Lets look more closely at the protein buffer system to illustrate how this works.
Proteins are composed of amino acids. Amino acids consist of an acid group (COOH) and an amino group (NH2). These groups form bonds with neighbouring amino but some amino acids in proteins have additional groups.
If the body fluid is exposed to acids (from e.g. biochemical degradation of substances) H+ of such acids will be bound by the amino group (NH2) of a protein by the following reaction (R indicates the rest of the protein):
R-NH2 + H+ → R-NH3+
The H+ is thus bound by the protein and so by buffering the free H+ ion by binding with it, the protein opposes a potential pH change, caused by H+.
If the body fluid is exposed to a high pH, i.e. bases which gives up OH– ions , an acid group in a
protein can release H+ according to the following reaction:
R-COOH → R-COO– + H+
where H+ will react with the OH– and form water (H2O, see above). Again we see that a potential change in pH is compensated by the buffer.