Kinetic studies on β-d-fucosidase of Littorina littorea L

1. 1. The enzyme studied shows β- d -fucosidase, β- d -glucosidase and β- d -galactosidase activities, always associated in a single peak in all the chromatographic steps. 2. 2. The enzyme shows, at low and high substrate concentrations, two different K m and V max , suggesting a substrate-activation model; the highest V max /K m values are for β- d -fucosidase activity. 3. 3. β- d -Fucosides and β- d -glucosides completely compete for a common active site in mixed-substrates experiments, while β- d -galactosides only partially compete with both glycosides. With d -fucose, glucose and galactose as inhibitors, the enzyme shows, at low substrate concentrations, coincident K i values for β- d -fucosidase and β- d -glucosidase activities, different from those for β- d -galactosidase activity. With those inhibitors, the enzyme shows a partial competitive-type inhibition. 4. 4. All the kinetic evidences suggest that this enzyme has two different active sites. 5. 5. At high substrate concentrations some activities are activated by d -fucose, glucose and galactose, probably in relation with a transglycosylation mechanism.