Functions of Vitamin A
Free online lectures on Medical Biochemistry for medical students by Biochemistry Club
The major biochemical role or you can say the functions of Vitamin A are:
- Vision/Visual cycle.
- Maintenance of epithelial cells and many more.
Wald’s Visual Cycle
Generation of Nerve Impulse
- Rhodopsin with a mol.wt. 35,000 D is a membrane protein found in the photoreceptor cells of the retina.
- Rhodopsin is made up of the protein opsin & 11-cis-retinal
- When light falls on the retina, the 11-cis-retinal isomerizes to all-trans-retinal. A single photon can excite the rod cell.
- The photon produces immediate conformational change.
- The unstable intermediates produced are:
- Rhodopsin → Bathorhodopsin → Lumirhodopsin → Metarhodopsin-I → Metarhodopsin-II → and finally Opsin + all-transretinal.
- Each of these intermediaries has a lifespan of only few picoseconds to microseconds.
- The all-transretinal is then released from the protein.
Mechanism of Action of Vitamin A
- Visual pigments are G-protein-coupled receptors and 11-cis retinal locks the receptor protein (opsin) in its inactive form.
- The isomerization and photo-excitation leads to activation of G-protein and generation of cyclic-GMP.
- Cyclic GMP acts as the gate for cation specific channels.
- Transducin is the G-protein in retina.
- The nerve impulse thus generated in the retina is transmitted to visual centers in the brain.
- The signal is terminated by phosphorylation of a serine residue of activated rhodopsin, by an enzyme rhodopsin kinase, so that the inhibitory …
FOR COMPLETE LESSON PLEASE VISIT THE TOPIC OF VITAMINS IN “BIOCHEMISTRY I”. IN THE VITAMINS, CLICK CURRICULUM TO VIEW THE FULL LIST.