Which of the following is not true about buffering action of haemoglobin (Hb)?

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.