Biochemistry of the Visual System

Question 1

What are the types of photoreceptor cells?

Answer 1

Rods and cones

Question 2

What do rods respond to?

Answer 2

Light detection

Question 3

What do cones respond to?

Answer 3

Color detection

Question 4

What receptor protein are rods associated with?

Answer 4

Rhodopsin

Question 5

What receptor protein are cones associated with?

Answer 5

3 different opsins - red, green, blue

Question 6

Between rods and cones, which has a high sensitivity but low resolution?

Answer 6

Rods

Question 7

Between rods and cones, which has a low sensitivity but high resolution?

Answer 7

Cones

Question 8

Why do cones have a low sensitivity but high resolution?

Answer 8

A single cone associates with a single bipolar cell

Question 9

Why do rods have a high sensitivity but low resolution?

Answer 9

Many rods converge upon a single bipolar cell

Question 10

Describe the organization or rods and cones

Answer 10

Synaptic ending, axon, nucleus, inner segment, outer segment that includes discs

Question 11

Disc membranes in the outer segment include a GPCR system that initiates the pathway, what is in that system?

Answer 11

Opsin receptor protein, Transducin G protein, and cGMP Phosphodiesterase

Question 12

Describe what happens when light enters the disc membrane of the outer segment of a rod

Answer 12

• 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

Question 13

What is necessary to open the na/ca channel on the outer segment membrane?

Answer 13

cGMP, which is why the channels are closed when light is present and actively leading to its decrease

Question 14

What will recreate cGMP leading to the cells depolarization and thus more release of glutamate?

Answer 14

Guanylate cyclase (GC)

Question 15

Light leads to?

Answer 15

Hyperpolarization and thus less release of glutamate

Question 16

Darkness leads to?

Answer 16

Depolarization and more release of glutamate

Question 17

What forms a covalent link (schiff base) with 11-Retinal?

Answer 17

Lysine 296

Question 18

Photo-activated lysine in the 7TM is ____

Answer 18

Pronated! It can now absorb light’s wavelength

Question 19

What creates Retinal?

Answer 19

Vitamin A

Question 20

When light is added to 11-cis-retinal what results?

Answer 20

11-trans-retinal conformational change

Question 21

What opsin receptor proteins on cones are X chromosome inherited?

Answer 21

Red and green

Question 22

What wavelength is blue light?

Answer 22

420 nm

Question 23

What wavelength is red light?

Answer 23

560 nm

Question 24

What wavelength is green light?

Answer 24

530 nm

Question 25

Red/Green color blindness is inherited in what way and primarily affects what gender?

Answer 25

X-linked recessive, so carrier females often produce affected male sons

Question 26

What are 3 ways to terminate the signal that light started?

Answer 26

Rhodopsin Kinase, Innate GTPase activity of Transducin, and Guanylate cyclase

Question 27

Rhodopsin kinase phosphorylates C-terminus and results in what?

Answer 27

Arrestin binds and does not allow Metarhodopsin to interact with Transducin

Question 28

GTPase activity of transducin causes what?

Answer 28

Alpha subunit to inactivate and dissociate from phosphodiesterase

Question 29

Guanylate cyclase synthesizes cGMP from GTP and causes what?

Answer 29

Na channels to open and depolarize the cell

Question 30

What deficiency most commonly affects the eye/cornea?

Answer 30

Vitamin A/beta carotene

Question 31

Abnormal dryness of conjunctiva and cornea of eye

Answer 31

Xeropthalmia

Question 32

What are some things that vitamin A deficiency can cause?

Answer 32

Xeropthalmia, night blindness, visual impairment, Bitot's spots, failure of wounds to heal

Question 33

Infants exposed to _____ in womb may be born with birth defects such as cleft palates and heart abnormalties

Answer 33

Accutane/Isotretinoin

Question 34

Vitamin A produces?

Answer 34

Retinal in the retina

Question 35

Vitamin A deficiencies are most common in developing nations, what food was fortified with beta carotene to try and prevent that?

Answer 35

Golden rice

Question 36

What is the purpose of the retinoid cycle?

Answer 36

Regenerate 11-cis-retinal

Question 37

Mutations in the retinoid cycle can cause?

Answer 37

Blindness

Question 38

Where does the retinoid cycle take place?

Answer 38

In the rod cells/photoreceptor cells and the Retinal Pigmented Epithelium (RPE)

Question 39

In a rod cell, what transports all-trans-retinal to the cytoplasm to prevent buildup?

Answer 39

ABC transporter

Question 40

Mutations in ABC transporter can cause?

Answer 40

Macular Degeneration

Question 41

Macular Degeneration

Answer 41

Loss of central field of vision due to buildup of all-trans-retinal

Question 42

Once the all-trans-retinal is transported to the RPE, what converts it to All-trans-retinyl

Answer 42

LRAT

Question 43

What converts All-trans-retinyl in the RPE to 11-cis-retinol?

Answer 43

RPE65

Question 44

Mutations in LRAT, RPE65 cause?

Answer 44

Retinitis Pigmentosa

Question 45

Retinitis Pigmentosa

Answer 45

Loss of night vision and peripheral vision due to accumulation of retinoid metabolites - Causes photoreceptor degeneration and bony spicule formation

Question 46

Describe the Retinoid cycle

Answer 46

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

Question 47

Risk factors for macular degeneration?

Answer 47

Caucasion, obesity, smoker, old, diet