Molecular and kinetic properties of Aspergillus ficuum inulinases

The beta-fructosidases from the thermotolerant fungus, A. ficuum, were characterized. All were active as to inulin and sucrose hydrolysis with different specificity constants (kcat/Km). The enzymes were classified according to the ratio (alpha) of the specificity constants for inulin (I) and sucrose (S). The invertase showed an alpha value of lower than one, while the inulinases had alpha values of higher than one. The alpha ratio is proposed for the classification of beta-fructosidases into the inulinase (EC 3.2.1.7) and invertase (EC 3.2.1.26) groups. The amino acid composition, pH and temperature profiles, ultracentrifugation analysis results and effects of metal ions and effectors were studied. The data confirmed the preceding classification based on the I/S ratio, on the mode of action of inulinases (exo or endo) during inulin hydrolysis and on the molecular properties of the proteins.