WlbB,one of the enzymes required for the biosynthesis of UDP-2,3-diacetamido-2,3-dideoxy-D-mannuronic acid(UDP-ManNAc3NAcA),is an N-acetyltransferase that catalyze N-acetylation of UDP-2-acetamido-3-amino-2,3-dideoxy-D-glucuronic acid(UDP-GlcNAc3NA)to form UDP-2,3-diacetamido-2,3-dideoxy-D-glucuronic acid(UDP-GlcNAc3NAcA).With combined QM/MM method,we employed six snapshots taken from MD trajectories as initial models to provide insight into the detailed reaction pathway of WlbB and the effects of starting geometries.The calculation results suggested that WlbB-catalyzed process involves two sequent steps.The nucleophilic attack of C3-amino group of substrate on the carbonyl carbon of acetyl-CoA and the departure of CoA from acetyl-CoA occur simultaneously to generate a negatively charged CoA and a positively charged intermediate,which is inconsistent with previous proposals that reaction undergoes an oxyanion tetrahedral intermediate.The sulfur of CoA subsequently functions as a proton acceptor of the intermediate to yield the final product.Although Ans84 is not essential,it is important for promoting catalysis by forming hydrogen bonding with C3-amino group,which is manifested in two aspects: positioning the lone pair of electrons of C3-amino group in ideal orientation for nucleophilic attack,and stabilizing the transition states and intermediate.The selection of initial geometries apparently affects the individual energy barriers for the reaction pathways,suggesting that a reasonable starting geometry is crucial for getting accurate results.