Conserved Calcium-Binding Residues at the Ca-I Site Involved in Fructooligosaccharide Synthesis by 121 Inulosucrase.

Inulosucrase is an enzyme that synthesizes inulin-type β-2,1-linked fructooligosaccharides (IFOS) from sucrose. Previous studies have shown that calcium is important for the activity and stability of 121 inulosucrase (LrInu). Here, mutational analyses of four conserved calcium-binding site I (Ca-I) residues of LrInu, Asp, Gln, Asn, and Asp were performed. Alanine substitution for these residues not only reduced the stability and activity of LrInu, but also modulated the pattern of the IFOS produced. Circular dichroism spectroscopy and molecular dynamics simulation indicated that these mutations had limited impact on the overall conformation of the enzyme. One of Ca-I residues most critical for controlling LrInu-mediated polymerization of IFOS, Asp, was also subjected to mutagenesis, generating D418E, D418H, D418L, D418N, D418S, and D418W. The activity of these mutants demonstrated that the IFOS chain length could be controlled by a single mutation at the Ca-I site.