lipoproteins

Despite the high heterogeneity found among
lipids, either in terms of their different classes
(fatty acids, triglycerides, cholesterol,
cholesterol esters and phospholipids), or even
within each of these classes, there is a feature
common to all of them - its high insolubility in
water. Indeed, although some of the lipids have
an amphipathic behavior (phospholipids and
cholesterol) they are predominantly apolar.
Since in our body it is necessary to transport
lipids from one organ to another, and the
solvent of all our fluids, including plasma, is
water, we have a potential problem... If the
lipids were able to circulate in their free forms
in the bloodstream, it would be the tendency of
the lipids to cluster in lipid droplets (such as
when olive oil is dropped in a glass of water),
which would, ultimately, lead to the occlusion of
blood vessels.
It is precisely to avoid this situation that the
plasma lipoproteins are synthesized. As the
name implies, the lipoproteins are
macromolecular complexes composed of lipids
and proteins and have the function of
transporting lipids (the only exception are fatty
acids! ) in the bloodstream, keeping them in a
partially soluble state. Basically, the idea is that
these are spherical structures with an extremely
hydrophobic interior (mostly composed of the
more nonpolar lipids - triglycerides and
phospholipids) and a polar surface to enable
interactions with water. Thus, in the surface
there are the polar groups of the phospholipids
and cholesterol. Therefore, by being able to
interact with water, lipoproteins can be in a
partially soluble state, preventing the formation
of hydrophobic lipid droplets that occur to
minimize the contacts of lipids with water.

Classes of lip protein

There are several classes of lipoproteins that
are grouped according to their density. Thus , in
order of increasing density, we have the
chylomicrons, VLDL, IDL (not a “true” class of
lipoproteins), LDL and HDL. Since the lipids are
less dense than water, the greater the fat
content of a lipoprotein, the less its density.
Regarding the size of the different lipoprotein
classes, this varies inversely with the density,
that is, the denser lipoproteins are the smaller ones.