Using site-directed mutagenesis, we investigated the roles of Ile66 and Ala107 of d-psicose 3-epimerase from Agrobacterium tumefaciens in binding O6 of its true substrate, d-fructose. When Ile66 was substituted with alanine, glycine, cysteine, leucine, phenylalanine, tryptophan, tyrosine or valine, all the mutants dramatically increased the K (m) for d-tagatose but slightly decreased the K (m) for d-fructose, indicating that Ile66 is involved in substrate recognition. When Ala107 was substituted by either isoleucine or valine, the substituted mutants had lower thermostability than the wild-type enzyme whereas the proline-substituted mutant had higher thermostability. Thus, Ala107 is involved in enzyme stability.