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29. Amino Acids

Acid-Base Properties of Amino Acids

29. Amino Acids

Acid-Base Properties of Amino Acids

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Why Amino Acids Exist as Zwitterions

Why Amino Acids Exist as Zwitterions

Johnny Betancourt

Determining Predominant Forms

Determining Predominant Forms

Johnny Betancourt

The 7 Ionizable Amino Acids

The 7 Ionizable Amino Acids

Johnny Betancourt

Predicting Predominant Form of Lysine

Predicting Predominant Form of Lysine

Johnny Betancourt

Practice this topic

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

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

Multiple Choice

Calculate the pI for arginine (the pK_a values—or the pK_a values for the conjugate acids of bases—are given).

Multiple Choice

Why do amino acids exist in the Zwitterionic form at physiological pH?

Textbook Question

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

Textbook Question

Alanine has pKa values of 2.34 and 9.69. Therefore, alanine exists predominately as a zwitterion in an aqueous solution with pH >____ and pH 6 <____.

Textbook Question

Identify the location and type of charge on the hexapeptide Lys-Ser-Asp-Cys-His-Tyr at each of the following pH values: d. pH=12

Textbook Question

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

Textbook Question

Draw the form of aspartate that predominates at the following pH values: d. pH=11.0

Textbook Question

Draw the form of aspartate that predominates at the following pH values: c. pH=6.0

Textbook Question

Draw the predominant form for glutamate in a solution with the following pH: b. 3

Textbook Question

Draw the predominant form for glutamate in a solution with the following pH: a. 0

Textbook Question

c. Which amino acid has the greatest amount of negative charge at pH = 6.20?

Textbook Question

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 pKa = 5.6.

Textbook Question

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.

Textbook Question

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

Textbook Question

Draw the structure of the predominant form of (b) proline at pH 2.

Textbook Question

Draw the structure of the predominant form of (e) a mixture of alanine, lysine, and aspartic acid at (i) pH 6;

Textbook Question

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

Textbook Question

Draw the structure of the predominant form of

(d) glutamic acid at pH 7.

Textbook Question

Draw the structure of the predominant form of

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

Textbook Question

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.

Textbook Question

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.

Textbook Question

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

b. His

Textbook Question

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

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