Light Detection- The Visual Cycle Flashcards
In the dark, chromophore/visual pigment is in which configuration?
II-CIS
In the light, with photon absorption, photo-isomerization changes the chromophore/visual pigment to which configuration?
All-trans. Can not go activate G proteins in the photoreceptor cytoplasm.
Active and inactive forms of chromophore/Visual pigment
Inactive: II-CIS
Active: All trans
Activating rhodopsin is ___, but recovery takes ___
Activating is fast. Recovering is slow.
Resting potential of a typical non-retinal neuron is ___
In the dark, the membrane potential of a photoreceptor cell is ____
What does this mean?
- 70mv
- 40 to -50 mv
Thus, a photoreceptor is relatively depolarized in the dark.
The neurotransmitter used by photoreceptors is
Glutamate. Maximum glutamate outflow is in the dark.
Two main channels on rods
- Cyclic nucleotide-gated cation channel. Not operated by a pump. Either open or closed depending on the amount of the cGMP attached.
- Na+/K+ pump via active transport.
How are the cGMP gated channels regulated in the dark?
In the dark, the channels are kept relatively open by 4 cGMP molecules bound to the subunits of each channel.. The cGMP molecules on the channel are not highly bound, but attach and detach at a random rate due to the molecular collisions.
The rate at which cGMP molecules attach and detach is relatively fixed, thus establishing the membrane potential of about -40 to -50 mv.
How are cGMP gated channels regulated in the light?
In the light, fewer circulating cGMP molecules leads to closure of some of the cGMP channels. The effect is relative hyper polarization of the photoreceptor. Meaning that in the light, the voltage of the photoreceptor becomes more negative. The concentration of circulating cGMP is reduced via phototransduction.
G proteins bind to either __ or __
What are the 3 subunits?
Which subunit binds to the nucleotide?
GDP or GTP
Alpha, beta and gamma
Alpha. Can dissociate.
G protein molecules can freely diffuse ACROSS the disc membranes whereas rhodopsin can diffuse WITHIN the membrane.
R*
Activated rhodopsin
Once R* is activated,
It remains activated and continuous to diffuse through the membrane, encountering and activating other G-proteins. In about 100 milliseconds, a single R* will create about 700 activated G-alpha subunits.
Once the G-alpha protein is activated by R* and dissociated from the G-beta/gama portion of the G protein, it will bind to
A PDE (phosphodiesterase) on the photoreceptor disc membrane.
What does the PDE molecule look like?
Contains and alpha and beta active sites, both of which are covered by gamma subunits.
Activated alpha subunit from a G protein can bind to the gamma subunit of the PDE molecule, which displaces it and exposes the catalytic site.
Activated PDE does what?
It breaks a bond in cGMP, converting it to GMP. This reduces the concentration of cGMP.
Results in disc amplification. A single R* causes the number of cGMP molecules in the outer segment to be reduced by about 1400.
