Extracellular exo-inulinase has been isolated from a solid-phase culture of the filamentous fungus Aspergillus awamori var. 2250. The apparent molecular mass of the monomer enzyme was 69±1kDa, with a pI of 4.4 and a pH optimum of 4.5. The enzyme hydrolysed the β-(2 → 1)-fructan (inulin) and β-(2 → 6)-fructan (levan) via exo-cleavage, releasing fructose. The values for the Michaelis constants Km and Vmax in the hydrolysis of inulin were 0.003±0.0001mM and 175±5μmol·min−1·mg−1. The same parameters in the hydrolysis of levan were 2.08±0.04mg/ml and 1.2±0.02μmol/min per mg, respectively. The gene and cDNA encoding the A. awamori exo-inulinase were cloned and sequenced. The amino acid sequence indicated that the protein belongs to glycoside hydrolase family 32. A surprisingly high similarity was found to fructosyltransferase from Aspergillus foetidus (90.7% on the level of the amino acid sequence), despite the fact that the latter enzyme is unable to hydrolyse inulin and levan. Crystals of the native exo-inulinase were obtained and found to belong to the orthorhombic space group P212121 with cell parameters a = 64.726 Å (1Å = 0.1 nm), b = 82.041 Å and c = 136.075 Å. Crystals diffracted beyond 1.54 Å, and useful X-ray data were collected to a resolution of 1.73 Å.
Abbreviations used: Endo H, endo-β-N-acetylglucosaminidase H.
The nucleotide sequence for A. awamori inulinase was submitted to the EMBL Nucleotide Sequence Database under the accession number AJ315793.