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β-Glucosidase (β-D-Glucosidase, EC3.2.1.21), also known as β-D-glucosidase glucohydrolase, alias gentiobiase, cellobiase (cellobias, CB or β-G ) And amygdalase. It belongs to the cellulase class and is an important component of the cellulolytic enzyme system. It can hydrolyze the non-reducing β-D-glucose bond bound to the terminal and release β-D-glucose and the corresponding ligand at the same time.
Figure 1. Enzyme structure of β-Glucosidase.
In 1837, Liebig and Wohler first discovered β-glucosidase in bitter almond juice. The enzyme is widely distributed in nature, especially in plant seeds and microorganisms. It is also found in animals and fungi. The plant sources of β-glucosidase are ginseng, soybeans, etc.; there are many reports of microbial sources, such as Flavobacterium meningosepticum, Flavobacterium johnsonae, etc., eukaryotic sources.
According to the way different glucosidase hydrolyze the oligosaccharide substrate, it can be divided into exo-glucosidase and endo-glucosidase. Exoglucosidase refers to the glucosidase that hydrolyzes from one end (reducing or non-reducing end) of the oligosaccharide substrate, while endoglucosidase refers to the glucose that hydrolyzes from the middle part of the oligosaccharide substrate Glucidase.
Since the glucosidic bond is divided into two types: α-type and β-type, the corresponding glucosidase is called α-glucosidase and β-glucosidase, respectively.
According to the specific classification of CAZy, α-glucosidase is mainly distributed in the six families of GH4, GH13, GH31, GH63, GH97 and GH122; while β-glucosidase is mainly distributed in GH1, GH3, GH5, GH9, GH17, GH30, GH116 and other seven families.
According to the comparison of whether the configuration of the anomeric carbon in the hydrolyzed glucosyl group has changed before and after the hydrolysis, glucosidase can be divided into retention glucosidase and flip glucosidase. The former does not change the configuration of the anomeric carbon of the glucosyl group before and after the hydrolysis process; while the latter will reverse the configuration of the substrate anomeric carbon.
Most glucosidases are reserved glucosidases, and their catalytic mechanism usually follows the "two-step" mechanism.
Step 1: the carboxyl anion as a nucleophilic group nucleophilically attacks the anomeric carbon on the glycosidic bond, and at the same time, as another generalized acid-base pair, it catalyzes the formation of hydrogen bonds between the hydrogen on the carboxyl group and the oxygen atom on the glycosidic bond. The formation of Oxocarbenium-like transition state (Oxocarbenium-like transition state). After the bond is formed and broken, the anomeric carbon configuration of the glycosyl molecule is reversed for the first time and forms an ester bond with the nucleophilic carboxyl group to generate a glycosyl-enzyme covalent intermediate, and at the same time release a molecule of glycoside.
Step 2: The active hydroxyl hydrogen of the glycosyl acceptor molecule interacts with the dissociated generalized acid-base pair carboxyl ion, and the active hydroxyl oxygen of the acceptor molecule nucleophilically attacks the glycosyl molecule in the glycosyl-enzyme covalent intermediate The anomeric carbon once again forms an oxygen-containing carbocation-like transition state, and finally the configuration of the anomeric carbon is reversed for the second time and forms a covalent bond with the acceptor hydroxyl oxygen to complete the reaction.
Glucosidase is one of the important members in the carbohydrate metabolism pathway in organisms. β-glucosidase can participate in the metabolism of cellulose and a variety of physiological and biochemical pathways, and α-glucosidase is directly involved in the metabolic pathways of starch and glycogen. Abnormal functions of these enzymes can lead to metabolic diseases. At the same time, these enzymes are also targeted for various drugs and inhibitors to regulate the chemical metabolism of sugar in the human body.
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