Only cysteine has a side chain with a thiol group. A thiol group is a sulfur with a bonded hydrogen. Through an oxidation reaction, the sulfurs from two different cysteine amino acids can bond together to form a disulfide bridge. Methionine, the other sulfur containing amino acid, is incapable of forming disulfide bridges in biological systems.

Alpha-helices, beta-sheets, and less regular turns, loops, and coils are all of the secondary structure elements. Greek keys and Rossman folds are examples of structural motifs that are comprised of specifically arranged secondary structure elements.

Chaperone proteins assist in proper protein folding and prevent aggregate formation. Kinases add phosphate groups to target proteins, phosphatases remove them, and proteases are involved in protein degradation and recycling. Hydrolases use water to facilitate the breakage of a bond.

All amino acids have the following main components: carbon backbone, an amine group, a carboxylic acid group, and a specific side chain to further define the particular amino acids. The other groups listed can serve as functional groups in other molecules, but an amino acid, at its baseline, is defined a a molecule with an amine and a carboxylic acid group. 

Serine's R-chain contains a hydroxyl group , which is polar but uncharged, and cysteine's side chain contains a thiol (sulfhydryl)  R-group which is also polar but uncharged. Each of the the other answers contain amino acids that have side groups with different functional properties, but only serine and cysteine can be classified as polar and uncharged. 

Proline does not have an aromatic ring, and its ring is incorporated into its backbone. The human body is able to manufacture it, and hence it is not an essential amino acid which humans must consume. Hydroxyproline is, indeed, fabricated from proline, and is used in collagen synthesis. Phenylalanine, tryptophan, and tyrosine all have an aromatic cyclical structure. All aromatic rings of amino acids absorb ultraviolet light, including the imidazole ring of histidine, which absorbs in the lower UV range more than some amino acids.

Glutamate, not aspartate, is a natural precursor of arginine (and not vice versa). The natural precursor for histidine is a ribose derivative, and cysteine can be produced from serine or via methionine degradation.

For this question, we're provided with the pKa values of the alpha-carboxyl and alpha-amino groups of glycine. We're then asked to determine the pI value for glycine.

For any given amino acid, the pI value describes that amino acid's isoelectric point. This number represents the pH value where the amino acid will exist predominately as a zwitterion with a net charge of zero. For amino acids that don't have any ionizable R-groups, the pI value is simply the average of the pKa values of the alpha-amino and alpha-carboxyl groups.

For glycine, the pI value is:

In this question, we are given information concerning the acidity of glutamic acid's side chain, which is equal to . We're then asked to determine what pH is necessary in order for  to be in the acidic, protonated form.

For starters, we'll need to use the Henderson-Hasselbalch equation, which relates the pH of a solution to the pKa of an acid as well as to the relative amounts of the acidic and basic forms of the compound in question.

From the question stem, we're given the pKa. Moreover, we're told that the concentration of the acidic form of the R-group will be , or . Therefore, the concentration of the basic form of the R-group must be . Plugging these values into the above equation allows us to solve for pH.

An amino acid with an aromatic side group means that the group must have at least one aromatic ring. Of the choices, the only choice that has amino acids all containing aromatic rings is Phenylalanine, tryptophan, tyrosine. Each other selection has at least one amino acid that does not contain an aromatic group in its chemical makeup.