Which of the following amino acid responsible for buffering action of hemoglobin is a major intracellular buffer of blood?

Hemoglobin is the major intracellular buffer of blood which is present in erythrocytes. Histidine residues of albumin and hemoglobin play a significant role in buffering action. Histidine is a semi essential basic amino acid and its pK value is 6.1; and hence in proteins, histidine is responsible for the maximum buffering action. In addition, the iron of the heme moiety of hemoglobin is attached to globin through two specific histidine residues. Ref: Textbook of Biochemistry for Medical Students By D. M. Vasudevan, M.D., Dm, 2010, Page 201.