Figure 4.

Overall reaction scheme for β-galactosidase. A galactoside (Gal-OR) binds to free enzyme (E_s) initially in a shallow mode (E_s•Gal-OR) which progresses to a deep mode (E_d•Gal-OR). The first chemical step (galactosylation, k₂) is shown here to begin with E_s•Gal-OR and follows through the transition state E_d•TS1 to a covalent intermediate, E_d-Gal, normally releasing the first product HOR (not shown). The intermediate is released to water (degalactosylation, k₃) through the second transition state E_d•TS2 to a product complex E_d•Gal, finally releasing the second product, galactose. In the case of allolactose production, the first product (glucose) is not released and acts as the acceptor for the degalactosylation reaction, releasing the disaccharide, allolactose.