Evidence for a single catalytic site on the "beta-D-glucosidase-beta-D-galactosidase" of almond emulsin.

An isoenzyme of glycosidase obtained from almond emulsin, which is both a β-d-glucosidase and a β-d-galactosidase, has now been shown to possess β-D-fucosidase activity. It has been concluded that all three activities reside in a single catalytic site for the following reasons. (i) d-Glucosylamine, d-galactosylamine, and d-fucosylamine (a newly discovered potent inhibitor of this enzyme) each act competitively against all three of the substrates. (ii) Any given inhibitor exhibits the same K_i value when tested in the presence of any of the three substrates, (iii) When the enzyme is incubated with any two of the p-nitrophenyl glycoside substrates, at or above their respective K_m values, the rate of p-nitrophenol formation is not additive, but rather is equal to the value calculated on the basis of the individual K_m values and relative maximum velocities.