MALTASA: CHARACTERISTICS, SYNTHESIS AND FUNCTIONS - BIOLOGY - 2025

The maltase, also known as α-glucosidase, maltase acid, glucose invertase, glucosidosucrasa, α-glucosidase lysosomal or maltase-glucoamylase is the enzyme responsible for the hydrolysis of maltose in cells of the intestinal epithelium during the final steps of digestion of starch.

It belongs to the class of hydrolases, specifically to the subclass of glycosidases, which are capable of breaking the α-glucosidic bonds between glucose residues (EC. 3.2.1.20). This category groups several enzymes whose specificity is directed to the exo-hydrolysis of terminal glycosides linked by α-1,4 bonds.

Maltase catalyzed reaction. On the left a maltose molecule and on the right the two glucose molecules resulting from hydrolysis (Source: Dapantazis.jpg via Wikimedia Commons)

Some maltases are capable of hydrolyzing polysaccharides, but at a much slower rate. In general, after the action of maltase, α-D-glucose residues are released, however, enzymes of the same subclass can hydrolyze β-glucans, thus releasing β-D-glucose residues.

The existence of maltase enzymes was initially demonstrated in the year 1880 and it is now known that it is not only present in mammals, but also in microorganisms such as yeast and bacteria, as well as in many higher plants and cereals.

An example of the importance of the activity of these enzymes is related to Saccharomyces cerevisiae, the microorganism responsible for the production of beer and bread, which is capable of degrading maltose and maltotriose thanks to the fact that it has maltose enzymes, whose products are metabolized into products. characteristic fermentatives of this organism.

characteristics

In mammals

Maltase is an amphipathic protein associated with the membrane of intestinal brush cells. An isozyme known as acid maltase is also known, located in lysosomes and capable of hydrolyzing different types of glycosidic bonds on different substrates, not just maltose and α-1,4 bonds. Both enzymes share many structural characteristics.

The lysosomal enzyme is approximately 952 amino acids long and is post-translationally processed by glycosylation and removal of peptides at the N- and C-termini.

Studies carried out with the enzyme from the intestine of rats and pigs establish that in these animals the enzyme consists of two subunits that differ from each other in terms of some physical properties. These two subunits arise from the same polypeptide precursor that is proteolytically cleaved.

Unlike pigs and rats, the enzyme in humans does not have two subunits, but is a single, high molecular weight and highly glycosylated (by N- and O-glycosylation).

In yeasts

Yeast maltase, encoded by the MAL62 gene, weighs 68 kDa and is a cytoplasmic protein that exists as a monomer and hydrolyzes a broad spectrum of α-glucosides.

In yeast there are five isoenzymes encoded in the telomeric zones of five different chromosomes. Each coding locus of the MAL gene also comprises a gene complex of all the genes involved in maltose metabolism, including permease and regulatory proteins, as if it were an operon.

In plants

The enzyme present in plants has been shown to be sensitive to temperatures above 50 ° C and that maltase occurs in large quantities in sprouted and non-sprouted cereals.

Furthermore, during the degradation of starch, this enzyme is specific for maltose, since it does not act on other oligosaccharides, but it always ends with the formation of glucose.

Synthesis

In mammals

Human intestinal maltase is synthesized as a single polypeptide chain. Carbohydrates rich in mannose residues are co-translationally added by glycosylation, which seems to protect the sequence from proteolytic degradation.

Studies on the biogenesis of this enzyme establish that it is assembled as a high molecular weight molecule in a “membrane-bound” state of the endoplasmic reticulum, and that it is subsequently processed by pancreatic enzymes and “re-glycosylated” in the Golgi complex.

In yeasts

In yeast there are five isoenzymes encoded in the telomeric zones of five different chromosomes. Each coding locus of the MAL gene also comprises a gene complex of all genes involved in maltose metabolism, including permease and regulatory proteins.

In bacteria

The maltose metabolism system in bacteria, such as E. coli, is very similar to the lactose system, especially in the genetic organization of the operon responsible for the synthesis of regulatory, transporter, and enzyme-active proteins on the substrate (maltases).

Features

In most organisms where the presence of enzymes such as maltase has been detected, this enzyme plays the same role: the degradation of disaccharides such as maltose in order to obtain soluble carbohydrate products that are more easily metabolized.

In the intestine of mammals, maltase plays a key role in the final steps of starch degradation. Deficiencies in this enzyme are generally seen in conditions such as type II glycogenosis, which is related to the storage of glycogen.

In bacteria and yeasts, reactions catalyzed by enzymes of this type represent an important source of energy in the form of glucose that enters the glycolytic pathway, for fermentation purposes or not.

In plants, maltase, together with amylases, participates in the degradation of the endosperm in seeds that are "asleep", and which are activated by gibberellins, plant growth regulating hormones, as a prerequisite for germination.

In addition, many transient starch-producing plants during the day have specific maltases that contribute to the degradation of intermediates in their metabolism at night, and chloroplasts have been found to be the main maltose storage sites in these organisms.

References

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