Cleavage and synthesis of sialic acids with aldolase. NMR studies of stereochemistry, kinetics, and mechanisms

1H-NMR spectroscopy was used to study cleavage and synthesis of N-acetyl- and N-glycoloyl-D-neuraminic acid by Clostridium perfringens aldolase. Whereas the alpha-anomers of Neu5Ac and Neu5Gc serve as substrate in the cleavage reaction, alpha-ManNAc and alpha-ManNGc are its primary products. The same alpha-anomers are needed by the aldolase for the synthesis of Neu5Ac and Neu5Gc. During the enzyme reaction in D2O both H-atoms at C-3 of Neu5Ac are exchanged by deuterium, H-3e reacting faster than H-3a. Rate constants and concentrations at equilibrium of reactants are temperature- and pH-dependent: The amount of Neu5Ac in equilibrium increases with decreasing temperature and increasing pH-value. Based on these results a mechanism of aldolase action is discussed.