Protein Buffers in Blood Plasma and Cells

Protein buffer systems rely on the ability of amino acids to respond to pH alterations.

If the pH increases, the carboxyl group in an amino acid can dissociate, acting as a weak acid and releasing hydrogen ions.

Some amino acids, such as histidine and cysteine, have side chains that may donate hydrogen ions.

In specific pH conditions, an amino acid carries both positive and negative charges, a state known as a zwitterion.

Conversely, when the pH decreases, the amino group can act as a weak base and accept hydrogen ions, forming an amino ion.

For instance, the protein hemoglobin acts as a buffer in red blood cells.

When blood flows through capillaries, carbon dioxide from tissue cells enters the red blood cells and combines with water to form carbonic acid. This acid then dissociates into hydrogen and bicarbonate ions.

Simultaneously, oxyhemoglobin gives up its oxygen to tissue cells, and the resulting reduced hemoglobin or deoxyhemoglobin accepts the free hydrogen ions, increasing the pH.