sentences:  The water molecule is then activated by both the zinc ion and the Glu143 residue and attacks the carbonyl carbon to form a tetrahedral transition state (see figure).Holden and coworkers then mimicked that tetrahedral transition state to design a series of phosphonamidate peptide analogues.Among the synthesized analogues, R = L-Leu possesses the most potent inhibitory activity ("K"i = 9.1 nM).