GUSB

β-glucuronidase (GUSB, EC3.2.1.31) is a glycoside hydrolase, belonging to glycosyl hydrolase family. It can catalyze the hydrolysis of various types of β-glucuronide to produce a variety of derivatives, while releasing β-glucuronic acid and the corresponding ligands. In 1934, Masamune et al. discovered GUSB in animal tissues for the first time, and thereafter detected the activity of the enzyme in bacteria, fungi, higher plants and animals. The study found that GUSB exists as a tetramer in the Enterobacteriaceae family, with a relative molecular mass of 68 kDa per unit and a monomeric peptide chain consisting of 603 amino acids.

Molecular Structure

In 1996, Jains et al. first reported on the crystal structure of human GUSB in Nature. The GUSB monomer consists of three distinct domains. The first domain is a jelly roll barrel that is highly distorted and forms a barrel-like structure with two β-hairpin structures. The hairpin loop of the jelly roll barrel is considered to be an important part of GUSB's targeting of lysosomes, so this domain is also known as the lysosomal targeting motif. The second domain is structurally similar to an immunoglobulin constant domain. The third domain is the α/β or TIM barrel domain. This domain has the active site of the enzyme and is the catalytic site, which is a feature of glycosyl hydrolases.

The TIM barrel domain contains approximately 200 amino acids and is a spatial structure consisting of 8 β-sheets and 8 α-helices alternately arranged. The structure that connects the individual β-sheets and the next α-helix is called the βα-loop. The structure that connects the individual α-helix and the next β-sheet is called the αβ-loop, which folds to form a barrel-like structure with an internal β-sheet and an external α-helix, while the βα- and αβ-loops form the catalytic surface and stability of the TIM barrel domain. In the course of biological evolution, the amino acid residues of the loop located on the catalytic surface are prone to change, and thus the catalytic surface plays a catalytic role; while the stable surface is relatively less likely to change, and serves to stabilize the basic structural function of the protein. The active site of each monomer exists at the interface of the oligomer, so the tetramer complex GUSB has four active sites.

Physiological Function

GUSB is widely found in human tissues and body fluids. It is an important enzyme involved in the metabolism of sterols and plays an important role in the metabolism of bilirubin. At the same time, GUSB belongs to lysosomal hydrolase, which is widely found in mammalian tissues, especially in liver, spleen, adrenal, intestinal mucosa and gastric mucosa. As a housekeeping enzyme, GUSB is expressed in most tissues and is actively involved in the hydrolysis of proteoglycans in lysosomes. It catalyzes he fifth step of the degradation of glycosaniboglycans (GAGs) and plays an important role in the degradation of dermatan sulfate and keratin sulfate. GUSB is also involved in cation binding and the interconversion between various metabolites such as pentose, glucuronic acid, chlorophyll, porphyrin, starch and sucrose. GUSB plays an important role in the remodeling of extracellular matrix components under physiological and inflammatory conditions. In addition, GUSB plays a significant role in the release of active or inactive complexes by glucuronides, so it can alter the activity of pro-drugs. In summary, GUSB is an important hydrolase in the human body, and the lack of GUSB can cause various metabolic abnormalities.

Applications

GUSB is one of the many hydrolyzing enzymes that metabolizes chemotherapeutic drugs and is located in lysosomes. It has been used an important research object in the antibody-directed enzyme pro-drug therapy (ADEPT). The main purpose of using GUSB in ADEPT is to prevent the body from generating an immune response. GUSB can hydrolyze glucuronide pro-drugs at the tumor site or near the tumor to release the active site and is therefore used in ADEPT to treat cancer to avoid serious adverse reactions due to chemotherapy. GUSB has played an important role in the treatment of pancreatic cancer. In addition, some GUSB pro-drugs have been developed, such as glucuronidated daunorubicin DNR-GA3, glucuronidated doxorubicin (DOX-GA3) and glucuronidated epirubicin (Epi-bus).

GUSB is widely found in human internal organs and is closely related to human metabolism. The detection method of this enzyme is relatively intuitive. Therefore, changes in the enzyme activity of GUSB in different organs can detect the occurrence of lesions in the body organs. Changes in the enzyme activity of GUSB can be detected when common bile duct stones, periodontitis, abnormal cell proliferation, or carcinogenesis, therefore can be used in disease diagnosis.

Reference

1. Wallace B D; et al. Alleviating Cancer Drug Toxicity by Inhibiting a Bacterial Enzyme. Science, 2010, 330(6005):831-835.