Question

Question #7f247

Answer 1

WARNING! Long answer. (a) The C-terminus is glycine. (b) There are 4 charged groups at pH 7. (c) At pH 1 the net charge is +2. (d) The sequence using one-letter symbols is `"E-M-R-T-G"`.

Explanation:

(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"`.