Thermolysin is an enzyme produced by Bacillus thermoproteolyticus that catalyses the hydrolysis of peptides containing hydrophobic amino acids. Therefore, it is also a target for antibacterial agents. The enzymatic reaction mechanism starts form the small peptide molecule and replaces the zinc binding water molecule towards Glu143 of thermolysin. 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 = -Leu possesses the most potent inhibitory activity (K = 9.1 nM).