 The reactivity is enhanced when the thiol is ionized, and cysteine residues in proteins have pKa values close to neutrality, so are often in their reactive thiolate form in the cell.Because of its high reactivity, the sulfhydryl group of cysteine has numerous biological functions, and cysteine may have played an important role in the development of primitive life on Earth.Due to the ability of thiols to undergo redox reactions, cysteine has antioxidant properties.