visual transduction Flashcards
(22 cards)
what is rhodopsin and where is it found
- rhodopsin is a specialised 7TM receptor that absorbs photons
- found only in rods
what medium is vitamin A soluable in
vitamin A is a fat-soluable hydrophobic vitamin (belongs with lipids)
nb. fats are a type of lipids
what are the 2 major dietary sources of vitamin A
- beta-carotene
- carrots and sweet potatoes - retinyl esters
- animal esters
distinguish between rods and cones (3)
- Rods
- functions in dim light
- b/w
- 100 million - Cones
- functions in bright light
- colour perception
- 3 million (conc. in fovea)
what type of photoreceptors are in the eye for visual transduction
what pathway is utilised by this photoreceptor
- the photoreceptors that detect visible light are 7TM receptors
- monomeric receptors with 7 transmembrane domains (helices) that function via F protein signalling pathways
outline the SIGNAL transduction pathway
4.5) or (4
signal -> reception -> (amplification) -> transduction -> response
or
membrane receptors (transfer info from environment to cell’s interior)
- > second messengers (relay info from receptor-ligand complex)
- > protein phosphorylation (info transfer)
- > signal terminated
what type of signals do 7TM receptors transmit (5)
photons, odor, taste, hormones, neurotransmitters
outline the structure of a rod cell (2)
- outer segment contains a stact of 1000 discs (membrane-enclosed sacks packed with rhodopsin)
- detection of photon leads to closure of Na+/Ca2+ channels
- each rod responds to a single photon
outline rhodopsin structure
- membrane protein found in rod discs and plasma membranes in retina
- 7 alpha-helices
- N-terminals inside disc
- C terminals at phosphorylation sites to turn off activated protein
made up of:
- opsin (colourless protein)
- 11-cis-retinal (vitamin A -> non-protein prosthetic group component)
what happens to both dietary sources of vitamin A in the gut (3)
- both gets enzymatically converted to retinol (vitamin A alcohol)
- beta-carotene (carrots) -> 2 retinal -> retinol
- retinyl esters (animal sources) -> retinol
- stored in liver
- when needed, retinol is transported to target cells via the blood
what are the 4 forms of vitamin A in the body and their roles (6)
- retinal
- (as 11-cis form) binds to opsin to form rhodopsin (photoreceptor in rods) - retinol
- transport form of vitamin A
- acts as hormone to control certain types of protein synthesis - retinoic acid
- formation of glyoproteins
- maturation of corneal epithelial cells - retinyl ester
- animal dietary source
describe 3 occular symptoms of vitamin A deficiencies
- early vitamin A deficiency results in loss of night vision (night blindness)
- continued deficiency results in hardening of corneal conjunctiva
- > loss of conjunctival secretions (dry eyes) - degenration of the corneal epithelium and corneal perforation
identify 2 non-occular symptoms of vitamin A deficiencies
non-occular symptom:
1. adverse effects to tissues covered by epithelial cells
- inhibition of bone elongation (growth impairment)
how does a photon induce isomerisation in 11-cis-retinal
- 11-cis-retinal is covalently bound to lysine residue in opsin
- absorption of a light photon causes isomerisation of the 11-cis-retinal group (in rhodopsin) to an all-trans form
light photon + 11-cis-retinal = all-trans-retinal
outline the importance of metarhopodsin (3)
- the conversion of rhodopsin to metarhodopsin II activates a signal transduction pathway
- metarhodopsin then activates the G protein transducin
- transducin in turn activates cGMP phosphodiesterase
outline visual transduction summary (8)
photon (ligand) + rhodopsin (receptor)
- > metarhodopsin II (activated receptor)
- > activated transducin (G protein)
- > activated cGMP phosphorylase
- > decreased cGMP
- > closure of cGMP-gated ion channels (Na+/Ca2+ & K+ exchange stops)
- > membrane hyperpolarisation (negative potential generated)
- > nerve impulse (ganglion cells in the retina discharges bc of negative potential)
how are light-activated signals terminated (2)
- rhodopsin kinase catalyses phosphorylation of the C-terminus metarhodopsin
- phosphorlyated, light-activated rhodopsin binds to the protein arrestin to prevent further interaction with transducin
how is the signalling system recovered (4)
ie. how does cGMP-gated ion channels reopen
hydrolyse bound GTP to GDP
- > GDP form of transducin then leaves the phosphodiesterase
- > phosphodiesterase returns to inavtive state
- > guanylate cyclase synthesis cGMP from GTP to raise cGMP levels and reopen the cGMP-gated ion channels
(nb: when Ca2+ levels drop [i.e. cGMP gate closes], it stimulates guanylate cyclase activity)
outline the type of photoreceptor that cone cells contain
- cone cells contain rhodopsin homologs
- different bc it has 3 distinct proteins with absorption spectra at:
426nm (blue), 530nm (green), 560nm (red).
outline the similarity of the photoreceptor proteins
- each cone photoreceptor proetin is 40% identical to rhodopsin
- blue photoreceptor is 40% identical to red and green
- red and green are 95% identical (differ by 15 amino acids)
what structure is responsible for colour interpretation
amino acids in the 11-cis-retinal region of cone cells
how does colour blindness/deficiency occur (2)
rate of male population with colour blindness
- the photoreceptors in cone cells vary in amino acid sequence -> changes absorption spectra
- genes for red and green photoreceptors are adjacent to each other on the X chromosome -> susceptible to homologous recombination
- 3-8% of male population has red/green colour blindness
