The detection of light is a biochemical process involving a signal transduction.
ID: 265111 • Letter: T
Question
The detection of light is a biochemical process involving a signal transduction. The process starts with rod and cone cells, which have GPCRs on their membranes. The rod cells are involved in detection of the intensity of light through their GPCRs, called opsins. There are between 90 to 120 million rod cells in the human retina, which helps handle different shades of light and dark through their GPCR, called opsins, and its ligand, 11-cis-retinal. When 11-cis-retinal binds in to the opsin, it forms a schiff base with a lysine, as shown below: Protonated 11-cis Schiff baseK296 Counterion (E113) As a photon of light hits the 11-cis-retinal, it isomerizes it to all-trans-retinal. After this event, the G protein is able to bind with the activated GPCR, sending off a signal that results in the cell sending a nerve impulse to the brain. We have frequently talked about ligands binding to GPCRs through intermolecular forces, however in this case, the ligand binds with a covalent interaction. Why is this covalent interaction helpful in activating the G-protein? Why does the conversion of cis-retinal to all-trans-retinal enable the G-protein to be able to bind in? a.Explanation / Answer
1A)G-protein is one of the principal and active celprocess.Rho proteins that involves in cell signalling transduction system.Unless any stimulant binds the cell surface through G-protein the intracellular signalling becomes fatigue so as to that impact will be at the entire organsim level.Henceforth G-protein cell receptor system is to be involved inevitably in the conversion of 11-cis retinal to All trans retinal.
2A) Infact Rhodopsin is a conjugated protein present in Rods of eye.It contains 11-cis retinal and the protein opsin.Here the aldehyde group is linked up with epsilan amino group of lysin of opsin.When a beam of light hits the eye ball immediately Wald's visual cycle begins the dissociation of Rhodopsin in to the respective components where separated opsin is inevitable to trigger a neural impulse by the hyperpolarization of membrane potential.Here opsin is away as long as neural excitability is going on slowly when excitability goes to disappear opsin binds with 11-cis retinal again to form Rhodopsin.
3A) When exposed to light the color of Rhodopsin changes from Red to yellow by a process known as Photo bleaching.It persists for few milli seconds whilst no vision on retina feels full of blurredness.As we know Rods involve in making image during less intensity of light sometimes Rhodopsin forms All trans retinal in a complex step wise process instead of having a single step process.Rhodopsin leaves the following intermediates before it forms All trans retinal.Therefore we take darkness for milliseconds.
Rhodopsin---Prelumirhodopsin---Lumirhodopsin-----Metarhodopsin I ----Metarhodopsin II-----All trans retinal+opsin