Question

Which statement correctly explains the protonation states of these histidine residues?

A. Histidine residues are always positively charged in physiological conditions.
B. The protonation state of histidine residues depends on the pH of the environment.
C. Histidine residues are negatively charged in neutral pH.
D. Protonation of histidine residues occurs in the absence of oxygen.

Answer 1

The correct statement about histidine residue protonation is that it depends on the pH of the environment. Histidine is typically neutral in physiological conditions and can gain or lose protons depending on the pH, affecting its overall charge.

Step-by-step explanation:

The statement that correctly explains the protonation states of histidine residues at different pH levels is: B. The protonation state of histidine residues depends on the pH of the environment.

Histidine is an amino acid with a side chain that contains an amine group.

The side chain of histidine has a pKa value of around 6.0, which means that at physiological pH (7.4), it is not fully protonated and therefore remains neutral.

Amino acids, in general, can have different charges depending on the pH of their environment.

At low pH levels (acidic), they tend to be positively charged because they can accept protons.

At high pH levels (basic), they tend to be negatively charged as they can donate protons.

Additionally, at a certain pH known as the isoelectric point (pI), amino acids have no net charge because the positive and negative charges balance each other out, resulting in a zwitterion.