 The mechanism used to cleave a peptide bond involves making an amino acid residue that has the cysteine and threonine (proteases) or a water molecule (aspartic acid, metallo- and acid proteases) nucleophilic so that it can attack the peptide carboxyl group.One way to make a nucleophile is by a catalytic triad, where a histidine residue is used to activate serine, cysteine, or threonine as a nucleophile.This is not an evolutionary grouping, however, as the nucleophile types have evolved convergently in different superfamilies, and some superfamilies show divergent evolution to multiple different nucleophiles.