29. Amino Acids

Calculate the isoelectric point of tyrosine (Y)

Calculate the isoelectric point of glutamic acid (E)

Explain why the pI of lysine is the average of the pKa values of its two protonated amino groups.

a. What percentage of the a-amino group of lysine will be protonated at its pI?

<25%, 50%, >75%

b. Answer the same question for the e-amino group of lysine

a. Which amino acid has the lowest pI value?

b. Which amino acid has the highest pI value?

(a) The isoelectric point (pI) of phenylalanine is pH 5.5. Draw the structure of the major form of phenylalanine at pH values of 1, 5.5, and 11.

The isoelectric point of glutamic acid is pH 3.2. Draw the structures of the major forms of glutamic acid at pH values of 1, 3.2, 7, and 11. Explain why the side-chain carboxylic acid is a weaker acid than the acid group next to the α-carbon atom.

Most naturally occurring amino acids have chiral centers (the asymmetric α carbon atoms) that are named (S) by the Cahn–Ingold–Prelog convention (Section 5-3). The common naturally occurring form of cysteine has a chiral center that is named (R), however.

(b) (S)-Alanine is an L-amino acid (Figure 24-2). Is (R)-cysteine a d-amino acid or an L-amino acid?

d. Which amino acid has a greater negative charge at pH = 6.20, glycine or methionine?

Explain the order of elution (with a buffer of pH 4) of the following pairs of amino acids through a column packed with Dowex 50:

a. aspartate before serine

Explain the order of elution (with a buffer of pH 4) of the following pairs of amino acids through a column packed with Dowex 50:

b. serine before alanine

Which has a higher percentage of negative charge at physiological pH (7.4), leucine with pI = 5.98 or asparagine with pI = 5.43?

Draw the electrophoretic separation of Trp, Cys, and His at pH 6.0.