Homework Name Hena Jenki 1 10 pts The pKe value of His57 at

Homework Name: Hena Jenki 1. (10 pts) The pKe value of His-57 at the active site of chymotrypsin is 6.8. This histidine residue is one of the three residues of the catalytic triad essential for catalysis. Answer the following questions (a) Calculate the ratio of the neutral to positively charged side chain of this histidine residue at physiological phi of 7.4. (b) what is the percentage (%) of the neutral side chain? Calculate this from the answer in (a). 2 (4 pts) Answer each of the following questions regarding the tripeptide Asp-Cys-Ala. Aspartic acid Cysteine Ala 1.99 1.92 2.35 10 78 9.87 8.33 a) Give the net charge at pH 1. Explain. b) Give the net charge at pH 12. Explain.

Solution

1. His-57 pKa = 6.8

a. using Hendersen-Hasselbalck equation,

pH = pKa + log(neutral/protonated)

7.4 = 6.8 + log(neutral/protonated)

(neutral/protonated) Histidine residue = 4

b. % of neutral side chain

= 100 - (1 x 100/5)

= 80%

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2. For the tripeptide

Asp-Cys-Ala

a) at pH 1

charge on N-terminal end +1

charge on side chain Asp 0

charge on Cys side chain 0

charge on C-terminal 0

Net charge at pH 1 = +1

b) at pH 12

charge on N-terminal end 0

charge on side chain Asp -1

charge on Cys side chain -1

charge on C-terminal -1

Net charge at pH 1 = -3