2.1 System construction
Based on the crystal structure of NocTE bound to the mimic structure of the hydrolytic tetrahedral intermediate of nocardicin G (PDB ID: 6OJD)6, two catalytic systems of NocTE covalently bound to epi -nocardicin G (NocTE-LSub) and nocardicin G (NocTE-DSub) were modeled respectively. The protonation state for each residue was determined via pKa calculations on PDB2PQR web serve14. For parameter preparations, the substrates covalently bound to NocTE were capped with -CO-CH3 group in the N-terminal and -NH-CH3 group in the C-terminal. Then the capped geometric structure was optimized at M062X15/6-31G*16 level and electrostatic surface potential (ESP) charge calculations were carried out at HF17/6-31G* level using Gaussian 09 program18. The charge distribution information used for system construction was calculated by a two-step restrained electrostatic potential (RESP)19 method through Multiwfn20 program and the antechamber21 package implemented in AMBER 1822. These systems were set up within an octahedral box of TIP3P water molecules. Besides, 15 sodium ions were added to maintain the electroneutrality in each system.