Biochemistry of the Visual System Flashcards
rods are responsible for what type of vision?
light vision
cones are responsible for what type of vision?
color detection
what is the opsin associated with rods?
rhodopsin
what are the opsins associated with cones?
red, green, and blue
what are the general characteristics of rods?
they have a high sensitivity but a low spatial resolution
what are the general characteristics of cones?
they have low sensitivity but high spatial resolution
which photoreceptor can respond to a single photon?
rods
what type of system is found in the disc membranes?
the GPCR system
what is the membrane spanning protein found in the GPCR system in the disc membranes?
opsin (rhodopsin)
what is the specific G protein found in the disc membranes?
transducin
what is found on the surface membrane of rods?
cGMP-gated Na+ channel, Na+/Ca2+-exchanger, and guanylate cyclase
what is the status of the cGMP-gated Na+ channel in dark conditions?
they are open and the cell is depolarized
what is the status of the cGMP-gated Na+ channel in light conditions?
Na+ channels are closed and the cell is hyperpolarized
what kind of neurotransmitter is released under dark conditions?
in the dark, a photoreceptor (rod/cone) cell will release glutamate, which inhibits (hyperpolarizes) the ON bipolar cells and excites (depolarizes) the OFF bipolar cells.
what is rhodopsin homologous to?
the beta-adrenergic receptor
what is found right in the middle of the membrane spanning region of rhodopsin?
lysine-296
What is the effector protein that is found on the disc membranes in rods?
phosphodiesterase
what does the lysine-296 covalently bind to?
its chromophore-11-cis-retinal
what is formed when the lysine-296 covalently binds to the 11-cis-retinal?
a schiff base
when we are talking about the light-activated rhodopsin and its chromophore retinal we are looking specifically at what?
at the pronated schiff base version of rhodopsin covalently bounded with retinal at the lysine 296
what happens when a photon reacts with retinal bound to rhodopsin?
there is a energetically induced isomaerization from 11-cis-retinal to all-trans retinal
what is the activated version of rhodopsin referred to as?
metarhodopsin II (Rh*)
what chromosome is the rod opsin protein coded on?
3
what chromosome is the blue opsin protein coded on?
7
what chromosome is are the red and green opsin proteins coded on?
X chromosome
what is the absorption maxima of blue opsin protein?
420 nm
what is the absorption maxima of green opsin protein?
530 nm
what is the absorption maxima of red opsin protein?
> 560 nm
What is the first step of the physical blockage mechanism of signal termination/ desensitization?
rhodopsin kinase will phosphorylate the C-terminus of Rh* at the threonine and serine residues
what does phosphorylation of the threonine and serine residues at the C-terminus of the Rh* promote?
it promotes the binding of the protein Arrestin at the C-terminus of Rh*
what does binding of the Arrestin protein cause?
inhibition of the rhodopsin interacting with its G protein transducin
what does the innate GTPase activity of transducin cause?
rapid hydrolysis of GTP to GDP. This will cause the dissociation of the alpha-subunit from the PDE. Makes the PDE inactive once again and the alpha subunit will reassociate with the B- and Y- subunits
what makes the cGMP PDE inactive?
the Y-subunit
What triggers the activity of the guanylate cyclase?
low levels of intracellular Ca2+
what is the action of the guanylate cyclase?
it will use GTP to reform the cGMP
retinal is locally produced in the retina from what?
dietary vitamin a
how else is vitamin a produced?
by B-carotene
what happens when you have deficient levels of vitamin a?
you can get deficient levels of retinal, retinol, or retinoic acid
what are the symptoms of vitamin A deficiencies?
night blindness, visual impairment, dry eyes, and bitot’s spots
what are bitot’s spots?
a build up of keratin superficially in the conjunctiva; usually “foamy” in appearance
what is golden rice fortified with?
beta-carotene
once the 11-cis-retinal is transformed into all-trans-retinal, where is the all-trans-retinal shuttled to?
the retinal pigmented epithelium
what transports the all-trans-retinal from the disc membrane into the cytoplasm?
ABC transporter
what happens if you have a deficiency of the ABC transporter?
you will have a build up the all-trans-retinal in the disc membrane, which will affect the health of the photoreceptor cell itself
what happens to the all-trans-retinal once it gets into the cytoplasm?
it is reduced to all-trans-retinol
once the all-trans-retinol gets moved out of the photoreceptor cell, what is it bound to?
cellular retinoid binding protein (CRBP)
where does the CRBP transport the all-trans-retinol to?
to LRAT
what does LRAT do to the all-trans-retinol?
it esterifies it; so it will now be all-trans- retinyl ester
where is the all-trans-retinyl ester transported to?
to RPE65
what does RPE65 do to the all-trans- retinyl ester?
converts it to 11-cis-retinol
what shuttles the 11-cis-retinol to it’s next spot?
a binding protein CRALBP
where does the CRALBP take the 11-cis-retinol?
to 11cRDH
what does 11cRDH do to the 11-cis-retinol?
it oxidizes it to 11-cis-retinal
where is the 11-cis-retinal shuttled back to?
to the photoreceptor cell using iRBP
in the RPE, what can you have deficiencies in?
LRAT and RPE65
what does deficiencies in LRAT and RPE65 lead to?
retinitis pigmentosa
the high flux of photons by light exposure can lead to what?
elevated levels of toxic retinal metabolites
what are the classical presentation signs of retinitis pigmentosa?
decreased night vision and peripheral vision
upon a retinal examination, what would you find in a patient with retinitis pigmentosa?
bony spicule formation in the RPE
what does degeneration of the macula in the retina cause?
loss of central field vision
what are mutations in the ABC4 gene known to cause?
macular degeneration
