Lecture 18. Sight and Blue-Tinged Vision Flashcards
What are rod cells responsible for?
Non-colour vision at low light
What makes up the outer segment of rods?
~1000 discs not connected to the plasma membrane
Each is a closed sac of membrane with embedded photosensitive rhodopsin molecules
What is rhodopsin?
A visual pigment, is a specialised GPCR made of opsin (the GPCR protein component), linked to 11-cis-retinal (a prosthetic group that is the chromophore or light-absorbing group)
What happens to retinal during light capture?
- Alternating single and double bonds form a ‘polyene’ with a long unsaturated network of electrons that can absorb light energy
- Light absorption causes cis-trans isomerisation around the C12 and C13 bond
- The N of the key lysine moves 5 Å (0.5 nm)
How long does it take for light to be converted into atomic motion?
A few picoseconds
What does light absorption by retinal alter?
The conformation of the GPCR (inactive rhodopsin becomes activated metarhodopsin II)
What does metarhodopsin do?
Stimulates nucleotide exchange on the α-subunit of a specific heterotrimeric G protein called transducin (Gt)
What is the process of transducin Gαt activating cGMP phosphodiesterase?
Light activates rhodopsin which activated the Gt transducin (Gαt,Gβt,Gγt)
Gαt (GTP) stimulates cGMP phosphodiesterase (cGMP PDE) which removes cGMP from cGMP-gated ion channels
What is mammalian rhodopsin’s peak absorbance?
500nm
What does light close?
cGMP gated ion channels, reducing influx of Ca⁺⁺
How is rhodopsin terminated in very high light intensities?
Light activated rhodopsin
Light-activated rhodopsin can be phosphorylated by rhodopsin kinase
What does arrestin do?
Binds to fully phosphorylated rhodopsin: and this rhodopsin stops activation of transducin
What are the three mechanism to make rods insensitive to light?
- Prolonged cGMP-gated channel closure
- Phosphorylation of opsin reduces transducin activation
- Arrestin binding to phosphorylated opsin stops transducin activation
How long does it take for the rods to reset?
20-30 minutes
What are the three visual pigments human colour relies on?
‘Blue’ - peak absorbance at 416-426nm
‘Green’ - peak absorbance at 530-532nm
‘Red’ - peak absorbance at 560-563nm
How many individual pigments do each cone cell express?
One
What is the photoreceptor comprised of?
An opsin (a modified GPCR) with 11-cis-retinal as the chromophore and a (different) transducin
How are cephalopod eyes different from human eyes?
Light strikes the retina directly
There is no blind spot
The retina has only rod cells
Why do cephalopod have oddly shaped pupils?
Allow light to enter the eye through the lens from many directions at the same time, rather than just straight into the retina
What do dichromats have difficulty distinguishing?
Similarly sized objects where lightness varies in an unpredictable manner
What causes protanopia?
Mutation in L-cone (red)
What causes deuteranopia?
Mutation in M-cone (green)
What causes tritanopia?
Mutation in S-cone (blue)
What is sildenafil citrate?
A potent inhibitor of cGMP phosphodiesterase. It is most active against phosphodiesterase type 5 (PDE-5)
Sildenafil citrate also inhibits PDE-6. PDE-6 regulates blue-green colour discrimination in the retina
What is a side effect of using sildenafil?
A side-effect of sildenafil citrate can be blue-tinged vision
