# Question #7f247

Aug 28, 2016

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 $\text{E-M-R-T-G}$.

#### Explanation:

(a) C-terminus

In a peptide, the amino acids are written from left to right with the ${\text{NH}}_{2}$ group on the left and the $\text{C=O}$ 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 $\text{Glu-Met-Arg-Thr-Gly}$, the C-terminus is Gly (glycine).

(b) Charged groups at pH 7

There are three points to remember:

• If $\text{pH = p} {K}_{a}$, the neutral and ionic forms are present in equal amounts.
• If $\text{pH < p} {K}_{a}$ (i.e. more acidic), the protonated form will predominate.
• If $\text{pH > p} {K}_{a}$ (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 $\text{p"K_"a}$ values are

The red line marks the pH 7 division mark.

For Gly, $\text{pH > pK"_"a}$, so the non-protonated form ($\text{COO"^"-}$) will predominate.

For Arg, $\text{pH < pK"_"a}$, so the protonated form (${\text{NH}}_{3}^{+}$) will predominate.

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

However, for the N-terminal group of Glu, $\text{pH < pK"_"a}$, so the protonated form (${\text{NH}}_{3}^{+}$) will predominate.

Thus, there are four ionized groups at pH 7.

(c) Net charge at pH 1

At pH 1, all groups have $\text{pH < pK"_"a}$, so the protonated forms will predominate..

For the C-terminal group of Gly and the R-group of Glu, the protonated forms are $\text{COOH}$.

For the N-terminal group of Glu and the R-group of Arg, the protonated forms are ${\text{NH}}_{3}^{+}$.

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

(d) Sequence using one-letter symbols

The one-letter symbols for the amino acids are

$\text{Arg = R}$
$\text{Glu = E}$
$\text{Gly = G}$
$\text{Met = M}$
$\text{Thr = T}$

$\text{Glu-Met-Arg-Thr-Gly = E-M-R-T-G}$.