Question

Using the following peptide, answer the following questions: HWESPGHAKCNKL.

A.) This peptide has the following pKa values associated with it: 2.1, 4.3, 6.0, 8.1, 9.3 & 10.7. What is the pI for this peptide?

B.) What would be the overall charge on this peptide at: (i) pH 12? (ii) pH 7?

C.) If this protein has the following modifications: one K side chain which is acetylated and the S side chain is phosphorylated. What would be the approximate pI for this modified peptide?

D.) This peptide is not likely to form an α-helix structure, give two reasons that would best explain why this is true?

E.) Given this sequence what is the most likely secondary structural element you think this is likely to form?

Answer 1

The structure of peptide HWESPGHAKCNKL (His-Trp-Glu-Ser-Pro-Gly-His-Ala-Lys Cys-Asn-Lys-Leu) with its pKa values is as follows: 10.7 NH 8.1 OH SH 9.3 H 2.1 -NH2 6.0 H 10.7 NH 6.0 H A.) Write the pKa values from lowest to the highest as shown below: 2.1, 4.3, 6.0, 8.1, 9.3 & 10.7 The value of the pI depends only on the 8.1 and 9.3. So, the pI is the average of these two values 8.1 9.3 pl = 2 17.4 2 -8.7 Therefore, the pI of the HWESPGHAKCNKL is 8.7. B.) The overall charge on this peptide at pH 7 is as follows:

NH, OH SH 9.3 H N H H H -NH2 NH, Charge So, the overall charge on this peptide at pH 7 is +1 The overall charge on this peptide at pH 12 is as follows: NH2 8.1 OH H 2.1 -NH2 4.3 NH2 Charge -3 on this peptide at pH 12 is -3 So, the overall charge C.) The peptide with structural modifications (one K side chain which is acetylated, and the S side chain is phosphorylated) is as follows

y.7 NH, SH H -NH2 HN- Charge -2 So, the approximate pI for this modified peptide is -2 D.) Secondary structure refers to the spatial arrangement of amino acids in protein i.e. it brings conformational changes in some region of polypeptide chain. Alpha helix and beta sheets are two widely known secondary structure of protein. The amino acid would most likely NOT be found in an a helix is proline. The given peptide has the amino acid proline, so it won't allow the peptide to attain Alpha helical conformation E.) B turns or beta ends are tight turns, which are a form of secondary structures formed by H2 (hydrogen bonding). It would like to form B turns.