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.

Primary protein structures are composed of amino acids linked together by peptide bonds.  Secondary protein structures are held together by hydrogen bonds.  Phosphodiester bonds can be found between sugar and phosphate groups in the backbone of DNA.

The primary structure of a protein deals with its sequence while the tertiary structure deals with the folding of the protein. The folding of the protein is what determines its function, and because this is important in maintaining the life of organisms, the tertiary structure must be heavily conserved.

Amino acids are classified based on their charged (polar groups) at neutral pH (pH=7).Lysine (Lys, K), arginine (Arg, R) and histidine (His, H) are positively charged at neutral ph (pH=7), while aspartate and glutamate are negatively charged.

The structure represents guanine because of the characteristic carbonyl group at carbon 6 and amine group and carbon 2.

The structure is adenine because of the characteristic amine group on carbon 6 and lack of any other substituents.