Amino Acid Practice Problems

Calculate the number of possible pentapeptides that contain one resi-

due each of Ala, Gly, His, Lys, and Val

The number of possible pentapeptides is calculated by finding the number of permutations of 5 distinct amino acids, which is 5! = 120.

In some proteins, the side chain of serine appears to undergo ioniza-

tion. Explain why ionization would be facilitated by the presence of an

aspartate residue nearby

The presence of an aspartate residue nearby can facilitate the ionization of serine because aspartate is a negatively charged amino acid. This negative charge can stabilize the positive charge that results from serine's side chain being protonated, thereby promoting the ionization process.

Determine the net charge of the predominant form of Asp at

(a) pH 1.0, (b) pH 3.0, (c) pH 6.0, and (d) pH 11.0.

At pH 1.0, Asp is positively charged; at pH 3.0, it is neutral; at pH 6.0, it is negatively charged; and at pH 11.0, it remains negatively charged as both carboxyl groups are deprotonated.

Determine the net charge of the predominant form of Arg at (a) pH 1.0,

(b) pH 5.0, (c) pH 10.5, and (d) pH 13.5.

(a) +2, 

(b) +1, 

(c) 0, 

(d) -1​

Over time, the glutamine residues of polypeptides are susceptible

to deamidation, a reaction in which the amide group is replaced by

a carboxylate group. What amino acid is produced when glutamine is

deamidated?

Deamidation of glutamine produces glutamic acid.

A sample of the amino acid tyrosine is barely soluble in water.

Would a polypeptide containing only Tyr residues, poly(Tyr), be more

or less soluble, assuming the total number of Tyr groups remains con-

stant?

A polypeptide containing only Tyr residues would be less soluble in water than individual tyrosine molecules due to increased hydrophobic interactions.

(a) What is the net charge at neutral pH of a tripeptide containing

only alanine? (b) How does the total number of negative and positive

charges change following hydrolysis of the tripeptide?

(a) The net charge is 0 since alanine is a neutral amino acid at neutral pH. (b) Hydrolysis does not change the total charge, as it results in three neutral alanine molecules.

The ionic characters of which amino acids are likely to be sensitive to pH changes in the physiological range? Explain

Amino acids with ionizable side chains such as aspartic acid, glutamic acid, lysine, arginine, and histidine are sensitive to pH changes in the physiological range due to their ability to gain or lose protons, affecting their overall charge.

Why is the pK of the carboxyl group of glycine (pK = 2.3) less than that for acetic acid (pK = 4.76)?

The pK of the carboxyl group of glycine is lower than that of acetic acid because glycine can exist as a zwitterion, stabilizing the carboxylate form, while acetic acid does not have such stabilization, making it less likely to donate a proton.

Which amino acid found in proteins has no optical activity?

Glycine, due to its symmetrical structure with two hydrogen atoms as side chains.