β-Galactosidases: A great tool for synthesizing galactose-containing carbohydrates

β-Galactosidases, an important class of glycosidases, naturally catalyze the hydrolysis of β-galactosidic bonds in oligosaccharides and polysaccharides. Traditionally, these enzymes have been used to degrade lactose in dairy products, which are beneficial for lactose-intolerant people. Attractively, β-galactosidases exhibit glycosyl transfer activity under certain conditions in vitro. They are capable of synthesizing carbohydrates from cheap starting substrates in a facile, efficient, and environment-friendly way. The condensation of lactose into the well-known prebiotic galacto-oligosaccharides by β-galactosidases has become a key aspect of the industrial interest in the synthetic activity in recent years. At present, the transglycosylation activity of these enzymes has been greatly extended. It can be used not only in building glycan blocks of crucial glycoconjugates to elucidate their biological functions, but also in glycosylation of vital molecules, which have been applied in food, medicine and cosmetic industries to improve solubility, stability and bioactivity. Further molecular engineering of β-galactosidases has significantly improved their synthetic activity, expanded the substrate spectrum and made them more powerful in carbohydrate synthesis. This review covers the classification, structure and mechanism of β-galactosidases, galactosylation reactions catalyzed by these enzymes, and various strategies of enzyme engineering, with an emphasis on recent advances.