Q.NO.35. Considering that the three pKas of histidine are pK1=1.8, pK2=9.2 and pKR=6.O,
its isoelectric will be ____________ (rounded off to one decimal place).
The isoelectric point (pI) of histidine, given pK1=1.8, pK2=9.2, and pKR=6.0, is 7.6. This value is calculated by averaging the two pKa values flanking the zwitterionic form with net zero charge. Standard pKa assignments confirm pK1 for the carboxyl group, pKR for the imidazole side chain, and pK2 for the amino group.
pKa Assignments
Histidine has three ionizable groups: the α-carboxyl (pK1=1.8), the imidazole side chain (pKR=6.0), and the α-amino (pK2=9.2). These values match experimental data across biochemical references. The side chain pKR=6.0 is basic, classifying histidine as a basic amino acid despite its near-neutral pI.
Ionic Forms and Charges
At very low pH (<1.8), histidine is fully protonated: +2 net charge (–cooh, –nh3+, imidazole-h+). deprotonation occurs sequentially: carboxyl loses h+ between pk1 and pkr (net +1), imidazole pk2 0, zwitterion), amino above -1). the zero form predominates where positive (amino imidazole) negative (carboxyl) charges balance.
pI Calculation
For amino acids with ionizable side chains, pI is the average of the two pKa values surrounding the zero-charge species. For histidine (basic side chain), this is the average of pKR (6.0) and pK2 (9.2): pI = (6.0 + 9.2)/2 = 15.2/2 = 7.6. Rounding 7.60 to one decimal place gives 7.6, directly addressing the query.
Common Errors Explained
Averaging all three pKa values (1.8 + 6.0 + 9.2)/3 ≈ 5.7 is incorrect, as it ignores charge-specific pairing. Averaging pK1 and pKR (≈3.9) applies to acidic side chains like aspartic acid, not basic ones like histidine. Using standard literature values (pK2≈9.17, pKR=6.0) yields (9.17+6.0)/2=7.6, consistent here.
Introduction to Histidine Isoelectric Point Calculation
In histidine isoelectric point calculation, the pI is the pH where net charge is zero, vital for protein separation in electrophoresis and CSIR NET life sciences prep. With pK1=1.8 (carboxyl), pK2=9.2 (amino), pKR=6.0 (imidazole), the histidine isoelectric point is found by averaging pKR and pK2.
Step-by-Step Histidine pI Determination
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Identify zwitterion: deprotonated carboxyl (-1), protonated amino (+1), deprotonated imidazole (0).
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Flanking pKa: pKR=6.0 (imidazole deprotonates) and pK2=9.2 (amino deprotonates).
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Formula: pI = \frac{6.0 + 9.2}{2} = 7.6.
This matches exam standards for basic amino acids.
Applications in CSIR NET and Biochemistry
The histidine isoelectric point of 7.6 aids isoelectric focusing, enzyme activity studies, and peptide design. For competitive exams, remember: basic side chain → average side chain and amino pKa.
