Homework Name 1 10 pts The pKe value of His57 at the active

Homework Name: 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 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 egarding the tripeptide Asp-Cys-Ala. Aspartic acid Cysteine Ala 1.99 1.92 2.35 9.90 10.78 9.87 3.90 8.33 a) Give the net charge at pH 1. Explain. b) Give the net charge at pH 12. Explain.

Solution

(1)

(a)

Using Henderson Hasselbach equation:

pH = pKa + log(moles of neutral/moles of positively charged)

Putting values:

7.4 = 6.8 + log(moles of neutral/moles of positively charged)

Solving we get:

Ratio of neutral to positively charged = (moles of neutral/moles of positively charged) = 0.25

(b)

Ratio of neutral to positively charged = a/(1-a) = 0.25

Here, a = degree of dissociation of positively charged species to give neutral species

So,

a = 0.2

So, % of neutral side chain = % of positively charged chain that dissociated = a = 20%

Hope this helps !