Structure-guided saturation mutagenesis of N-acetylneuraminic acid lyase for the synthesis of sialic acid mimetics

Analogues of N-acetylneuraminic acid (sialic acid, NANA, Neu5Ac), including 6-dipropylcarboxamides, have been found to be selective and potent inhibitors of influenza sialidases. Sialic acid analogues are, however, difficult to synthesize by traditional chemical methods and the enzyme N-acetylneuraminic acid lyase (NAL) has previously been used for the synthesis of a number of analogues. The activity of this enzyme towards 6-dipropylcarboxamides is, however, low. Here, we used structure-guided saturation mutagenesis to produce variants of NAL with improved activity and specificity towards 6-dipropylcarboxamides. Three residues were targeted for mutagenesis, Asp191, Glu192 and Ser208. Only substitution at position 192 produced significant improvements in activity towards the dipropylamide. One variant, E192N, showed a 49-fold improvement in catalytic efficiency towards the target analogue and a 690-fold shift in specificity from sialic acid towards the analogue. These engineering efforts provide a scaffold for the further tailoring of NAL for the synthesis of sialic acid mimetics.