The side chain of has a pKa in the physiological pH range and is therefore often involved in proton transfer during enzymatic catalysis. a. Arg b. Asp c. His d. Glu

His

Explanation:

The side chain of Histidine has a pKa in the physiological pH range and is therefore often involved in proton transfer during enzymatic catalysis. It is a amino acid which is used for the biosynthesis of protein. Histidine was initially thought to be essential only for infants but now it has  been found to be essential for adults too.If we see the histidine structure contains an α-amino acid, a carboxylic acid group, and an imidazole side chain.

answer; i believe that the correct answer is (lung);