Chapter 3

Question 1

How can amino acids act as acids and bases?

Answer 1

Amino acids ionize in aqueous solution.
Amino acids contain at least 2 ionizable groups, each with its own pKa.
Carboxylic acid has a low pKa and will be protonated at an acidic (low) pH.
Amino group has a high pKa and will be protonated until basic pH (high) is achieved.
R groups of some amino acids can also ionize.
At low pH, amino acid exists in a positively charged form.
At high pH, amino acid exists in a negatively charged form.
Between pKa for each group, amino acid exists in a zwitterion form, in which a single molecule has both a positive and negative charge.

Question 2

At what pH do amino acids exist in solution as a zwitterion?

Answer 2

Neutral pH.

Question 3

Explain how amino acids carry a net charge of zero at a specific pH (the pI).

Answer 3

Zwitterions predominate at pH values between the pKa values of the amino and carboxyl groups.
Net charge is zero:
- Amino acid is least soluble in water.
- Amino acid does not migrate in electric field.

Question 4

How are amino acids classified?

Answer 4

Common amino acids can be placed in 5 basic categories depending on their R groups:
- nonpolar, aliphatic
- aromatic
- polar, uncharged
- positively charged
- negatively charged

Question 5

What are the nonpolar, aliphatic R groups?

Answer 5

Glycine, alanine, proline, valine, leucine, isoleucine, and methionine

Question 6

What are the aromatic R groups?

Answer 6

Phenylalanine, tyrosine, and tryptophan

Question 7

What are the polar, uncharged R groups?

Answer 7

Serine, threonine, cysteine, asparagine, and glutamine

Question 8

What can polar, uncharged R groups do?

Answer 8

Amino acid side chains can form hydrogen bonds. Cysteine can form disulfide bonds

Question 9

What are the positively charged R groups?

Answer 9

Lysine, arginine, histidine

Question 10

What are the negatively charged R groups?

Answer 10

Aspartate, glutamate