This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Hb is a good buffer because of: 
Select and explain. 
A. Histidine residues, B. Protein nature, C. Acidic nature, D. Iron molecule
A: Histidine residues
Answer: A: Histidine residues 
Explanation: Harper mentions …‘Hemoglobin also functions in CO2 and proton transport from tissues to lungs. Release of O2 from oxy-Hb at the tissues is accompanied by uptake of protons, due to lowering of the pKa of histidine residues.’

Decarboxylation of histidine to histamine is catalyzed by a broad-specificity of aromatic L-amino acid decarboxylase.
Histidine compounds present in the human body include ergothioneine, carnosine, and dietary anserine.