Which of the following statements regarding native carbonic anhydrase at pH 8.0 is true?

• A. It has a net charge of 0
• B. It has a net charge of +2.9
• C. It has a net charge of -2.9
• D. It has a net charge that depends on its concentration

Answer: (C)

The correct answer highlights that native carbonic anhydrase has a net charge of -2.9 at pH 8.0. This value can be attributed to the ionization states of the amino acids that compose the enzyme, particularly at physiological pH levels.

At pH 8.0, which is close to physiological pH, certain side chains of amino acids will either be protonated or deprotonated depending on their pKa values. Typically, amino acids like aspartic acid and glutamic acid have carboxyl groups that can lose protons and carry a negative charge, while others, such as lysine and arginine, tend to carry a positive charge due to their amino groups being protonated.

Considering the presence of more negatively charged residues at this pH, alongside any neutral and positively charged residues, the net charge calculated for carbonic anhydrase tends towards negative. This results in a net charge of -2.9 when accounting for the overall balance of charged residues at pH 8.0, which reflects the predominant ionization states of the side chains.

Thus, understanding the relationship between pH and the ionization states of amino acids is crucial in determining the overall charge of proteins like