Exercise : For the pentapeptide Glu - met-Arg - Thr-Gly (a) Name the carboxyl - terminal residue (b) give the # of charged groups at pH7 (c, give the net charge at pH 1 id, draw the peptide bond between Thr & Gly

A) The amino acid Glycine would have the carboxyl group.

B) 4

C) + 2

B) Figure 4

Explanation:

For A) we have to keep in mind the nomenclature of aminoacids. The aminoacid in the left would be the N-terminal (amino group) and the aminoacid in the right would be the C-terminal (carboxyl group) (Figure 1).

For B) we have to check the pka values for each amino acid (Figure 2). In this case the charged groups are the resdiual groups in the Glu and Arg aminoacids and the C and the N terminal groups. (Figure 1)

For C) we have to follow a similar procedure and we have to keep in mind that we have a very low pH value, so we will have a high concentration of hydronioum ions H+. So all the groups would be protonated. In this case the residual group of Arg and the N-terminal group. (Figure 3).

Finally we will have a peptidic bond between Thr and Gly (Figure 4)