Purification and characterization of an oat fructan exohydrolase that preferentially hydrolyzes beta-2,6-fructans

Oat (Avena sativa cv Fulghum) fructan hydrolase was purified by ammonium sulfate precipitation and anion-exchange, hydrophobic interaction, and size-exclusion chromatography. The enzyme was purified to homogeneity as determined by the presence of a single band (43 kD) on a silver-stained sodium dodecyl sulfate-polyacrylamide gel. A mixture of beta-2,6-linked fructan (neokestin) isolated from oat was used as the substrate to purify fructan hydrolase. Neokestin and small degree of polymerization fructan isomers were used to characterize the substrate specificity of the purified enzyme. The purified fructan hydrolase catalyzed hydrolysis of the terminal beta-2,6 linkage of 6(G),6-kestotetraose 3.5 times more rapidly than it hydrolyzed the terminal beta-2,6 linkage of 6(G)-kestotriose and approximately 10 times faster than it hydrolyzed the terminal beta-2,1 linkage of chicory inulin. Sucrose and 1-kestose were not substrates. The K-m for neokestin (beta-2,6-linked fructans with a degree of polymerization of 7-14) hydrolysis was 2.8% (w/v), and the V-max was 0.041 mu mol min(-1) mL(-1). The K-m for hydrolysis of 6(G),6-kestotetraose was 5.6% (w/v), and the V-max was 0.138 mu mol min(-1) mL(-1). Catalysis was exolytic acid by multiple chain attack. Hydrolysis of neokestin was maximal at pH 4.5 to 5.0.