General index
Isoprenoid Lipids: Steroids
Isoprenoid lipids are those lipids formed by the condensation of Isoprenoid units.
From an experimental point of view, they correspond to what we call insaponifiable lipids, that
remain in the organic phase after a saponification reaction. The isoprenoid unit is theoretically based
on the hydrocarbon known as Isoprene or Isopentene:
According to the number of isoprenoid units, these lipids can be classified as
monoprenoids (1 unit), diprenoids (2 units), triprenoids (3 units),
tetraprenoids (4 units), etc. An alternative nomenclature is based on the Terpene unit, that
is the same as a diprenoid. Thus we have hemiterpenes (monoprenoids), monoterpenes (diprenoids), sesquiterpenes
(triprenoids), diterpenes (tetraprenoids), etc.
A diprenoid (monoterpene) lipid is Geraniol:
A triprenoid (sesquiterpene) is Farnesol:
As an example of tetraprenoids (diterpenes) we have Retinol, a member of the family of
Retinoids or Vitamin A:
Note that in this case part of the molecule appear ciclyzed (b-Ionone ring).
A hexaprenoid compound is Squalene:
An octaprenoid compound is b-Carotene or Provitamin A:
An important group of isoprenoid lipids are the Steroids, hexaprenoid derivatives.
Many natural products and essential oils are also isoprenoid compounds, and of special interest to us, the
Liposoluble Vitamins (A, D, E and K).
Steroids
Steroids are hexaprenoid compounds derived from Squalene. This compound
undergoes a series of metabolic transformations giving a polyalicyclic system called cyclopentane phenantrene.
As a general example of steroid molecules, we'll study the structure of Cholestanol:
First, the polyalicyclic system (cyclopentane phenantrene):
Cholestanol is an
Sterol (Steroid alcohol), with a side chain of eight carbon atoms.
Cholestanol is a member of the allo- series, in which the first two rings (A and B) of the cyclic system are fused in trans-:
Note that substituents in C-10 (methyl) and in C-5 (hydrogen) are directed in opposite directions with respect to the rings. Substituents in the same direction as methyl are called substituents in b; those directed as the hydrogen are called substituents in a. In planar representations, substituents in a are represented with a dotted line, as in the figure:
Coprostanol is an isomer of cholestanol:
Coprostanol is a member of the normal- series, in which the first two rings (A and B) are fused in cis-:
Substituents in C10 (methyl) and C5 (hydrogen) are on the same side of the system; that is, both are in b. In planar representations, this is represented by a continuous line:
Both Cholestanol and Coprostanol are derivatives by reduction of Cholesterol:
It is the most abundant sterol in animal tissues, where it is an essential component of
plasma membranes, being a regulator of membrane fluidity. Cholesterol is also present in plasma lipoproteins, and
has a tendency to deposit in the intima of the arteria, giving ateroma plaques, the main lesion
in arteriosclerosis. Note that isomery normal/allo is not possible in cholesterol, having a double bond
between C5 and C6.
Other steroids are the Bile Acids, with a shorter side chain than sterols. Bile
acids have hydroxy functions in a, and a carboxyl terminal group in the
side chain. We present the structure of Cholic Acid:
Cholic acid is an planar amphipathic molecule, in which we distinguish between
a polar face and a hydrophobic face:
, different from other amphipathic molecules like lecithin, that are linear.
Other steroid families are very important hormonal signals. For example, among the
hormones of the adrenal cortex, we have, as a representative of Glucocorticoids, Cortisol:
Or, as a Mineralocorticoid, Aldosterone:
Sexual hormones are also steroids. Among the Gestagens, hormones of
the second phase of the female mensatrual cycle, we have Progesterone:
And among the Estrogens, hormones of the first phase of the female mensatrual cycle,17-b-Estradiol:
Note that in estrogens ring A is aromatic, this being the main difference from the rest of steroids.
Among the Androgens, the male sexual hormones, we have Testosterone: