Define Proteins as biological buffers?

Proteins have the ability to accept or donate hydrogen ions and by doing so they serve as biological buffers. In blood, there are three important buffering systems - plasma proteins, haemoglobin and carbonic acid bicarbonate. The equilibrium reactions for each of these buffering systems are as follows:

The first of these buffering systems, the plasma proteins, functions as a weak acid1 salt buffer when the free carboxyl groups on the protein dissociate, or as a weak base/salt buffer when the free amino groups dissociate. Although the buffering ability of the plasma protein is extremely important in maintaining blood pH, it is not as important as the other two systems, the second buffering system, carbonic acid-bicarbonate, is extremely effective because there are reactions which follow this equilibrium which will regulate either acids or bases.

The H₂CO₃ level in plasma never goes too high because it is in equilibrium with CO₂ (H₂CO₃ → CO₂ + H₂O), which is expired by the lungs. In blood, this equilibrium proceeds very quickly because of the presence of carbonic anhydrase, an enzyme found in red blood cells which catalyze it. If the carbonic acid-bicarbonate reaction goes in the opposite direction, the concentration of the HCO₃^- so formed will be regulated by the kidneys. The third important buffering system in blood results from haemoglobin. Haemoglobin has six times the buffering power of the plasma proteins. It functions well as a buffer because it is present in large amounts, it contains 38 histidine residues (Histidine residues are good buffers because they can dissociate to H⁺ and the imidazole group) and because haemoglobin exists in blood in two forms, reduced haemoglobin and oxy haemoglobin. It is thus a weaker acid and a better buffer.