29. Amino Acids

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

a. the catalytic group is a general-acid catalyst with a pK_a = 5.6.

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.

Draw the structure of the predominant form of

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

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.

Determine the net charge of the dipeptide R-C at pH 4.3. (Hint:Peptide bonds do not count)

Draw the electrophoretic separation of Ala, Lys, and Asp at pH 9.7.

Draw the structure of the predominant form of

(d) glutamic acid at pH 7.

Draw the structure of the predominant form of

(b) proline at pH 2.

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.

Glycine has pK_a values of 2.34 and 9.60. At what pH does glycine exist in the forms shown?

a.

Draw the predominant form for glutamate in a solution with the following

pH: b. 3

Identify the location and type of charge on the hexapeptide Lys-Ser-Asp-Cys-His-Tyr at each of the following pH values:

c. pH=7

Predict the predominant form and net charge of tyrosine (Y) at pH 10. What is the net charge? 

Draw the following amino acids in all possible protonation states as you move from high pH to low pH.

b. His