Tyrosine is a triprotic, dibasic amino acid with pKa of 2.17, 9.19, and 10.47 The first proton is removed from the carboxylic acid, the second from the ammonium group. The third proton, with a pKa of 10.47, is the phenolic proton from the amino acid side chain. This case is of interest because the acid is dibasic.