Low-density lipoproteins have the highest content of cholesterol and cholesterol esters. There are essentially five classes of blood lipoproteins: chylomicrons, very-low-density lipoproteins, intermediate-density lipoproteins, low-density lipoproteins, and high-density lipoproteins. Chylomicrons have the lowest density of the five classes of lipoproteins. This is because the have the highest proportion of triglycerides and the least lowest proportion of protein. Very-low-density lipoproteins are a bit more dense than chylomicrons; however, the relative amount of triglycerides is still high. Intermediate-density lipoproteins which are formed from the very-low-density lipoproteins have a higher density than very-low-density lipoproteins due to the fact that they have less than half of the amount of triglycerides as very-low-density lipoproteins. Low-density lipoproteins have the highest amount of cholesterol and an even lesser amount of triglycerides than intermediate-density lipoproteins. Lastly, high-density lipoproteins are the densest of the lipoproteins due to the fact that they have the highest amount of protein in relation to the amount of triglycerides they contain.

Low-density lipoproteins have the highest content of cholesterol and cholesterol esters. There are essentially five classes of blood lipoproteins: chylomicrons, very-low-density lipoproteins, intermediate-density lipoproteins, low-density lipoproteins, and high-density lipoproteins. Chylomicrons have the lowest density of the five classes of lipoproteins. This is because the have the highest proportion of triglycerides and the least lowest proportion of protein. Very-low-density lipoproteins are a bit more dense than chylomicrons; however, the relative amount of triglycerides is still high. Intermediate-density lipoproteins which are formed from the very-low-density lipoproteins have a higher density than very-low-density lipoproteins due to the fact that they have less than half of the amount of triglycerides as very-low-density lipoproteins. Low-density lipoproteins have the highest amount of cholesterol and an even lesser amount of triglycerides than intermediate-density lipoproteins. Lastly, high-density lipoproteins are the densest of the lipoproteins due to the fact that they have the highest amount of protein in relation to the amount of triglycerides they contain.

The exclusive apolipoprotein of low-density lipoproteins (LDL's) is apoB-100. LDL's are taken up by cells via IDL receptor-mediated endocytosis, as described above for IDL uptake. The uptake of LDL's occurs predominantly in liver (75%), adrenal glands, and adipose tissue. As with intermediate-density lipoproteins, the interaction of LDL's with LDL receptors requires the presence of apoB-100. The endocytosed membrane vesicles (endosomes) fuse with lysosomes, in which the apoproteins are degraded and the cholesterol esters are hydrolyzed to yield free cholesterol. Apo-48 is the exclusive apolipoprotein associated with chylomicrons.

A connexon is made up of 6 alpha subunits (connexins) arranged hexagonally and embedded in the plasma membrane. Two membrane connexins together form a connexon, which comprises a gap junction. Gap junctions are important for the movement of small ions, amino acids, sugars, and nucleotides between cells.

An 18-strand beta barrel is indicative of another membrane transport structure called a maltoporin.

Vitamin B12 is also known as cobalamin, and has cobalt at the center of a corrin ring. Cobalt is rarely found in biology, and the synthesis of cobalamin only naturally occurs in bacteria and archaea. For that reason, vitamin B12 has to be ingested in our diet; it is not synthesized in the human body. Iron is found, among other places, in hemoglobin; zinc, in carbonic anhydrase; magnesium, for example, in chlorophyll; and sulfur in iron-sulfur proteins.

Disulfide bonds are involved in the tertiary and quaternary structure of proteins, not the other structural levels. Primary structure consists of the amino acid sequence. Secondary structure consists of alpha helices and beta pleated sheets. Tertiary structure consists of bonds between hydrogen bonds between R-groups, nonpolar interactions, electrostatic interactions, and covalent bonds including disulfide bonds. Quaternary structure involves the arrangement of more than one polypeptide into a protein complex, and involves the same bonds as those in tertiary structure.

The interior portion of a transmembrane protein is most likely to be populated with smaller, hydrophobic amino acids.  This is because the interior of the transmembrane protein is in the hydrophobic environment of the lipid bilayer.  Thus, alanine, valine, and leucine - small, hydrophobic amino acids - are most likely to be found there.

The secondary structure of a protein can be either an alpha helix or a beta pleated sheet.  In either case, the structure forms due to intra-chain hydrogen bonding of the protein's backbone amino and carboxyl groups.

For this question, we're being asked the basics of how sugars can combine with proteins to create glycoproteins.

For starters, it's important to distinguish between glycoproteins and proteoglycans. Both of these are compounds that consist of carbohydrate and protein. The difference, however, is in the relative amounts of each. Glycoproteins are predominately protein, whereas proteoglycans are predominately carbohydrate.

Another important distinction is the difference between glycation and glycosylation. Both of these processes involve the addition of a sugar to a protein or polypeptide. In glycation, however, the process occurs on its own without the help of any enzymes. Glycosylation, on the other hand, is assisted by enzymes.

Generally speaking, reducing sugars that are capable of equilibrating between a closed chain form and an open chain form are able to add to polypeptides via glycation.

In fact, clinicians take advantage of this fact for more accurately diagnosing individuals with diabetes. This is because glucose in the bloodstream is able to naturally attach to proteins found within the blood, such as hemoglobin, via glycation. When glucose levels have been elevated for an extended period of time, as is the case in someone with diabetes, there will also tend to be elevated levels of glycated hemoglobin, otherwise known as hemoglobin A1C.

All 20 of the amino acids have on its central carbon a hydrogen, a carboxyl group, an amino group, and a distinctive R-group. These R-groups determine the properties of the amino acid and thus the polypeptide of which they are a part.