Loop engineering of amadoriase II and mutational cooperativity

Amadori compounds and their cross-linked products have been implicated in diabetic complications and some age-related diseases. Fructosyl amine oxidases (FAOXs) are a family of enzymes that can cleave the amadori compounds. However, the natural enzymes are only active on small substrates (fructosyl amino acids or dipeptides), which limits the therapeutic and diagnostic applications of these enzymes. In this study, amadoriase II, a member of the FAOX family from Aspergillus fumigatus was engineered to broaden its substrate range using a modified combinatorial active site saturation testing approach. The two loops at the entrance of the substrate channel were targeted. Saturation mutagenesis was carried out to search for hot-spot sites, followed by pairwise mutagenesis and subsequent combination of active mutations. Five sites on the loops were found to be critical for accessibility for two model bulky substrates, fructosyl adamantanamine and fructosyl-polylysine (3-13 lysines). Two best mutants (with three and five mutations, respectively) were obtained, with a specific activity toward the model substrates 20.6-fold and 16.8-fold that of the wild-type, respectively. Deconvolution experiments revealed the cooperativity of the mutations.