General Index
Fat-soluble Vitamins
The Fat-sluble vitamins are the Retinoids or vitamins A, the
Calcipherols or vitamins D, the Tocopherols or vitamins E and the Naphtoquinones or
vitamins K. They have very diverse functions in the organism: visual stimuli transductors (retinoids),
antioxidants (tocopherols), calcium absorption promoters (calcipherols) and procoagulant factors
(naphtoquinones). All have in common a polyprenoid structure. Calcipherols are modified sterols.
Retinoids or vitamins A are derived from the scission of a molecule of
b-carotene by means of a specific oxygenase. The main features of these molecules
can be seen in the structure of Retinol:
Retinoids are tetraprenoid derivatives (20 carbon atoms) forming a cyclic system
(b-ionone) and an unsaturated polyprenoid chain ending on an oxygen
function. If this is al alcohol, we have Retinol; an aldehyde, Retinal; and a carboxylic
acid, Retinoic acid.
Retinoids have an important function as gene transcription regulators. However, their
most known role is the transduction ov visual stimuli in the retina, being the main photosensitive pigment.
In photoreceptors, 11-cis retinal is the prosthetic group of Rhodopsin and the different conopsins.
When a visible light photon is captured by this molecule, it isomerizes to all-trans Retinal:
This isomerization is the ultimate cause of the visual impulse, through a complex
signal transduction system in which ionic channels and G-proteins are also required.
Retinoic Acid is another retinoid:
Calcipherols or vitamins D are modified sterols. They are formed in animals
from cholesterol by means of the solar UV radiation and a series of metabolic transformation in liver
and kidney. The active metabolite is 1,25-Dihydroxycholecalcipherol:
This compound promotes the intestinal absorption of calcium.
Tocopherols o vitamins E are also polyprenoid derivatives, whose function
has to do with antioxidant protection. This is the structure of a-Tocopherol:
Chemically, a-Tocopherol is a cyclic system (chromane),
and a saturated polyprenoid side chain.
Vitamins K are cofactors in the process of glutamic acid carboxylation to
give g-Carboxyglutamic Acid, a postranslational modification that is
required for the correct functioning of some blood coagulation factors, like Prothrombin. The
structure of Naphtoquinone or Vitamina K1 is:
Chemically is a naphtoquinone cycle with a polyprenoid side chain.