Adaptive string method QM/MM MD simulations. The potentials of mean force (PMF) for oxytocin and angiotensin IV hydrolysis were obtained with the adaptive string method (ASM), which provides a good, flexible description of complex reactions in enzymes. First, the reactant (E-R) and intermediate (E-I) structures for each peptide complex were extracted from the equilibrated QM/MM umbrella sampling windows at reaction coordinate (OWater-CPeptide distance) values of 2.7 Å and 1.5 Å, respectively. The product complexes were generated from the E-I structures by manually attaching the Glu465-bound proton to the nitrogen atom of the scissile bond. After a short unrestrained QM/MM MD simulation, this yielded a cleaved peptide bond and the second proton from the water molecule spontaneously transferred to Glu465. Five collective variables (CVs) for the ASM simulations were chosen (after testing) to represent the distances of the bonds that break and form during the reaction, as shown in Figure of the SI. Initial string optimization was carried out for 30 ps until the string had converged, after which the umbrella sampling phase was performed for up to 50 ps until the standard deviation for the individual PMF profile was under 1 kcal mol -1 . Within the ASM protocol, the E-R and E-P states were propagated using a total of 20 replica exchange simulations (10 replicas per state) for the oxytocin and angiotensin IV systems. All ASM simulations were carried out in triplicate under an NVT ensemble at 300 K using Langevin dynamics starting from different initial conditions. Structural snapshots were collected for analysis every 10 fs. The ASM simulations were performed using the sander.MPI module of AmberTools22. More details about the QM/MM simulation setup, umbrella sampling, high-level DFT/MM calculations of the potential energy surface, natural bond orbital (NBO) and non-covalent interaction (NCI) analysis are given in the SI.