retina 2; visual pigments Flashcards
where are visual pigment found?
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
how to extract pigments found within rods?
- detergents such as digitonin will extract pigments found within rods
- digitonin dissolves the membrane and the visual pigment comes out into solution
why did the frog retinae appear purple when put in a solution ?
this is due to the visual pigments within rods
what is Rhodopsin ?
visual pigment that you find in rods
how to characterise visual pigment ?
- 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
what is the wavelength with most absorption ?
green ( 500nm)
why does conventional scanning spectrophotometry of retinal extracts will only isolate pigments found within rods?
because they bigger outer segment and there is more rods than cones in the retina
how to characterise less abundant cone pigment ?
must perform microspectrophotometry ( MSP ) where light is passed through individual photoreceptors
what does MSP show us ?
- 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
is visual pigment different in animals ?
- different animals have different pigments
- for example the gold fish has 4 cone visual pigments and have maximum absorption at different wavelength
what is adaptive optics ?
- 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
what are visual pigments referred to sometimes?
- chromoproteins
- made of chromophore ( absorbs light ) + protein
what is chromophore ?
- 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
ways visual pigment are named ?
- initially named according to colour they appeared
- another method would be to use the colours absorbed by a pigment
what should retinal pigments be named?
- 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
what are the two classes of retinal pigment?
- rhodopsin
- porphyropsin
how to further identify visual pigment?
- 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
what is the protein component in visual pigment?
- 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
what other proteins have heptahelical structure ?
olfactory receptors
Acetylcholine receptors
GABA
dopamine
what is the protein component of visual pigment?
opsin which has a structure of 7 helical hoops going through the outer segment disc membrane
what is the retinal bound to ?
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
what can you find in a rod?
- 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
what is the point of having an opsin ?
- 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
how to tune retinal?
- retinal by it self absorbs maximally at around 380nm
- to tune it you have to combine it opsin
