Biochemistry of Vision Flashcards
List the various layers of the Retina.
- Outer Nuclear Layer
- Outer Plexiform Layer
- Inner Nuclear Layer
- Inner Plexiform Layer
- Ganglion Cell Layer
Describe the general sequence of events when light hits the eye.
Photoreceptors –> Interneurons (Vertical and Lateral Pathways) –> Ganglion Cells
Ganglion Cells: Output cells of the retina; Axons form optic nerve; Project to the Brain; Information transmitted by Action Potentials
Differentiate between Rods and Cones.
- *Rods:** Night Vision (differentiate between light and dark)
- Rhodopsin (cannot detect color)
- HIGH sensitivity and low spatial resolution
- *Cones:** Color Detection
- Three opsins (red, gree, and blue)
- LOW sensitivity and HIGH spatial resolution
Which tissue in the human body is going to have the HIGHEST respiratory rate?
Why?
Rods and Cones
Dark: Na+ channels are open and the cell is depolarized (Inhibitory neurotransmitter is released when your eyes are closed!)
Light: Na+ channels are closed and the cell is hyperpolarized
**** Constantly using ATP!
Describe the Disc Membrane of the Outer Segment.
- *GPCR System:**
1. Rhodopsin (Receptor)
2. Transducin (G-Protein)
3. Phosphodiesterase (cGMP –> GMP)
Structural Protein –> Peripherin
Describe the strucures associated with the surface membrane.
- cGMP-gated Na+ channel (Ca2+ leak channel; tonically OPEN in the dark)
- Na+/Ca2+ exchanger (Ca2+ continues out after light)
- Guanylate Cyclase (GTP –> cGMP + PPi)
Describe the structure of the Photoreceptor Protein.
7 Transmembrane Receptor
*** Analogous to the B2-Adrenergic Receptor!
Describe the different states of retinal and absorption levels in regards to Schiff-Bases.
Resting State: Schiff-Base is Pronated
Schiff-Base linkage occurs between Lysine 296 and Retinal!
Describe how one can become color-blind in regards to recombination pathways among visual pigments.
Which chromosomes are the different pigments located on?
You can straight up LOSE one of the color-genes through recombination.
You could form a hybrid structure that is going to effect the absorption characteristics if the gene.
*** Chance of getting color blindness is GREATER for the “Red and Green” because they are on the SAME chromosome!
\*\*\*Red opsonin (500 nm) --\> Chromosome 3 Blue opsonin (420 nm) --\> Chromosome 7 Red opsonin (560 nm) --\> X Green opsonin (530 nm) --\> X
Describe why the structure of the retina is a “perfect storm” for Macular Degeneration.
- High respiratory Quotient (high O2 flux)
- High Lipid Content
- UV Rays
What are some molecules that can protect you against macular degeneration?
- Lutein –> In Kale
- Zeaxanthin –> Eggs, Broccoli
List some good sources of food for Vitamin A.
- Carrots
- Dark green and leafy vegetables
- Sweet Potatoes
- Squash
- Broccoli
What are some consquences of Vitamin A deficiency?
- Night Blindness
- Xerophthalmia (Failure to produce tears)
- Keratinization of epithelium in GI, respiratory and genitourinary tract
- Skin becomes dry and scaly
Describe the production of Retinol.
Beta-Carotene is ingested and converted to Retinol (Vitamin A) via Dioxygenase (located in the intestinal mucosa)
How does Retinol travel in the blood stream?
Bound to Retinol Binding Protein (RBP)
Describe the Retinoid Cycle.
In the Rod Cell:
- Light-induced change from 11-cis to all-trans
- Release of all-trans-retinal from opsin
- Enzymatic reduction of all-trans-retinal to all-trans-retinol
- Exportation of all-trans-retinol (with help of iRBP)
In the Retinal Pigmented Epithelium (RPE):
- Uptake into RPE
- Translocation to ER
- Esterification to all-trans-retinyl ester by lecithin retinol acyl transferase (LRAT)
- Conversion to 11-cis-retinol by isomerohydrolase activity of RPE65
- Enzymatic oxidation from 11-cis-retinol to 11-cis-retinal by 11-cis RDH
- Exportation of 11-cis-retinal (with help of iRBP)
Back in the Rod Cell:
- Uptake of 11-cis-retinal into rod cell
- Covalent attachment (Schiff base) to opsin forming a functional rhodopsin
Describe the different structures that are within the G-Protein Couple Receptor Complex.
- Receptor –> Rhodopsin
- G-Protein –> Transducin
- Effector Enzyme –> cGMP Phosphodiesterase
What is the first event to happen after the photon hits the Rod Cell?
It hits the DOUBLE BOND and you have conversion of 11-CIS –> all-Trans
What are the two ways to terminate the signal initated by Rhodopsin?
- Dissociation of the signal molecule from the Receptor
- Phosphorylation of the cytoplasmic C-terminal tail of the receptor and binding of B-Arrestin
Describe how Ca2+ plays a role in controlling guanylate cyclase.
Calcium is going to inhibit the activity of the enzyme (guanylate cyclase)
In the Dark:
- Ca2+ as well as Na+ enter the rod outer segment through the cGMP-gated channels.
- Calcium ion influx is balanced by its efflux through an exchanger, a transport system that uses the thermodynamically favorable flow of four Na+ ions into the cell and one K+ ion out of the cell to pump one Ca2+ ion out of the cell.
In the Light:
- After illumination, the entry of Ca2+ through the cGMP-gated channels stops, but its export through the exchanger continues
- Thus, the cytoplasmic Ca2+ level drops from 500 nM to 50 nM after illumination.
- This drop markedly stimulates guanylate cyclase, rapidly restoring the concentration of cGMP to reopen the cGMP-gated channels.
Describe the concept of Amplification as it relates to absorbing light.
