Question #7f247

(a) C-terminus

In a peptide, the amino acids are written from left to right with the #"NH"_2# group on the left and the #"C=O"# group on the right.

The left hand amino acid is called the N-terminus, and the right hand amino acid is called the C-terminus.

Thus, in #"Glu-Met-Arg-Thr-Gly"#, the C-terminus is Gly (glycine).

(b) Charged groups at pH 7

There are three points to remember:

• If #"pH = p"K_a#, the neutral and ionic forms are present in equal amounts.
• If #"pH < p"K_a# (i.e. more acidic), the protonated form will predominate.
• If #"pH > p"K_a# (i.e. more basic), the non-protonated form will predominate.

Both Met (methionine) and Thr (threonine) have neutral R-groups, and they are neither N-terminal or C-terminal amino acids, so we can ignore them in our calculations.

Glu (glutamic acid) has both a basic N-terminus and an acidic R-group.

The corresponding #"p"K_"a"# values are

The red line marks the pH 7 division mark.

For Gly, #"pH > pK"_"a"#, so the non-protonated form (#"COO"^"-"#) will predominate.

For Arg, #"pH < pK"_"a"#, so the protonated form (#"NH"_3^+#) will predominate.

For the R-group of Glu, #"pH > pK"_"a"#, so the non-protonated form (#"COO"^"-"#) will predominate.

However, for the N-terminal group of Glu, #"pH < pK"_"a"#, so the protonated form (#"NH"_3^+#) will predominate.

Thus, there are four ionized groups at pH 7.

(c) Net charge at pH 1

At pH 1, all groups have #"pH < pK"_"a"#, so the protonated forms will predominate..

For the C-terminal group of Gly and the R-group of Glu, the protonated forms are #"COOH"#.

For the N-terminal group of Glu and the R-group of Arg, the protonated forms are #"NH"_3^+#.

Thus, at pH 1 the net charge is +2.

(d) Sequence using one-letter symbols

The one-letter symbols for the amino acids are

#"Arg = R"#
#"Glu = E"#
#"Gly = G"#
#"Met = M"#
#"Thr = T"#

∴ #"Glu-Met-Arg-Thr-Gly = E-M-R-T-G"#.