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

His

Explanation:

His is the short-form for the amino acid Histidine.Histidine has imidazole as its functional group. When it is in the protonated then the pKa of the side chain is 6 which lies in the physiological pH range of around ph6.Due to this the catalysis of many enzymes have involves histidine for proton transfer. Histidine has the ability to abstract protons from amino acids such as cysteine, serine and threonine.The abstraction of proton takes place with the help of the basic nitrogen present in histidine which abstracts the protons from the targets and then another molecule is used to act as a buffer and proton is abstracted from the acidic nitrogen.