text: The mechanism of NA has been shown to proceed with the retention of configuration which means it preserves the absolute configuration on the atom in the stereocenter. There are four steps of catalytic pathways. In the first step, the binding step, the carboxylate group changes from the axial position into the pseudo-equatorial position. The second step is the proton donation from water molecule and formation of the endocyclic sialosyl cation transition-state intermediate. Step three involves nucleophilic attack of tyrosine on the sialosyl cation. The fourth step is the formation and release of Neu5Ac. A similar mechanism has been proposed by Janakiraman et al. where the double bond of Neu5Ac2en forces the pyranose of sugar ring into a planar structure were resembled the transition-state structure.