BETA GALACTOSIDASE: CHARACTERISTICS, STRUCTURE, FUNCTIONS - BIOLOGY - 2025

The Beta galactosidase, β-galactosidase also called β-D-or galactohydrolase is an enzyme belonging to the family of hydrolases capable of hydrolyzing glycosyl galactosyl residues different types of molecules: polymers, oligosaccharides and secondary metabolites, among others.

Previously known as «lactase», its distribution, as well as that of the β-galactosidated oligo- and polysaccharides that serve as its substrate, is extremely wide. It is found in bacteria, fungi, and yeasts; in plants it is common in almonds, peaches, apricots and apples, and in animals it is present in organs such as the stomach and intestines.

Graphic representation of the structure of the B-galactosidase enzyme (Source: Jawahar Swaminathan and MSD staff at the European Bioinformatics Institute via Wikimedia Commons)

The most studied enzyme is that of the Lac operon of E. coli, encoded by the lacZ gene, whose studies have been key to understanding the functioning of genetic operons and many regulatory aspects of them.

It currently belongs to the group of the best-studied enzymes and its best known function is that of hydrolysis of the glycosidic bonds of lactose. It fulfills essential metabolic functions in the organisms that express it and is also used for different industrial purposes.

Industrial applications include the removal of lactose from dairy products for lactose intolerant people and the production of various galactosidate compounds. They are also used to improve the sweetness, flavor and digestion of many dairy products.

characteristics

In addition to galactosidated substrates such as lactose, most of the known β-galactosidases require divalent metal ions such as magnesium and sodium. This has been proven with the discovery of binding sites for these metals in their structure.

The β-galactosidases present in nature have a wide variety of pH ranges at which they can work. Fungal enzymes work in acidic environments (2.5 to 5.4), while yeast and bacterial enzymes work between 6 and 7 pH units.

Bacterial β-galactosidases

Bacteria have large galacto-hydrolytic enzymes compared to other galactosidases analyzed. In these organisms, the same enzyme catalyzes three types of enzymatic reactions:

- Hydrolyzes lactose to its constitutive monosaccharides: galactose and glucose.

- It catalyzes the transgalactosylation of lactose to allolactose, a disaccharide sugar that participates in the positive regulation of the expression of the genes belonging to the Lac operon, eç.

- Hydrolyzes allolactose in a similar way as it does with lactose.

Fungal β-galactosidases

Fungi possess β-galactosidase enzymes more susceptible to inhibition by galactose than enzymes belonging to other organisms. However, they are thermostable and work in acidic pH ranges.

The lactose metabolism mediated by these enzymes in fungi is divided into extracellular and cytosolic, since these organisms can use β-galactosidase to extracellularly hydrolyze lactose and introduce the products into the cells or they can take the disaccharide directly and process it internally.

Structure

Bacterial β-galactosidase enzyme is a tetrameric enzyme (of four identical subunits, AD) and each of its monomers has more than 1,000 amino acid residues, which means a molecular weight of more than 100 kDa for each and more than 400 kDa for the complexed protein.

In plants, by contrast, the enzyme is considerably smaller in size and can commonly be found as a dimer of identical subunits.

The domains of each monomer are distinguished by the numbers 1 to 5. Domain 3 has an α / β "TIM" barrel structure and has the active site at the C-terminal end of the barrel.

It is presumed that the active sites of the enzyme complex are shared between monomers, so this enzyme is biologically active only when it is complexed as a tetramer.

Its active site has the ability to bind to D-glucose and D-galactose, the two monosaccharides that make up lactose. It is particularly specific for D-galactose, but it is not so specific for glucose, so the enzyme can act on other galactosides.

Features

In animals

In the intestine of humans, the main function of this enzyme has to do with the absorption of lactose ingested with food, as it is located on the luminal side of the plasma membrane of the intestinal brush-shaped cells.

Furthermore, lysosomal isoforms of this enzyme have been shown to participate in the degradation of many glycolipids, mucopolysaccharides, and galactosidated glycoproteins, serving multiple purposes in different cellular pathways.

In plants

Plants possess β-galactosidase enzymes in leaves and seeds. These perform important functions in the catabolism of galactolipids, which are characteristic of algae and plants in general.

In these organisms, β-galactosidase participates in the processes of plant growth, fruit ripening and, in higher plants, this is the only known enzyme capable of hydrolyzing galactosyl residues from the galacosidated polysaccharides of the cell wall.

In industry and research

In the food industry related to dairy products, the enzyme β-galactosidase is used to catalyze the hydrolysis of lactose present in dairy products, which is responsible for many of the defects related to the storage of these products.

The hydrolysis of this sugar seeks to avoid the sedimentation of particles, the crystallization of frozen dairy desserts and the presence of “sandy” textures in most of the commercial derivatives of milk.

The β-galactosidase that is used industrially is commonly obtained from the fungus Aspergillus sp., Although the enzyme produced by the yeast Kluyveromyces lactis is also widely used.

The β-galactosidase activity, which is translated in scientific terms as "lactose fermentation", is routinely tested for the identification of gram negative Enterobacteriaceae present in different types of samples.

In addition, in medical terms, it is used for the production of lactose-free dairy products and for the formulation of tablets used by lactose intolerant people to digest milk and its derivatives (yogurt, cheese, ice cream, butter, creams, etc.).

It is used as a "biosensor" or "biomarker" for a variety of purposes, from immunoassays and toxicological analyzes to analysis of gene expression and diagnosis of pathologies thanks to the chemical immobilization of this enzyme on special supports.

References

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