Histidine Isoelectric Point (pI) Calculation
The isoelectric point (pI) is the pH at which an amino acid has no net electrical charge. For amino acids with ionizable side chains, like histidine, the pI is calculated by averaging the pKa values of the two relevant ionizable groups that surround the zwitterionic form.
Identifying Relevant pKa Values
Histidine has three pKa values:
- pK1 = 1.8 (associated with the alpha-carboxyl group)
- pKR = 6.0 (associated with the imidazole side chain)
- pK2 = 9.2 (associated with the alpha-amino group)
The zwitterionic form of histidine is stable between the deprotonation of the imidazole group (pKR) and the deprotonation of the alpha-amino group (pK2). Therefore, these are the two pKa values used to calculate the pI.
Calculating the Isoelectric Point (pI)
The formula for pI when three pKa values are involved is:
$pI = (pKa_lower + pKa_upper) / 2$
In the case of histidine, the relevant pKa values are pKR and pK2.
Calculation:
- pI = (pKR + pK2) / 2
- pI = (6.0 + 9.2) / 2
- pI = 15.2 / 2
- pI = 7.6
The calculated pI is 7.6. Rounding to one decimal place gives 7.6.
Important Questions from Chemical Biology {p}K_a Isoelectric Point
- Consider the two $pK_a$ values of valine as 2.32 and 9.62. The isoelectric point (pI) of this amino acid is __________. (rounded off to two decimal places)
- The $pK_a$ values of the carboxylic and amino groups of an amino acid with a non-ionizable side chain are 2.17 and 9.13, respectively. The isoelectric point (rounded off to two places of decimals) of this amino acid is ________.
- Consider the given peptide, Ala-Glu-Val-Asn-Ile-Asp-Pro-Asp-Gln-Gly-Asp. The net charge on the peptide at pH 1.0 will be ________
- The C-terminal carboxyl group and the N-terminal amino group in amino acids have a dissociation constant ($pK_a$) of 2.2 and 9.2, respectively. The $pK_a$ of side chain carboxyl group in glutamic acid is 4.2 and side chain amino group in lysine is 10.2. The difference in isoelectric point ($pI$) of lysine and glutamic acid (rounded off to two decimal places) is ________.
- An amino-acid has one proton donating group in the side chain (R). The $pK_{COOH}$, $pK_{NH2}$ and $pK_R$ values for this amino-acid are 2.19, 9.67 and 4.25, respectively. Which one of the following statements about this amino-acid is CORRECT?
