Example 2

Introduction to Protein Structure

Overall Charge on a Peptide -- Example 2

Serylaspartylvalinylglutaminyllysine
(Ser-Asp-Val-Glu-Lys; SEVQK)

Note: ionizable groups are "boxed"

In addition to the amino group (pK_a ~ 9) and the carboxyl group (pK_a ~ 3) at each end of the molecule, the sidechains of the aspartic acid (carboxyl group; pK_a ~ 4.4) and the lysine (amino group; pK_a ~ 10) residues are ionizable.

Thus, the net charge on this peptide at three different pH's ( 2, 7 and 11) is as follows:

	Charge on Functional Group
pH	N-terminus	Asp -COOH	Lys -NH2	C-terminus	Net Charge
2.0	1+	0	1+	0	2+
7.0	1+	1-	1+	1-	0
11.0	0	1-	0	1-	2-