Isolation and characterization of a 1,4-beta-D-glucan glucohydrolase from the yeast, Torulopsis wickerhamii

1,4-beta-D-Glucan glucohydrolase (exo-1,4-beta-D-glucosidase) (EC 3.2.1.74) was isolated from growth supernatants of Torulopsis wickerhamii and was subjected to hydrodynamic, optical (CD), and kinetic analysis after purification to homogeneity by ammonium sulfate precipitation, size exclusion chromatography, ion exchange chromatography, and isopycnic banding centrifugation in cesium chloride. The last step was found to separate the enzyme from strongly associating, high molecular weight polysaccharide. Enzyme homogeneity was established by isoelectric focusing, sodium dodecyl sulfate-gel electrophoresis, and analytical high performance size exclusion chromatography using dual detection. The native exo-1,4-beta-D-glucosidase was found to be a dimer of 151,000 +/- 21,100 daltons by high performance size exclusion chromatography and 143,600 +/- 1,800 daltons by sedimentation equilibrium. The enzyme has a 12% linked carbohydrate content (mostly mannose) and no essential metal ions. Hydrolysis of p-nitrophenyl-beta-D-glucopyranoside was found to be optimal at pH 4.25 and 50 degrees C. The enzyme was found to produce beta-D-glucose from cellodextrins (indicating retention of anomeric configuration during hydrolysis) and demonstrated depolymerization from the non-reducing polymer terminus. The enzyme followed competitive type inhibition with p-nitrophenyl-beta-D-glucopyranoside as substrate and demonstrated high values of Ki for D-glucose and D-cellobiose inhibition (190 and 230 mM, respectively). The exo-1,4-beta-D-glucosidase was found to hydrolyze cellotetraose more rapidly than D-cellobiose and aryl-beta-D-glycosides more rapidly than all other substrates. Low levels of activity were found for the polymeric substrates beta-glucan (yeast cell walls), Avicel, and Walseth cellulose. Although this enzyme demonstrates broad disaccharide substrate specificity, a characteristic common to beta-D-glucosidases from many sources, the ability to hydrolyze higher cellodextrins more rapidly than cellobiose renders this enzyme the first exo-1,4-beta-D-glucosidase purified from yeast.