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Neural Biology 1 > retina 2; visual pigments > Flashcards

retina 2; visual pigments Flashcards

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1
Q

where are visual pigment found?

A

the visual pigment is found within the membranes of outer segment discs

  • 99% of visual pigment are bound to disc membranes
  • 1% bound to plasma membrane
  • 85% of protein in rod outer segment discs is visual pigment
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2
Q

how to extract pigments found within rods?

A
  • detergents such as digitonin will extract pigments found within rods
  • digitonin dissolves the membrane and the visual pigment comes out into solution
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3
Q

why did the frog retinae appear purple when put in a solution ?

A

this is due to the visual pigments within rods

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4
Q

what is Rhodopsin ?

A

visual pigment that you find in rods

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5
Q

how to characterise visual pigment ?

A
  • see what wavelengths of light it absorbs
  • you can do that using a spectrophotometer
  • in spectrophotometer you have a light source which is split into two identical beams
  • one beam goes through a reference solution and other beam goes through extracted visual pigment
  • compare light at different wavelength coming from the two beams
  • you can see which wavelength of light is absorbed by visual pigment
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6
Q

what is the wavelength with most absorption ?

A

green ( 500nm)

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7
Q

why does conventional scanning spectrophotometry of retinal extracts will only isolate pigments found within rods?

A

because they bigger outer segment and there is more rods than cones in the retina

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8
Q

how to characterise less abundant cone pigment ?

A

must perform microspectrophotometry ( MSP ) where light is passed through individual photoreceptors

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9
Q

what does MSP show us ?

A
  • its possible to identify four visual pigments in the human retina
  • all rods contain the same visual pigment with maximum sensitivity of 498nm
  • we have three different types of cones which contain three different visual pigments
  • ( blue cones ) have visual pigment with maximum sensitivity of 420 nm
  • ( green cones ) have visual pigment that absorbs at 534nm
  • ( red cones )which has visual pigment that maximum absorption at 563nm
  • referred to as S CONE , M CONE AND L CONE
  • note that blue , red and green cone absorb at a range of wavelength
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10
Q

is visual pigment different in animals ?

A
  • different animals have different pigments

- for example the gold fish has 4 cone visual pigments and have maximum absorption at different wavelength

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11
Q

what is adaptive optics ?

A
  • technique allows us to look down at the photoreceptors from above
  • what you can see when you do adaptive optics on the retina that 5-10% of our cones are the S-CONE and are evenly distributed around the retina- absent from fovea
  • M and L cones appear less randomly arranged
  • ration of M to L cone differs in people - in some 1:1 and other its 4:1
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12
Q

what are visual pigments referred to sometimes?

A
  • chromoproteins

- made of chromophore ( absorbs light ) + protein

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13
Q

what is chromophore ?

A
  • in humans and most other vertebrates, the chromophore is an aldehyde of vitamin A1, called retinal
  • in some animals chromophore is an aldehyde of vitamin A2 which is known as 3-dehydroretinal
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14
Q

ways visual pigment are named ?

A
  • initially named according to colour they appeared

- another method would be to use the colours absorbed by a pigment

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15
Q

what should retinal pigments be named?

A
  • any pigment that uses retinal as it chromophore is referred to as a rhodopsin
  • any pigment that uses 3-dehydroretinal is referred to as porphyropsin
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16
Q

what are the two classes of retinal pigment?

A
  • rhodopsin

- porphyropsin

17
Q

how to further identify visual pigment?

A
  • state its λmax
  • the human retina contains 4 rhoddosins
  • 3 in cones with λ max at 420, 534 and 563 nm
  • one rod with λ max at 489nm
18
Q

what is the protein component in visual pigment?

A
  • in all pigments the protein is known as opsin and consists of a chain of around 350 - 450 amino acids
  • the amino acid chain crosses the disc membrane 7 times.
  • each time it passes through the membrane it does so as a helix
  • the 7 helices are joined by straight bits
19
Q

what other proteins have heptahelical structure ?

A

olfactory receptors
Acetylcholine receptors
GABA
dopamine

20
Q

what is the protein component of visual pigment?

A

opsin which has a structure of 7 helical hoops going through the outer segment disc membrane

21
Q

what is the retinal bound to ?

A

the retinal is bound to a lysine in the 7th transmembrane helix nestling within a pocket formed by the opsin
chromophore is always bound to opsin at lysin residue

22
Q

what can you find in a rod?

A
  • disc in outer segment disc
  • membrane of the disc
  • have phospholipid bi layer
  • opsin part of visual pigment
  • opsin has structure of 7 helical hoops
  • within the loops you have binding pocket where retinal ( chromophore ) is attached to lysin residue - which is where light acts
23
Q

what is the point of having an opsin ?

A
  • the opsin tunes the retinal
  • when retinal is bound to opsin as part of a visual pigment its maximum absorption is shifted towards longer wavelengths
24
Q

how to tune retinal?

A
  • retinal by it self absorbs maximally at around 380nm

- to tune it you have to combine it opsin

25
Q

why is the retinal tuned by the opsin?

A
  • the tunning of the retinal by the opsin is due to the identity and changes of the amino acids surrounding the chromophore within the binding pockets
26
Q

what is the function of chromophore (retinal)?

A

to absorb the light

27
Q

what is the function of opsin ?

A

to tune the retinal to determine at what point in spectrum light is absorbed

28
Q

what is the chromophore absorption determined by ?

A

the chromophore absorption is determined by the amino acids surrounding it

29
Q

what do different visual pigments have ( i.e. the 4 human pigments )?

A

they have different opsins but same chromophore ( retinal )

30
Q

how can the visual pigment chromophore ( retinal ) exist )

A
  • retinal can exist in a number of isomeric forms
    meaning retinal can take a number of different shapes
  • in darkness it is bound to the opsin in the bent 11- cis configuration
  • in light the absorption of a photon isomerises the chromophore to the straight all trans form
31
Q

how can the visual pigment chromophore ( retinal ) exist )

A
  • retinal can exist in a number of isomeric forms
    meaning retinal can take a number of different shapes
  • in darkness it is bound to the opsin in the bent 11- cis configuration
  • in light the absorption of a photon isomerises the chromophore to the straight all trans form
32
Q

what does photoisomerization cause?

A

all trans molecule no longer fits into the opsin

  • retinal and opsin separate
  • results in loss of colour
33
Q

explain the Wald cycle ?

A
  • the visual pigment bleaching occurs via a number of intermediate steps, mostly representing different conformational states of the opsin. these can be studied by cooling the pigment
  • only the first stage requires light
  • final stage is when retinal and opsin separate
  • metarhodopsin is the rigger for transduction
  • rhodopsin - darkness and absorbs a photon
34
Q

what happens if you return the bleached frog retina back to darkness?

A
  • it regains its purple colour

- given the right conditions, visual pigment will regenerate

35
Q

how does visual pigment regeneration occur`/

A
  • regeneration will not occur unless the photoreceptors are in contact with the RPE
  • RPE is needed for visual pigment regeneration ( in rods)
36
Q

explain visual pigment regeneration?

A
  • 11- cis retinal( bent retinal ) is hit by light which converts to all trans retinal ( straight retinal)
  • all trans retinal diffuses into outer segment cystol and gets converted into all trans retinol
  • in the all trans retinol it leaves the photoreceptors into the interphotoreceptor matrix
  • all trans retinol binds to the IRBP
  • IRPB transports all trans retinol into the RPE
  • in the the RPE all trans retinol is converted to 11 - cis retinal
  • 11-cis retinal binds to IRBP which then transports it back to photoreceptor
  • once in the photoreceptor 11- cis retinal binds to opsin and visual pigment is regenerated
37
Q

what is the area around photoreceptor outer and inner segment filled by ?

A

-filled by the interphotoreceptor matrix containing IRBP

38
Q

what can cone visual pigments do ?

A
  • cone visual pigments can regenerate without RPE

Neural Biology 1 (58 decks)

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