Elimination of substrate inhibition of a β-N-acetyl-d-hexosaminidase by single site mutation

Substrate inhibition hinders chitinolytic β-N-acetyl-d-hexosaminidases in producing N-acetyl-d-glucosamine (GlcNAc), the valuable chemical widely applied in medical and food industries. Here we focused on a promising chitinolytic enzyme, OfHex1 from the insect, Ostrinia furnacalis. By structural analysis of OfHex1, five residues nearby the active pocket including V327, E328, Y471, V484 and W490 were chosen and nine mutants including V327G, E328Q, E328A, Y471V, V484R, W490A, W490H, V327G/V484R/W490A and V327G/Y471V/W490H were constructed and recombinantly expressed in Pichia pastoris. The best-performing mutant, W490A, obtained by a higher yield of 5 mg/L, did not show substrate inhibition even when 5 mM of the substrates, (GlcNAc)2–4, were applied. The kcat/Km values for (GlcNAc)2–4 are 239.8, 111.3 and 79.8 s−1 mM−1, respectively. Besides, the pH stability of the mutant ranges from pH 4 to 11 and the thermal stability is up to 50 °C. This work suggests the W490A mutant might be an ideal biocatalyst for GlcNAc production from chitin.

► Substrate inhibition hinders enzymatic preparation of GlcNAc. ► The mutant W490A by site-directed mutagenesis is devoid of substrate inhibition. ► It is with high yield of 5 mg/L of culture medium. ► It exhibits promising activity with kcat/Km value of 239.8 s−1 mM−1 for (GlcNAc)2.