PHSL 3061 Lecture Notes - Lecture 7: Transducin, Visual Phototransduction, Phosphodiesterase

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9 Oct 2018
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Phototransduction
rod outer segment
The steps in phototransduction:
1. Only light-dependent step in vision: Photopigment absorbs light conformational change
photopigments: opsin (protein) + retinal (co-factor)
light: 11-cis-retinal all-trans-retinal)
G-protein-coupled receptor
2. Photopigment activates transducin (G protein)
GDP exchanged for GTP
-GTP dissociates
Amplification: 1 photopigment molecule can activate hundreds of transducin molecules.
3. Transducin (-GTP) stimulates cGMP phosphodiesterase.
4. cGMP phosphodiesterase breaks down cGMP.
5. [cGMP] in cytosol of outer segment
6. cGMP dissociates from cGMP-gated cation (Na+) channel, due to equilibrium between cGMP in
cytosol and at channel
7. cGMP-gated cation (Na+) channels close photoreceptor hyperpolarizes
Cyclic-nucleotide-gated
cation (Na+) channel open in
dark
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