Substrate-assisted catalysis in sialic acid aldolase

Sialic acid aldolase catalyses the reversible aldol condensation of pyruvat e and N-acetylman-nosamine with an apparent lack of stereospecificity. Cons istent with this, modeling of Schiff base and enamine intermediates in the active site of this enzyme yields two conformations, corresponding to si- a nd re-face attack in the aldol condensation reaction. The acceptor-aldehyde group is found on different, sides of the enamine in the two conformations , but with the remainder of the substrate having very similar geometries in the protein. No histidine residue previously speculated to function as a g eneral base in the mechanism is found near the enzyme active site. In the a bsence of functionally active groups in the active site, the carboxylate of the substrate is proposed to function as the general acid/base. Molecular orbital calculations indicate that the barrier to aldol cleavage via this m echanism in the gas phase of the related system, 4-hydroxy-2-methyiminopent anoic acid, is 74 kJ mol(-1).