Question

In what range would you expect the pI to be for Ala-Val-Lys?

a) close to 7

b)less than 7

c)greater than 7

d)greater than 10

Answer 1

Method 1 :
pI = (pKa + pKb) / 2
pI of Ala = 6.11
pI of Val = 6.02
pI of Lys = 9.47
average pI of Ala-Val-Lys = (6.11 + 6.02 + 9.47) / 3 = 7.2 (pI is greater than 7)
This is one way to determine the pI, however there is another reasonable way to solve this problem since
it is difficult to remember the pI and Pk values of each.

Method 2 :
For this you have to look at the side chain character of each amino acid i.e) whether it is acidic,
basic or neutral.
The side chain character of Ala is neutral ( by looking at the structure), Val also has neutral side chain
and Lys has basic side chain. So, all in all it makes Ala - Val - Lys a Basic amino acid.
All Basic amino acids have pI greater than 7 (which is known fact). Hence the expected pI range
for Ala-Val-Lys ( together it forms a basic amino acid) would be greater than 7.
So, option c) is the correct answer.