Threonine proteases, such as the proteasome protease subunit and ornithine acyltransferases use the secondary hydroxyl of threonine in a manner analogous to the use of the serine primary hydroxyl. However, due to the steric interference of the extra methyl group of threonine, the base member of the triad is the N-terminal amide which polarises an ordered water which, in turn, deprotonates the catalytic hydroxyl to increase its reactivity. Similarly, there exist equivalent serine only and cysteine only configurations such as penicillin acylase G and penicillin acylase V which are evolutionarily related to the proteasome proteases. Again, these use their N-terminal amide as a base.