Homework SourseWhat is the net charge of the protein below at the following
What is the net charge of the protein below at the following pHs:
met-gly-leu-asp-glu-ile-arg-ala-his-val-glu-leu-stop
a. pH 2
b. pH 4
c. pH 5
d. pH 7
e. pH 9
f. pH 11
Solution
All amino acids are amphoteric in nature, the alpha amino -NH2 group is weakly basic (pKa ~ 9-10.5) and the alpha carboxylic -COOH group is weakly acidic (pKa ~ 1.7-2.4 ). Depending on the prevailing pH of the aqueous solution, all amino acids are ionized and possess a net charge. Amino acids having dissociable charged group also contribute to the net charge according to their pKa.
In peptides having multiple amino acids, like the one given above, only the N-terminal -NH3+ and C-terminal -COO- remain ionizable, as the rest of the ionizable alpha -NH3+ and alpha -COO- are lost in peptide bond formation. The net charge on this peptide is mainly due to the ionizable \'R\' groups. Hence you should be familiar with their pKa values.
Note: An easy way to calculate the net charge is that, if pH<pKa , then NH3+ will give +1 charge whereas COOH will be neutral or 0 charge. If pH>pKa, NH2 will be neutral or 0 charge whereas COO- will give -1 charge.
For the peptide, met-gly-leu-asp-glu-ile-arg-ala-his-val-glu-leu-stop
| Ionizable group | pH 2 | pH 4 | pH 5 | pH 7 | pH 9 | pH 11 |
| alpha-NH3+ of met | +1 | +1 | +1 | +1 | +1 | 0 |
| alpha -COO- of leu | 0 | -1 | -1 | -1 | -1 | -1 |
| R of asp | 0 | -1 | -1 | -1 | -1 | -1 |
| R of glu | 0 | 0 | -1 | -1 | -1 | -1 |
| R of arg | +1 | +1 | +1 | +1 | +1 | +1 |
| R of his | +1 | +1 | +1 | 0 | 0 | 0 |
| R of glu | 0 | 0 | -1 | -1 | -1 | -1 |
| Net charge of the peptide | +3 | +1 | -1 | -2 | -2 | -3 |
