Alpha subunit to inactivate and dissociate from phosphodiesterase
Guanylate cyclase synthesizes cGMP from GTP and causes what?
Na channels to open and depolarize the cell
What deficiency most commonly affects the eye/cornea?
Vitamin A/beta carotene
Abnormal dryness of conjunctiva and cornea of eye
Xeropthalmia
What are some things that vitamin A deficiency can cause?
Xeropthalmia, night blindness, visual impairment, Bitot’s spots, failure of wounds to heal
Infants exposed to _____ in womb may be born with birth defects such as cleft palates and heart abnormalties
Accutane/Isotretinoin
Vitamin A produces?
Retinal in the retina
Vitamin A deficiencies are most common in developing nations, what food was fortified with beta carotene to try and prevent that?
Golden rice
What is the purpose of the retinoid cycle?
Regenerate 11-cis-retinal
Mutations in the retinoid cycle can cause?
Blindness
Where does the retinoid cycle take place?
In the rod cells/photoreceptor cells and the Retinal Pigmented Epithelium (RPE)
In a rod cell, what transports all-trans-retinal to the cytoplasm to prevent buildup?
ABC transporter
Mutations in ABC transporter can cause?
Macular Degeneration
Macular Degeneration
Loss of central field of vision due to buildup of all-trans-retinal
Once the all-trans-retinal is transported to the RPE, what converts it to All-trans-retinyl
LRAT
What converts All-trans-retinyl in the RPE to 11-cis-retinol?
RPE65
Mutations in LRAT, RPE65 cause?
Retinitis Pigmentosa
Retinitis Pigmentosa
Loss of night vision and peripheral vision due to accumulation of retinoid metabolites
- Causes photoreceptor degeneration and bony spicule formation
Describe the Retinoid cycle
In Rod cell:
- 11-cis-retinal goes to all-trans-retinal and is released from opsin
- ABC transporter takes it to cytoplasm
- Converted to all-trans-retinol and taken to RPE
In RPE:
- LRAT takes all-trans-retinol to all-trans-retinyl ester
- RPE65 takes all-trans-retinyl to 11-cis-retinol
- Oxidized to 11-cis-retinal and taken back to rod cell
Risk factors for macular degeneration?
Caucasion, obesity, smoker, old, diet
