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.
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.
Regarding the proteic part of lipoproteins, their components are called apolipoproteins, or apoproteins. In biochemistry, the prefix "apo " means "incomplete" or "alone". Therefore, the concept of apoprotein applies to the protein part of the lipoprotein in the absence of lipids. The name of the apoproteins is given as follows: prefix "apo", a capital letter, and, in some cases, a number that may reflect the order of discovery or the molecular mass. Examples include the apoE or apoB -100. The apoproteins play several important roles in lipoproteins that will be addressed in a future post...