Thursday, 4-7-Biochemistry Of Vision (Seidler) Flashcards
What are the 3 cell types of the retina?
- photoreceptors
- interneurons (bipolar cells, horizontal cells, amacrine cells)
- ganglion cells
List the pathway for the processing of visual signals:
Photoreceptors —> interneurons —> ganglion cells
__ are the output cells of the retina whose axons form the optic nerve. They project to the brain and information is transmitted by action potentials
Ganglion cells
These photoreceptors are for night vision, have rhodopsin, and have high sensitivity and low spatial resolution
Rods
These photoreceptors are for color detection and have 3 opsins (red, green, blue). They have low sensitivity and high spatial resolution
Cones
When are Rods depolarized?
In the DARK: Na+ channels open and cell depolarizes
In the LIGHT: Na+ channels close and cell is hyperpolarized
The __ segment of rods are specialized for photoreception.
The photoreceptor molecule in rods is __, which consists of the protein opsin linked to 11-cis-retinal, a prosthetic group
Outer
Rhodopsin
The disc membrane of the outer segment of the Rods contain a GPCR system. List the components of the GPCR system:
- Rhodopsin: the receptor
- Transducin: Trimeric G protein
- Phosphodiesterase: the effector enzyme
What is a homologue to Rhodopsin?
Beta 2-adrenergic receptor
What is the ligand for Rhodopsin?
What is the wavelength absorbance of Rhodopsin?
11-cis-retinal
500 nm
The 500 nm absorption maximum for rhodopsin strongly suggests that the schiff base is ___
Retinal is linked to this AA by a schiff-base linkage:
Protonated
-Lysine (Lys 296)
What is the absorption maxima of the blue opsin protein?
What is the absorption maxima of the red opsin protein?
What is the absorption maxima of the green opsin protein?
420
560
530
Describe how color blindness occurs:
- Recombination between genes–> rearrangement in the course of DNA replication may lead to loss of pigment genes
- Recombination within genes –> formation of hybrid pigment genes that encode photoreceptors with anomolous absorption spectra.
The macular carotenoids (xanthophylls) include:
- Lutein
- Zeaxanthin
What are consequences of vitamin A deficiency?
- Night blindness
- Xerophthalmia
- Keratinization of epithelium in GI, respiratory, and genitourinary tract
- Skin becomes dry and scaly
Describe metabolism of ingested beta-carotene:
Beta carotene ———————————-> 2 Retinol
Dioxygenase (intestinal mucosa)
How is Retinol transported in the blood plasma?
Retinol bound to RBP (Retinol Binding Protein)
Describe the pathway of a single photon of light leading to cell hyperpolarization:
1 rhodopsin absorbs 1 photon –> 500 transducin molecules activated –> 500 phosphodiesterase molecules activated –> 10 to the 5th cGMP molecules hydrolyzed –> 250 Na+ channels close –> 10 to the 6th - 10 to the 7th Na+ ions/sec are prevented for a period of ~1 second –> Rod cell membrane is hyperpolarized by 1 mV
Regarding the Retinoid cycle, identify where the following takes place:
-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 ROD CELL
Regarding the Retinoid Cycle, identify where the following takes place:
-Uptake of all-trans-retinol into the retinal pigmented epithelium (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)
In the RPE
Regarding the Retinoid cycle, identify where the following takes place:
-Uptake of 11-cis-retinal from RPE –> Covalent attachment (Schiff base) to opsin forming a functional rhodopsin
In the ROD CELL
List the 5 layers of the retina:
- Outer nuclear
- Plexiform
- Inner nuclear
- Plexiform
- Ganglion cell
Infants exposed to ___ in the womb may have birth defects such as cleft palates and heart abnormalities
Isoretinoin
___ is a cancer of the blood an BM marked by buildup of immature WBCs (promyelocytes). It is linked to the presence of PML-RARalpha, the product of a fusion betweeen the promyelocytic leukemia (PML) gene and the retinoic acid receptor alpha (RARalpha) gene.
Acute promyelocytic leukemia (APL)
PML-RARalpha acts to repress the transcription of certain genes and thus prevent cell differentiation. This results in accumulation of immature promyelocytes. Treatment with ___ relieves PML-RARalphas repression of transcription and thus stimulates differentiation and maturation of promyelocytes and contributes to APL remission
All-trans-retinoic acid
