1 A tripeptide is composed of the following AspHisLys Determ

1. A tripeptide is composed of the following: Asp-His-Lys. Determine the isoelectric point for this protein.

Solution

Answer: 4.98

Asp-His-Lys.

Estimate the pH at which this peptide is neutral

Let’s say 7

At pH 7, this peptide is +1 due to Histidine side (pKa 6.10) chain ionized.

Decrease pH to make this charge zero, the next close pKa is 3.86.

Isoelectric pH is the average of two pKa values directly above and below the estimate.

PI is =(6.10 + 3.86)/2 =4.98

Amino Acid

a-carboxylic acid

a-amino

Side chain

Aspartic Acid

2.10

9.82

3.86

Histidine

1.77

9.18

6.10

Lysine

2.18

8.95

10.53

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