Overproduction of CMP-sialic acid synthetase for organic synthesis

The gene coding for Escherichia coli CMP-sialic acid synthetase (E.C. 2.7.7.43) was cloned and overexpressed in E. coli through a primer-directed polymerase chain reaction. Two plasmids were constructed to produce the native enzyme and a modified enzyme fused with a decapeptide at the C-terminus. The decapeptide tag was used for detection of the enzyme production. Both enzymes produced from E. coli were isolated and purified with an Orange A dye resin and FPLC. Their properties were compared with respect to their kinetic parameters, stability, pH profiles, and substrate specificities. Both enzymes have similar k(cat) and K(m) for NeuAc and CTP but different pH profiles. Contrary to the native enzyme, the modified enzyme is more active at higher pH. Studies on specificity indicate that both enzymes have a high specific activity for C-9 modified NeuAc derivatives at neutral pH. Some C-5 modified (hydroxy, deoxy, and deoxyfluoro) NeuAc derivatives are not acceptable as substrates. The modified enzyme has been used in the synthesis of CMP-NeuAc from ManNAc and CMP and sialyl N-acetyllactosamine (Neu-alpha-2,6Gal-beta-1,4GlcNAc) with in situ generation of NeuAc and regeneration of CMP-NeuAc. The 6-O-acyl derivatives of ManNAc were prepared via transesterification in anhydrous dimethylformamide by using an engineered stable subtilisin variant as a catalyst, and the products were used as substrates in sialic acid aldolase-catalyzed synthesis of 9-O-acyl-NeuAc derivatives.