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

Question

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 c 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 Side Acid Chain pK on the peptide population. The Arg Asp Cys Glu 12.5 3.7 8.2 4.3 6.0 10.5 10.5 8.0 Trp-His-Glu-Tyr-Gly-Asp Number Lys Tyr peptide- NH3 tide- 3.4 OH Amino and carboxy terminal values differ from the amino and carboxy values of a single amino acid

Explanation / Answer

pK values of the amino acids and amino and carboxy terminal peptide:

N-terminal = 8.0

C-terminal = 3.4

His = 6.0

Glu = 4.3

Tyr = 10.5

Asp = 3.7

Trp and Gly does not have any charge.

Now we need to order in either ascending order or descending order. The number in the brackets indicate the net charge at pH 7

=> C-terminal (+1), Asp(+1), Glu(+1), His(+1), N-terminal(-1), Tyr(-1)

As indicated above, the pK of the peptide should result in zero net charge, i.e, the positively charged elements should be equal to the negatively charged elements. At pH 7, we have 4 positively charged elements and 2 negatively charged elements. At pI, there will be 3 positively charged elements and 3 negatively charged elements. So pI will take a value between the pK of Glu (4.3) and the pK of His (6.0), so that the His will take a negative charge, giving rise to 3 positively charged elements and 3 negatively charged elements.

So, pI =(4.3+6.0)/2 = 10.3/2 = 5.15

pI = 5.15

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