α-glucosidase is a glucosidase located in the brush border of the small intestine that acts upon α(1→6) bonds. It breaks down starch and disaccharides to glucose. β-glucosidase catalyzes the hydrolysis of the glycosidic bonds to terminal non-reducing residues in beta-D-glucosides and oligosaccharides, with release of glucose.
β-Glucosidases catalyze the hydrolysis of glycosidic linkages in aryl and alkyl β-glucosides and cellobiose and occur ubiquitously in plants, fungi, animals, and bacteria. Because β-glucosides and β-glucosidases are ubiquitous in the living world, one expects to find structural and catalytic properties shared by all β-glucosidases. In fact, a review of the literature indicates that almost all β-glucosidases have subunit molecular weights of 55 to 65 kD, acidic pH optima (pH 5-6), and an absolute requirement for a β-glycoside (i.e., glucoside, and to a much lesser extent fucoside and galactoside) as substrate. β-Glucosidases from widely different sources show remarkable similarity in substrate specificity for glycone (glucose) and some nonphysiological aglycones (e.g., nitrophenols and umbelliferone), although they may have widely different physiological glucosidic substrates with different aglycone moieties. In general, β-glucosidases from different orders and kingdoms appear to differ in their specificities for the aglycone (an aryl or alkyl group) linked to the glucosyl group by a β-glycosidic bond. β-Glucosidases of fungi, bacteria, humans, and dicotylodenous plants have been shown to be glycosylated, while those of monocots (maize and sorghum) are not. In plants, the β-glucosidases of dicots are localized to the cell wall or protein bodies while the β-glucosidases of monocots are localized to the plastid. In mammals (e.g., humans and mice), acid β-glucosidase is localized to the lysosome while its neutral counterpart is a soluble protein (cytosolic). One would also expect that an enzyme such as β-glucosidase that is not involved in a mainstream metabolic pathway would differ considerably in its structure and specificity from organism to organism due to divergent or convergent evolution. This would be especially true among members of different kingdoms as well as among members of the higher taxonomic groups in the same kingdom.
Human and Mammalian β-Glucosidases and β-Glucosides
Among the mammalian β-glucosidases, the human acid β-glucosidase or glucocerebrosidase has been the most studied and best characterized both at the protein and gene level mostly due to its role in Gaucher's disease. The natural substrates for the enzyme in humans are sphingosylglucosides and glucosylceramide. Accumulation of these substances in the absence of the enzyme is responsible for the three clinical Gaucher phenotypes. The enzyme also hydrolyzes a variety of artificial substrates.
Plant β-Glucosidases and Their Functions
In plants, one of the functions of β-glucosidase is thought to be in cyanogenesis, the release of HCN upon the hydrolysis of cyanogenic glucosides. Cyanogenesis has been shown to occur in over 3000 plant species belonging to 110 different families. It was shown in well-studied model systems that the enzyme (β-glucosidase) and the substrates (cyanogenic glucosides) are present in different cellular compartments. The compartmentalization has to be disrupted, as would be the case after injury to cells and tissues by herbivores and pathogens, for the enzyme and the substrate to come in contact and lead to the hydrolysis of cyanogenic glucosides and the release of HCN. This would suggest that cyanogenesis is a chemical defense response to organisms feeding on intact plant parts or attacking the plant through a site of injury. One needs both laboratory and field studies to document unequivocally the involvement of β-glucosidases and their cyanogenic glucoside substrates in deterring either general or specific herbivores and pathogens by producing HCN and other toxic products.