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.

Catecholamines are derived from tyrosine and are epinephrine and norepinephrine. Testosterone and cortisol are steroid hormones; insulin and glucagon are polypeptide hormones.

Amphoteric substances can act as an acid or a base. An example of an amphoteric substance is an amino acid. Amphipathic is the term assigned to molecules that possesses both hydrophilic and hydrophobic regions. An example of an amphipathic molecule is a phospholipid. Compounds that do not contain carbon are typically referred to as inorganic.

In phenylketonurics, the body cannot convert phenylalanine into tyrosine due to deficiency in phenylalanine hydroxylase. This can be observed by looking at the structures of these amino acids, as they only differ by the hydroxyl group on the benzene ring. Tyrosine is a necessary precursor to L-DOPA (dihydroxyphenylalanine), dopamine, catecholamine neurotransmitters, melanin, thyroid hormones, and other biologically relevant substances.

The one letter code for asparagine (asn) is N.  The one letter code for aspartic acid (asp) is D. The one letter code for histidine (his) is H. The one letter code for glutamine (gln) is Q.

Cysteine is the only amino acid that forms disulfide bonds. Methionine also contains sulfur but does form these bonds.

Aromatic amino acids are those which contain one or more aromatic rings. There are four examples of this: tryptophan, phenylalanine, histidine, and tyrosine. Basic amino acids are those which contain basic side chains at neutral pH, and may carry a positive charge. There are three examples of this: histidine, arginine, and lysine. Hydrophobic amino acids are those which contain "water-fearing" side chains; these amino acids are found within the hydrophobic protein core. There are seven examples of this: alanine, isoleucine, leucine, phenylalanine, valine, proline, and glycine. Acidic amino acids are those which contain acidic side chains at neutral pH and may carry a net negative charge. There are two examples of this: glutamic acid and aspartic acid.

From the molecular structure shown in the question stem, we need to be able to recognize that this is tyrosine. Furthermore, the correct one letter abbreviation for tyrosine is Y, and its three letter abbreviation is Tyr.

T is the one letter abbreviation for threonine.

I is the one letter abbreviation for isoleucine.

R is the one letter abbreviation for arginine.

C is the one letter abbreviation for cysteine.

Just as with many things in biology, structure determines function. Proteins are no exception. Even though there are other things that can contribute to the functioning of a protein, such as cofactors or allosteric regulators (in the case of enzymes), a protein's unique structure can grant it unique functions.

As an example, let's look at enzymes. For an enzyme to function, it must be able to bind its substrate. A big part of what makes this possible is for the enzyme's active site to be structured in such a way that it can accept the substrate molecule. Furthermore, the enzyme's active site structure also plays a role in how that enzyme distorts the substrate and puts strain on its bonds in order to catalyze the transformation of that substrate into product.

At physiological pH, the amino group of an amino acid is protonated and carries a  charge, whereas the carboxyl group is deprotonated and carries a  charge. Therefore, only side chains contribute to the overall charge (the amino and carboxyl group charges cancel out). pKa refers to the pH at which half of the side chain functional group in question will be protonated and the other half deprotonated. For acidic amino acids (aspartate and glutamate), the deprotonated form will carry a  charge (protonated neutral), as it has donated a proton. For basic amino acids (histidine, arginine, and lysine), the protonated form will carry a  charge (deprotonated neutral), as it has accepted a proton. At pH = 7.4, there are more protons in solution than at a pH of 12.48, so arginine will be mostly protonated . There are more protons as well than at a pH of 10.54, so Lysine will also be protonated . But there are fewer protons than at a pH of 3.90, so aspartate will be deprotonated . Therefore, the overall charge is: