Biochemistry of the Visual System

Question 1

rods are responsible for what type of vision?

Answer 1

light vision

Question 2

cones are responsible for what type of vision?

Answer 2

color detection

Question 3

what is the opsin associated with rods?

Answer 3

rhodopsin

Question 4

what are the opsins associated with cones?

Answer 4

red, green, and blue

Question 5

what are the general characteristics of rods?

Answer 5

they have a high sensitivity but a low spatial resolution

Question 6

what are the general characteristics of cones?

Answer 6

they have low sensitivity but high spatial resolution

Question 7

which photoreceptor can respond to a single photon?

Answer 7

rods

Question 8

what type of system is found in the disc membranes?

Answer 8

the GPCR system

Question 9

what is the membrane spanning protein found in the GPCR system in the disc membranes?

Answer 9

opsin (rhodopsin)

Question 10

what is the specific G protein found in the disc membranes?

Answer 10

transducin

Question 11

what is found on the surface membrane of rods?

Answer 11

cGMP-gated Na+ channel, Na+/Ca2+-exchanger, and guanylate cyclase

Question 12

what is the status of the cGMP-gated Na+ channel in dark conditions?

Answer 12

they are open and the cell is depolarized

Question 13

what is the status of the cGMP-gated Na+ channel in light conditions?

Answer 13

Na+ channels are closed and the cell is hyperpolarized

Question 14

what kind of neurotransmitter is released under dark conditions?

Answer 14

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.

Question 15

what is rhodopsin homologous to?

Answer 15

the beta-adrenergic receptor

Question 16

what is found right in the middle of the membrane spanning region of rhodopsin?

Answer 16

lysine-296

Question 17

What is the effector protein that is found on the disc membranes in rods?

Answer 17

phosphodiesterase

Question 18

what does the lysine-296 covalently bind to?

Answer 18

its chromophore-11-cis-retinal

Question 19

what is formed when the lysine-296 covalently binds to the 11-cis-retinal?

Answer 19

a schiff base

Question 20

when we are talking about the light-activated rhodopsin and its chromophore retinal we are looking specifically at what?

Answer 20

at the pronated schiff base version of rhodopsin covalently bounded with retinal at the lysine 296

Question 21

what happens when a photon reacts with retinal bound to rhodopsin?

Answer 21

there is a energetically induced isomaerization from 11-cis-retinal to all-trans retinal

Question 22

what is the activated version of rhodopsin referred to as?

Answer 22

metarhodopsin II (Rh*)

Question 23

what chromosome is the rod opsin protein coded on?

Answer 23

3

Question 24

what chromosome is the blue opsin protein coded on?

Answer 24

7

Question 25

what chromosome is are the red and green opsin proteins coded on?

Answer 25

X chromosome

Question 26

what is the absorption maxima of blue opsin protein?

Answer 26

420 nm

Question 27

what is the absorption maxima of green opsin protein?

Answer 27

530 nm

Question 28

what is the absorption maxima of red opsin protein?

Answer 28

>560 nm

Question 29

What is the first step of the physical blockage mechanism of signal termination/ desensitization?

Answer 29

rhodopsin kinase will phosphorylate the C-terminus of Rh* at the threonine and serine residues

Question 30

what does phosphorylation of the threonine and serine residues at the C-terminus of the Rh* promote?

Answer 30

it promotes the binding of the protein Arrestin at the C-terminus of Rh*

Question 31

what does binding of the Arrestin protein cause?

Answer 31

inhibition of the rhodopsin interacting with its G protein transducin

Question 32

what does the innate GTPase activity of transducin cause?

Answer 32

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

Question 33

what makes the cGMP PDE inactive?

Answer 33

the Y-subunit

Question 34

What triggers the activity of the guanylate cyclase?

Answer 34

low levels of intracellular Ca2+

Question 35

what is the action of the guanylate cyclase?

Answer 35

it will use GTP to reform the cGMP

Question 36

retinal is locally produced in the retina from what?

Answer 36

dietary vitamin a

Question 37

how else is vitamin a produced?

Answer 37

by B-carotene

Question 38

what happens when you have deficient levels of vitamin a?

Answer 38

you can get deficient levels of retinal, retinol, or retinoic acid

Question 39

what are the symptoms of vitamin A deficiencies?

Answer 39

night blindness, visual impairment, dry eyes, and bitot's spots

Question 40

what are bitot's spots?

Answer 40

a build up of keratin superficially in the conjunctiva; usually "foamy" in appearance

Question 41

what is golden rice fortified with?

Answer 41

beta-carotene

Question 42

once the 11-cis-retinal is transformed into all-trans-retinal, where is the all-trans-retinal shuttled to?

Answer 42

the retinal pigmented epithelium

Question 43

what transports the all-trans-retinal from the disc membrane into the cytoplasm?

Answer 43

ABC transporter

Question 44

what happens if you have a deficiency of the ABC transporter?

Answer 44

you will have a build up the all-trans-retinal in the disc membrane, which will affect the health of the photoreceptor cell itself

Question 45

what happens to the all-trans-retinal once it gets into the cytoplasm?

Answer 45

it is reduced to all-trans-retinol

Question 46

once the all-trans-retinol gets moved out of the photoreceptor cell, what is it bound to?

Answer 46

cellular retinoid binding protein (CRBP)

Question 47

where does the CRBP transport the all-trans-retinol to?

Answer 47

to LRAT

Question 48

what does LRAT do to the all-trans-retinol?

Answer 48

it esterifies it; so it will now be all-trans- retinyl ester

Question 49

where is the all-trans-retinyl ester transported to?

Answer 49

to RPE65

Question 50

what does RPE65 do to the all-trans- retinyl ester?

Answer 50

converts it to 11-cis-retinol

Question 51

what shuttles the 11-cis-retinol to it's next spot?

Answer 51

a binding protein CRALBP

Question 52

where does the CRALBP take the 11-cis-retinol?

Answer 52

to 11cRDH

Question 53

what does 11cRDH do to the 11-cis-retinol?

Answer 53

it oxidizes it to 11-cis-retinal

Question 54

where is the 11-cis-retinal shuttled back to?

Answer 54

to the photoreceptor cell using iRBP

Question 55

in the RPE, what can you have deficiencies in?

Answer 55

LRAT and RPE65

Question 56

what does deficiencies in LRAT and RPE65 lead to?

Answer 56

retinitis pigmentosa

Question 57

the high flux of photons by light exposure can lead to what?

Answer 57

elevated levels of toxic retinal metabolites

Question 58

what are the classical presentation signs of retinitis pigmentosa?

Answer 58

decreased night vision and peripheral vision

Question 59

upon a retinal examination, what would you find in a patient with retinitis pigmentosa?

Answer 59

bony spicule formation in the RPE

Question 60

what does degeneration of the macula in the retina cause?

Answer 60

loss of central field vision

Question 61

what are mutations in the ABC4 gene known to cause?

Answer 61

macular degeneration