Homework Name: I.(10 pts) The pKe value of His-57 at the active site of chymotry

Question

Homework Name: I.(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 pH 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 1.92 2.35 0.78 9.87 8.33 a) Give the net charge at pH 1. Explain. b) Give the net charge at pH 12. Explain.

Explanation / Answer

1.
pH = pKa + log[salt]/[acid]
7.4 = 6.8 + log [salt]/[acid]
log [salt]/[acid] = 0.6
[salt]/[acid] = 1.8
[charged]/[uncharged] = 1.8
[uncharged]/[charged] = 0.55

[uncharged] = 0.55 X [charged]
[uncharged] = 1/1.55
[uncharged] = 0.645

2.
At pH = 1,
All the amino and carboxyl groups are protonated.
There is only one free amino group (NH3+ at the N-terminus).
So, the net charge = +1

At pH = 12,
All the amino and carboxyl groups are unprotonated.
So, the net charge = -3
-1 from side chain carboxyl group of Asp
-1 from S- group of Cys
-1 from alpha carboxyl group of Ala

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