3 RESULTS AND DISCUSSION
3.1 Deprotonation conformation distribution
Both pentapeptides and tripeptides containing the β-lactam ring can be catalyzed efficiently by NocTE, and more importantly, tripeptides have simpler structure and more detailed experimental observations.5 Therefore, epi -nocardicin G covalently bound to NocTE (NocTE-LSub) system was constructed to explore the catalytic mechanism of NocTE. According to the hypothesis from Andrew M. Gulick’s studies6, the proton acceptor H1901 may be involved in not only hydrolysis but also epimerization. Considering the conformation difference of surrounding residues between the epimerization reaction and hydrolysis reaction, the deprotonation distance d(H1901-LSub) was restrained between 3.00 Å and 3.50 Å to gain an appropriate initial structure, and then six replicas of 100 ns MD simulations without this restraint were performed to capture deprotonation conformations for epimerization (Figure S2).