Each ionizable group of an amino acid can exist in one of two states, charged or neutral

Each ionizable group of an amino acid can exist in one of two states, charged or neutral. The electric charge on the functional group is determined by the relationship between its pK and the pH of the solution. This relationship is described by the Henderson-Hasselbalch equation. Histidine has three ionizable functional groups. Complete the equilibrium equations by assigning the proper pK1 for each ionization and the net charge on the histidine molecule in each ionization state. 1-1-OH H2N-C-C-OH 1 CH2 + HN -NH O H || H2N-C-C-0- | CH 2 HN -NH O O H || H2N-C-C-0- 1 CH2 -NH ? H H2N-C-C-0- Lio CH 2 O -NH

The Correct Answer and Explanation is :

Histidine Ionization Groups:

1. Amino Group (-NH₃⁺): This group is typically protonated at physiological pH (around 7.4). The pK for the amino group is approximately pK₁ ≈ 9.2.
2. Carboxyl Group (-COOH): This group loses a proton at a relatively low pH, typically around 2. The pK for the carboxyl group is about pK₂ ≈ 1.8.
3. Imidazole Group: Histidine’s imidazole side chain has a pK that allows it to be protonated or deprotonated in the physiological pH range. The pK for the imidazole group is around pK₃ ≈ 6.0.

Henderson-Hasselbalch Equation:

The relationship between the pH and the protonation state of ionizable groups in an amino acid can be described using the Henderson-Hasselbalch equation:

[
\text{pH} = \text{pK} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right)
]

Where:

• pK is the acid dissociation constant for a given functional group.
• [(A^-)] is the concentration of the deprotonated form of the group.
• [(HA)] is the concentration of the protonated form.

This equation shows how the ionization of a group changes based on the pH of the solution relative to the pK of that group.

Ionization States and Net Charge:

1. At low pH (around 2):

• The carboxyl group will be protonated (–COOH), the imidazole group will be protonated (–NH), and the amino group will be protonated (–NH₃⁺).
• Net charge: +1 (due to the protonated amino group).

1. At physiological pH (around 7.4):

• The carboxyl group will be deprotonated (–COO⁻), the imidazole group may be protonated or deprotonated, and the amino group will still be protonated (–NH₃⁺).
• Net charge: 0 or +1, depending on whether the imidazole group is protonated or neutral.

1. At high pH (around 10):

• The carboxyl group will be deprotonated (–COO⁻), the imidazole group will be neutral or deprotonated, and the amino group will be deprotonated (–NH₂).
• Net charge: -1 (due to the negatively charged carboxyl group).

Final Net Charge at Different pH Values:

• At low pH, the net charge is +1.
• At physiological pH, the net charge can be 0 or +1 depending on the imidazole group.
• At high pH, the net charge will be -1 due to the deprotonation of the amino and carboxyl groups.

Conclusion:

The Henderson-Hasselbalch equation is used to understand the relationship between pH and the charge state of histidine’s functional groups. The ionization states of each group depend on the pH, with the amino group having the highest pK and the carboxyl group the lowest. The imidazole side chain of histidine is important in biological processes because its pK allows it to function effectively in a wide range of pH conditions.