Question

For each question, explain how you got the answer.

Which of the following have a net charge of +2 at pH of 1?

A. Asparagine

B.Arginine

C.Glutamine

D.Lysine

E.More than one of the above

2. A 200-amino acid polythreonine protein would have a net charge of _____ at pH 10.9

A.+200

B.-1

C.0

D.-200

E.+199

3. Which of the following amino acids has a side chain that is neutral at pH10?
A. Lysine

B. Aspartate

C. Histidine

D. Cysteine

E. None of the above

4. The net charge of the tripeptide Asn-Asn-Asn at pH=4.5 will be:

A. +2

B. +1

C. 0

D. -3

E. None of these answers are correct

Answer 1

1. Option E. More than one of the above.

We can calculate the net charge of given amino acids, by looking at the pKa values of the ionizable groups of the individual amino acids. The pKa values have been given below-

	pKa1 (CARBOXYL GROUP)	pKa2 (AMINO GROUP)	pKa3 (SIDE CHAIN)	CHARGE AT pH 1
ASPARAGINE	2	8.8		+1 (Since, none of amino or carboxyl group leaves their hydrogen ion and there is only one positively charged amino group)
ARGININE	2.1	9	12.48	+2 (None of the ionizable groups leave their hydrogen ion and there are two positively charged amino groups)
GLUTAMINE	2.1	9.1		+1 (None of the amino or carboxyl groups leaves there hydrogen ion. Only one positively charged amino group will be there)
LYSINE	2.18	8.9	10.53	+2 (Two positively charged amino groups will be there)

Thus, Arginine and lysine will have a net charge of +2 at pH 1.

2. Option B. -1

All of the 200 threonine residues in the protein are involved in peptide bond formation. Only the amino group of the first residue and the carboxyl group of the last residue are free. Therefore, only the first and the last threonine will be available for ionization at pH 10.9. The amino group of the first threonine will leave it's hydrogen ion at pH 10.9 (since, it's pKa value is 9). Carboxyl group of the first threonine will not be ionized, since it is involved in forming peptide bond with the next threonine residue. The amino group of the 200th threonine residue is involved in forming peptide bond with the 199th residue, therefore, it will not get ionized. It's carboxyl group will leave it's hydrogen ion at pH 10.9 (since, it's pKa is 2.6), giving the protein a net charge of -1.

3. Histidine. (pKa value of the side chain of histidine is 6. Thus, it gets ionized at pH 10, and becomes neutral. At pH 10, side chain of lysine is positively charged, whereas, side chain of aspartate and cysteine are negatively charged).

4. Option C.

Only the amino group of the first asparagine residue and the carboxyl group of the third asparagine residue will be available for ionization, rest of the groups will be involved in peptide bond formation. At pH 4.5, amino group of the first residue will remain positively charged (+1) (since, it's pKa value is 8.8), while, the carboxyl group of the third residue will get ionized (-1). Thus, overall charge of the tripeptide at pH 4.5 will be zero.