Add time:08/28/2019         Source:sciencedirect.com

Phosphate-catalyzed epimerization of N-acetyl-d-glucosamine to N-acetyl-d-mannosamine is reported in this study. The epimerization of N-acetyl-d-glucosamine was found to be facilitated by proton acceptors via the deprotonation of the amide group. Among the proton acceptors tested in this study phosphate exhibited the highest epimerization activity due to its nucleophilicity and its ability in promoting ring-opening reaction. The standard free energy change and standard enthalpy change for the epimerization of N-acetyl-d-glucosamine to N-acetyl-d-mannosamine are +2.32 kJ/mole and +1.83 kJ/mole, respectively. Based on the Arrhenius equation, the activation energy of the phosphate-catalyzed epimerization was estimated as 56.36 ± 2.89 kJ/mole. At 80 °C, the initial rate of the phosphate-catalyzed epimerization, 19.70 mM/h, was more than 47-fold that of the control. Under the optimal conditions, the phosphate-catalyzed epimerization reached equilibrium with a conversion of 0.316 within 7 h, comparable with a conversion of 0.284 after 70 min for the epimerase-catalyzed reaction. Due to its compatibility with the subsequent enzyme reaction, it is possible to conduct the phosphate-catalyzed epimerization and the lyase-catalyzed biotransformation in one single bioreactor for the quantitative transformation of N-acetyl-d-glucosamine to N-acetylneuraminic acid.

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