Example 3

Introduction to Protein Structure

Overall Charge on a Peptide -- Example 3

Cysteinylglutamylprolylthreonylleucinyllysylglutaminylarginine
(Cys-Glu-Pro-Thr-Leu-Lys-Gln-Arg; SEVQK)

Note: the 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 cystiene (sulfhydryl; pK_a ~ 8.5), the glutamic acid (carboxyl group; pK_a ~ 4.4), the lysine (amino group; pK_a ~ 10) and the arginine (guanidinium group; pK_a ~ 12) 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	Cys (-SH)	Glu (-COOH)	Lys (-NH₂)	Arg -C(NH₂)NH	C-terminus	Net Charge
2.0	1+	0	0	1+	1+	0	3+
7.0	1+	0	1-	1+	1+	1-	1+
11.0	0	1-	1-	0	1+	1-	2-