Which of the following is not true about buffering action of haemoglobin (Hb)?: (A) Hb is technically intracellular buffer, (B) Hb is functionally a plasma buffer, (C) Hb's buffering action is due to its histidine content, (D) Oxygenated Hb is a strong base
Answer is B. Protein buffers in blood include haemoglobin (150g/l) and plasma proteins (70g/l). Buffering is by the imidazole group of the histidine residues which has a pKa of about 6.8. This is suitable for effective buffering at physiological pH. Haemoglobin is quantitatively about 6 times more impoant than the plasma proteins as it is present in about twice the concentration and contains about three times the number of histidine residues per molecule.