1. Calculate the charge on the side chain of the amino acid histidine using the Henderson-Hasselbach equation pH = pKa + log (A/HA+). Use pH =7 and a pKa = 6 and solve to 2 significant figures.
2. Cysteine proteases have unusually reactive cysteine side chains. For one of these enzymes in solution at pH 7 you are able to measure the amount of the deprotonated sulfhydryl (ie, the negatively charged species) as 40% of the total. Calculate the pKa of this side chain to 2 significant figures using the Henderson-Hasselbach equation pH = pKa + log (A-/HA)
Ans. #1. Putting the values in HH equation –
7 = 6 + log ([A] / [AH+])
Or, ([A] / [AH+]) = antilog (7-6) = 101 =
Or, [A] = 10 [AH+] - equation 1
Now, let the total amount of chemical species, i.e. [A] + [AH+] = 1 (on a fraction scale)
So, [A] + [AH+] = 1 - equation 2
Comparing equation 1 and 2 –
10 [AH+] + [AH+] = 1
Or, [AH+] = 1 / 11 = 0.091
Hence, relative charge on the side chain of histidine = + 0.091
#2. Reaction- C-SH ------> C-S- + H+
Where, C-SH = cysteine side chain
Given- [C-S-] = 40 % = 0.4 when the total amount of amino acid = 100% or 1.
Note that Cyst side chain acts as an acidic group (whereas that of His acted as a basic group).
So, HH equation is: pH = pKa + log ([A-] / [AH])
# [AH] = 1.0 - [A-] = 1.0 - 0.4 = 0.6
Putting the values in above equation-
7 – log (0.4 / 0.6) = pKa
Or, 7 – (-0.18) = pKa
Hence, pKa = 7.18 = 7.20
1. Calculate the charge on the side chain of the amino acid histidine using the Henderson-Hasselbach...
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