Calculate the pI of Histidine (pKa’s = 2.3, 6.04, 9.33)
6.11
7.68
3.08
2.35
4.07

The correct answer is 7.68

Explanation:

Isoelectric point (pI) is the pH at which the amino acid has net a charge of zero. We have to choose the two values of pKa at which the neutral form of the amino acid is predominant. Histidine (Hys) is a basic amino acid. From pKa's data, we deduce:

pKa1 = 2.3 (corresponds to α- carboxyl group)

pKa2= 6.04 (corresponds to R group)

pKa3= 9.33 (corresponds to α- amino group)

Since it is a basic amino acid, the neutral form will exist between the higher pKa's values. Thus, the pI will be in a pH value between 6.3 and 9.33.

So, we have to calculate the average of pKa2 and pKa3:

pI= (pKa2+pKa3)/2= (6.04+9.33)/2= 7.68

Therefore, the pI of Hys is 7.68.

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