Biochemical Role of Vitamin A
The major biochemical role or you can say the functions of Vitamin A are:
- Vision/Visual cycle.
- Maintenance of epithelial cells and many more.
- We will discuss them later.
- First we will discuss the visual cycle that is the most important topic (also from the examination view point)
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…
FOR COMPLETE LESSON VISIT THE PAGE OF BIOCHEMISTRY I, CLICK VITAMINS THEN CLICK CURRICULUM TAB TO VIEW THE LESSONS.