Draw the structure of the predominant form of
(a) isoleucine at pH 11.

Draw the pH–activity profile for an enzyme that has one catalytic group at the active site:

b. the catalytic group is a general-base catalyst with a pKa = 7.2.

The pH–activity profile for glucose-6-phosphate isomerase indicates the participation of a group with a pK_a = 6.7 as a basic catalyst and a group with a pK_a = 9.3 as an acid catalyst. Draw the pH–activity profile and identify the amino acids that participate in the catalysis.

Draw the structure of the predominant form of

(b) proline at pH 2.

Draw the structure of the predominant form of

(e) a mixture of alanine, lysine, and aspartic acid at (i) pH 6;

Draw the structure of the predominant form of

(d) glutamic acid at pH 7.

Draw the structure of the predominant form of

(e) a mixture of alanine, lysine, and aspartic acid at (iii) pH 2.

Although tryptophan contains a heterocyclic amine, it is considered a neutral amino acid. Explain why the indole nitrogen of tryptophan is more weakly basic than one of the imidazole nitrogens of histidine.

Aspartame (Nutrasweet®) is a remarkably sweet-tasting dipeptide ester. Complete hydrolysis of aspartame gives phenyl alanine, aspartic acid, and methanol. Mild incubation with carboxypeptidase has no effect on aspartame. Treatment of aspartame with phenyl isothiocyanate, followed by mild hydrolysis, gives the phenylthiohydantoin of aspartic acid. Propose a structure for aspartame.