Hemoglobin acts as a buffer because of its

The correct answer is Histidine residue

Explanation:

• Hemoglobin, a protein found in red blood cells, is primarily responsible for transporting oxygen and carbon dioxide in the bloodstream.
• In addition to its role in gas transport, hemoglobin also acts as a buffer in the blood, helping to maintain pH stability, which is crucial for physiological functions.
• The buffering capacity of hemoglobin arises largely due to the presence of histidine residues in its structure.

• Histidine has an imidazole side chain with a pKa close to physiological pH (~6.0–7.0). This enables it to act as both a proton donor and acceptor, making it effective in stabilizing pH changes within the blood.
• When blood pH decreases (becomes more acidic), histidine can accept protons, and when pH increases (becomes more basic), histidine can donate protons, thereby maintaining pH balance.

Other options:

• Glycoprotein nature: Hemoglobin is not a glycoprotein; it is a conjugated protein made of globin (protein) and heme (iron-containing group). Glycoproteins are proteins attached to carbohydrate molecules, which are not relevant to hemoglobin's buffering capacity.
• Iron molecule: The iron molecule in hemoglobin is responsible for binding oxygen and carbon dioxide but does not contribute to its buffering properties. It is part of the heme group and plays no role in proton exchange or pH stabilization.
• Weak acidic nature: Hemoglobin itself is not weakly acidic. While proteins can exhibit varying levels of acidity or basicity depending on their amino acid composition, the buffering action of hemoglobin is specifically due to the histidine residues.