The greatest buffering capacity at physiologic pH would be provided by a protein rich in which of the following amino acids?
Ans. B. HistidineProteins can be effective buffers of body and intracellular fluids. Buffering capacity is dependent on the presence of amino acids having ionizable side chains with pKs near physiologic pH. In the example given, only histidine has an ionizable imidazolium group that has a pK close to neutrality (pK=6.0). Valine and leucine are amino acids with uncharged, branched side chains. Lysine has a very basic amino group (pK=10.5) on its aliphatic side chain that is positively charged at physiologic pH, and aspartic acid has a side chain carboxyl (pK=3.8) that is negative at pH7.Amino acidpK of the side chain groupAspartic acid3.9Glutamic acid4.2Lysine10.5Arginine12.5Histidine6.0
Lysine
Histidine
Aspartic acid
Valine
Proteins and Amino Acids