Analysis of the QM/MM MD trajectories revealed conformational change of the Glu465 sidechain, particularly prononuced in the oxytocin system. This catalytic glutamate plays a dual role: deprotonating the zinc-bound water molecule in the initial reaction step and facilitating the proton transfer to the scissile peptode bond nitrogen. Two distinct Glu465 rotamers were observed, designated as OE1 and OE2 conformations based on which carboxylate oxygen acts as a hydrogen bond acceptor to the catalytic water molecule (see Figure ). These conformations were characterised by dihedral angles of approximately -10° (eclipsed, OE1) and -60° (staggered, OE2) formed by the sidechain carbons (Figure ). In both oxytocin and angiotensin IV complexes, Glu465 predominantly adopted the staggered (OE2) conformation. Notably, reversible proton transfer from the catalytic water molecule to Glu465 was observed during simulations, generating a zinc-bound hydroxide, especially in the OE1 conformation. These findings suggest that subtle conformational changes of the catalytic glutamate might play a critical role in the peptide hydrolysis mechanism. Representative structures of the peptides in the active site with the Glu465 in the OE1 and OE2 conformations are depicted in Figures 3b for oxytocin and angiotensin IV.