Lipids, which make up the structure of biological membranes, are the storehouse of energy for the body. However, being insoluble in water, they cannot be easily transported through our body fluids. Hence the need of lipoproteins, which are conjugates of proteins and lipids. The lipid component of lipoproteins comprises neutral fats, namely triglycerides and cholesterol, whereas the protein component comprises apolipoproteins which have the ability to bind to lipid molecules. Apolipoproteins perform an important role in the human body by reacting with other proteins and enzymes present in the blood, to transport fats and lipids around.

Structurally, lipoproteins consist of a shell of apolipoproteins oriented such that the protein part is pointed towards the periphery and the lipid part is towards the inner side. The neutral fats are present inside this shell. The resultant molecule is a water-soluble structure and can be easily transported through the body fluids. Lipoproteins are classified based on their density. The more the protein content, denser the particle. High-density lipoproteins are the densest kind, but smallest in size; whereas, chylomicrons are the largest, but least dense. Let us have a brief overview of the different types of lipoproteins and their functions in the human body.

Classification, Structure and Function of Lipoproteins

Classification Structure and Functions
Chylomicrons Structure: These are tiny globules consisting of fats and proteins, having a density less than 0.95 g/mL, measuring anywhere from 100 to 1000 nm in diameter.

Function: They contain a very high concentration of fats as compared to proteins and hence, are thin in nature. They are synthesized in the epithelial cells in the small intestine, from where they are secreted into the bloodstream and lymphatic fluid. They circulate in the blood and lymph and react with high-density lipoproteins, exchanging apolipoproteins (apolipoprotein C-II and apolipoprotein E), turning mature. The mature chylomicrons are involved in transporting triglycerides that are exogenous in nature, all over the body. Once this function is over, they decrease in size, but gain density and are termed as remnant chylomicrons.
Very-low-density Lipoproteins (VLDL) Structure: VLDL molecules have a density of approximately 1 g/mL and diameter of roughly 80 nm or less.

Function: Processed by the liver, they serve to transport triglycerides, phospholipids and cholesterol that are endogenously produced by the human body. At the time they are processed, they contain apolipoproteins (apolipoprotein B-100, apolipoprotein C1 and apolipoprotein E). Similar to chylomicrons, they move around in the blood, exchanging apolipoprotein molecules (namely apolipoprotein E and apolipoprotein C-II) with high-density lipoproteins and converting to mature VLDLs. By the time they have disseminated the triglycerides throughout the body, they turn dense, and change to IDLs or intermediate-density lipoproteins.
Intermediate-density Lipoproteins (IDL) Structure: This class of lipoproteins is generated when very-low-density lipoproteins or VLDLs degenerate. These molecules are denser than VLDLs, but thinner than low-density lipoproteins, with a size that lies between 25 and 50 nm and a density of 1.006 to 1.019 g/mL.

Function: Consisting primarily of triglycerides and cholesterol, they transport these fat particles to the adipose tissues and muscles all over the body. They contain apolipoprotein E in a larger concentration, which aids in their degradation and conversion to low-density lipoproteins, which leaves them with apolipoprotein B-100.
Low-density Lipoproteins (LDL) Structure: LDLs typically have a density in the range of 1.02 to 1.06 g/mL and a diameter measuring between 18 to 28 nm.

Function: This class of lipoproteins is something most of us are familiar with. Low-density lipoproteins, or LDL as they are commonly known, are responsible for carrying cholesterol throughout the body, from the liver to the muscles. These lipoproteins are also referred to as bad cholesterol, due to the fact that a higher concentration of them in blood is linked to cardiovascular disease. These particles are composed of a complex heterogeneous structure and possess a single copy of apolipoprotein B-100. Low-density lipoproteins can be further subdivided into two subtypes: LDL Pattern A and LDL Pattern B. Pattern B molecules are denser than Pattern A, and are thus labeled bad cholesterol. The reason these are termed "bad" is, because of lesser density, these lipoproteins are able to pass through the endothelium, the cells that cover the inner surface of blood and lymphatic vessels. This implies that cholesterol can be deposited in artery walls, leading to plaque formation, and eventually raising the risk for arteriosclerosis and coronary artery disease.
High-density Lipoproteins (HDL) Structure: The "good cholesterol" lipoproteins, as they are usually known, are the densest (their density is usually greater than 1.06 g/mL) and the smallest (they measure no more than 15 nm in diameter) of all lipoprotein particles.

Function: The main reason why they are referred to as "good" is because, they carry the triglycerides and cholesterol from the muscles back to the liver, which is then discharged from the body or reprocessed and utilized again. This means that it prevents the cholesterol from depositing in the arteries, lowering the chances of coronary artery disease. Full of apolipoproteins A-1 and A-2, HDLs travel through blood, collecting cholesterol and lipids and expanding in size, growing spherical in turn, and transporting it to the liver.
Lipoprotein(a) Structure: Another subclass of lipoproteins, a lipoprotein(a) particle is composed of apolipoprotein(a) and a particle similar in structure to a low-density lipoprotein. The former contains apolipoprotein(b), and both apolipoprotein(a) and apolipoprotein(b) in the lipoprotein(a) share a covalent bond. The exact structure of this subclass is still not fully understood, but research has shown that apolipoprotein(a) is very similar in structure and function to plasminogen, the enzyme that is involved in the clotting of blood.

Function: The exact mechanism of lipoprotein(a) particles are still not known very well, but because of its structure, it is speculated that high levels of lipoprotein(a) in blood can increase the chances of blood clot formation. This is a reason why higher levels of this lipoprotein in blood can increase the risk of arteriosclerosis.

So, now you have a fair idea about the structure of different lipoproteins and the roles they perform. To summarize, LDL distributes cholesterol all over the body, which can get deposited in the vessels and arteries; whereas, HDL absorbs it from the blood vessels and sends it back to the liver. Thus, you should aim at maintaining a high level of HDL and reduce the level of LDL by including vegetables, fruits, and low-fat foods in your diet; and combine it with regular exercise to keep your heart in top shape.