Question 21.4: Calculating the Charge on an Amino Acid What is the overall ...
Calculating the Charge on an Amino Acid
What is the overall charge (+, 0, or -) for the following amino acids at pH 7?
(a) Lysine
(b) Alanine
(c) Glutamic acid
Strategy
Since the actual numeric charge is not required, just whether overall it is positive, negative, or neutral, the easiest way to handle this is to compare the isoelectric point, pI, of the amino acid to the pH of the solution it is in.
From Table 21.1, the pI for lysine is 9.74. Since pH 7 is more acidic than the pI of 9.74, that means starting from the center molecule below where the molecule is at its pI:
| Table 21.1 The 20 Amino Acids Commonly Found in Proteins | |||
| Isoelectric Point | 1-Letter Abbreviation | 3-Letter Abbreviation | Name |
| 6.01 | A | Ala | Alanine |
| 10.76 | R | Arg | Arginine |
| 5.41 | N | Asn | Asparagine |
| 2.77 | D | Asp | Aspartic acid |
| 5.07 | C | Cys | Cysteine |
| 3.22 | E | Glu | Glutamic acid |
| 5.65 | Q | Gln | Glutamine |
| 5.97 | G | Gly | Glycine |
| 7.59 | H | His | Histidine |
| 6.02 | I | Ile | Isoleucine |
| 5.98 | L | Leu | Leucine |
| 9.74 | K | Lys | Lysine |
| 5.74 | M | Met | Methionine |
| 5.48 | F | Phe | Phenylalanine |
| 6.48 | P | Pro | Proline |
| 5.68 | S | Ser | Serine |
| 5.87 | T | Thr | Threonine |
| 5.88 | W | Trp | Tryptophan |
| 5.66 | Y | Tyr | Tyrosine |
| 5.97 | V | Val | Valine |
More \text{H}^{+} is present, pushing the reaction to the left and creating a molecule with a net + charge.
With alanine, the pI from the table is 6.01. So, at a pH of 6.01, the molecule would exist as the center molecule in the figure above. A pH of 7 is more basic, so the equilibrium will have shifted to the right, giving a molecule that is negatively charged, although only slightly so given the small difference between the pH and the pI.
The pI for glutamic acid is 3.22. Therefore, the pH is significantly higher, leading to a net negative charge at pH 7.
■ Quick Check 21.3
What is the overall charge (+, 0, or -) for the following amino acids at a pH of 6.02
(a) Isoleucine
(b) Arginine
(c) Aspartic acid