Question

Your protein of interest contains 4 D, 5 E, 2 Y, 1 A, and 8 G. What is its charge at a pH of 1?

a) None of the above
b) Neutral
c)-11
d)+2
e) -9

Answer 1

At a pH of 1, the acidic side chains of a protein (D and E) would be protonated and carry no negative charge, while the basic side chain (Y) would also remain neutral. The protein would thus have a net neutral charge.

Step-by-step explanation:

The charge of a protein at a specific pH can be calculated based on the pKa values of its ionizable groups and the pH of the environment. At a pH of 1, which is very acidic, we can expect the acidic side chains (aspartic acid (D) and glutamic acid (E)) and the amino termini to be protonated (i.e., not carry a negative charge). On the other hand, the basic side chain of tyrosine (Y) would not be protonated and thus not carry a positive charge. The non-ionizable side chains, such as alanine (A) and glycine (G), do not contribute to the overall charge under these conditions.

Calculating the contributions, we have:

• Aspartic acid (D), 4 residues, protonated, no negative charge
• Glutamic acid (E), 5 residues, protonated, no negative charge
• Tyrosine (Y), 2 residues, neutral, no positive charge
• Alanine (A), 1 residue, neutral
• Glycine (G), 8 residues, neutral

Summing these up, at pH 1, the protein will have no negative or positive charges from the side chains, thus the net charge will be neutral (Option b).