The diverse nature of carbohydrate structures and linkages entails a variety of enzymes responsible for degradation of sugars. E. coli periplasmic protein encoded by the bglX gene is assigned to glycosyl hydrolases family 3 with the predicted function of β-glucosidase. Substrate specificity tests indicated that BglX enzyme hydrolyzes glycosidic bonds in β-aryl-saccharides and demonstrates greater activity toward galactose-containing substrates compared to glucose derivatives. In addition, the enzyme is capable of hydrolyzing disaccharide lactose but not cellobiose. Site-directed mutagenesis followed by the kinetic studies demonstrated the functional importance of two amino acid residues - D111 and D287. The ability of a mutant enzyme D287G to hydrolyze o-nitrophenyl-β-D-galactopyranoside but not its glucose analog suggested that different amino acid residues may be involved in the cleavage of glucose and galactose containing substrates. E. coli cells overexpressing wt-BglX or its mutants showed different growth rates on minimal media supplemented with lactose as a carbon source. Cells that overexpressed active BglX had a growing advantage, while cells that overexpressed mutant enzymes grew significantly slower. This correlates with the inability of mutant enzymes to hydrolyze lactose. Monomeric glucose and galactose inhibited enzyme activity to a different degree in a substrate-dependent manner. Relatively low activity with glucose-containing substrates in combination with much higher activity with galactosides suggests that β-glucosidase activity is not the primary function of BglX enzyme.

Periplasmic β-glucosidase BglX from E. coli demonstrates greater activity toward galactose-containing substrates

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