The isoelectric point (pI) of a peptide is the pH at which the peptide does not

Question

The isoelectric point (pI) of a peptide is the pH at which the peptide does not migrate in an electric field. Since the peptide is zwitterionic, there are the same number of positive charges as negative charges on the peptide population. The pI can be estimated fairly accurately (within 0.1 or 0.2 pH units) from the pK values of all the proton dissociable groups in the peptide. Using pK values from the table at the right, estimate the pI value of the following hexapeptide:

Trp-His-Glu-Tyr-Gly-Asp

The isoelectric point (pl) of a peptide is the pH at which the peptide does not migrate in an electric field. Since the peptide is zwitterionic, there are the same number of positive charges as negative charges on the peptide population. The pl can be estimated fairly accurately (within 0.1 or 0.2 pH units) from the pK values of all the proton dissociable groups in the peptide. Using pK values from the table at the right, estimate the pl value of the following hexapeptide Trp-His-Glu-Tyr-Gly-Asp Number pl 4.35 Amino Side Acid Chain pK Arg 12.5 Asp 3.7 8.2 Cys Glu 4.3 6.0 His Lys 10.5 Tyr 10.5 peptide- 8.0 NH3 peptide- 3.4 COOH *Amino and carboxy terminal values differ from the amino and carboxy values of a single amino acid

Explanation / Answer

Calculating pI of the peptide

the given peptide

looking at the pKa values in the table

pI = 1/2(4.3 + 3.7) = 4.0

at pH 4.0 the net charge on the peptide would be zero.

N-terminal charge +1

His side chain charge +1

Glu side chain 0

Tyr side chain charge 0

Gly charge 0

Asp side chain -1

C-terminal end charge -1

----------------------------------------

total charge = 0

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.