GLUCOSE OXIDASE: CHARACTERISTICS, STRUCTURE, FUNCTIONS - BIOLOGY - 2025

The glucose oxidase, also known as β-D-glucose: oxygen 1-oxidoreductase, glucose-1-oxidase or simply glucose oxidase is an oxidoreductase enzyme responsible for the oxidation of glucose β-D-producing D-gluconolactone and hydrogen peroxide.

It was discovered in the late 1920s in extracts of the fungus Aspergillus niger. Its presence has been proven in fungi and insects, where the permanent production of hydrogen peroxide, due to its catalytic action, has important functions in defense against pathogenic fungi and bacteria.

Schematic of the structure of the enzyme Glucose Oxidase (Source Arcadian, via Wikimedia Commons)

Currently, glucose oxidase has been purified from many different fungal sources, especially from the Aspergillus and Penicillium genera. Although it can employ other substrates, it is quite selective for the oxidation of β-D-glucose.

It has multiple uses in industrial and commercial contexts, which is due to its low production cost and great stability.

In this sense, this enzyme is used both in the food production industry and in cosmetology, pharmaceuticals and clinical diagnostics, not only as an additive, but also as a biosensor and / or analytical reagent for different solutions and body fluids.

characteristics

Glucose oxidase is a globular flavoprotein that uses molecular oxygen as an electron acceptor to produce, from glucose, D-glucono-δ-lactone and hydrogen peroxide.

In a cellular system, the hydrogen peroxide produced can be consumed by the enzyme catalase to produce oxygen and water. In turn, in some organisms, D-gluconolactone is hydrolyzed to gluconic acid, which can perform different functions.

The glucose oxidase enzymes described so far are capable of oxidizing monosaccharides and other classes of compounds, however, and as previously discussed, they are quite specific for the β anomer of D-glucose.

They work in acidic pH ranges, from 3.5 to 6.5, and depending on the microorganism, this range can vary considerably. Furthermore, fungal glucose oxidases are one of the three types of proteins that are bound to orthophosphates.

Like other biological catalysts, these enzymes can be inhibited by different molecules, including silver, copper and mercury ions, hydrazine and hydroxylamine, phenylhydrazine, sodium bisulfate, among others.

Structure

Glucose oxidase is a dimeric protein with two identical monomers of 80 kDa each, encoded by the same gene, covalently linked by two disulfide bridges and whose dynamism is involved in the catalytic mechanism of the enzyme.

Depending on the organism, the average molecular weight of the homodimer varies between 130 and 175 kDa and to each monomer is attached, through a non-covalent bond, a flavin adenine nucleotide (FAD), which is a coenzyme that functions as an electron transporter during catalysis..

Structure of monomers

The analysis of the monomers of different glucosas oxidases found in nature reveals that they are divided into two different regions or domains: one that binds to FAD and another that binds to glucose.

The FAD-binding domain is composed of β-folded sheets, while the glucose-binding domain consists of 4 alpha helices, which support several antiparallel β-folded sheets.

Glycosylation

The first studies carried out using the A. niger enzyme establish that this protein has 20% of its fresh weight composed of amino sugars and that another 16-19% corresponds to carbohydrates, of which more than 80% are mannose residues attached to the protein by N- or O-glycosidic bonds.

Although these carbohydrates are not essential for catalysis, there are reports that the elimination or removal of these sugary residues decreases the structural stability of the protein. This may be due to the solubility and resistance to proteases that this “layer” of carbohydrates confers on it.

Features

In fungi and insects, as discussed, glucose oxidase plays an essential defense function against pathogenic fungi and bacteria by maintaining a constant source of oxidative stress through the permanent production of hydrogen peroxide.

Talking about other general functions of the glucose oxidase enzyme is not so simple, as it has very particular utilities in the different organisms that express it. In bees, for example, its secretion from the hypopharyngeal glands into saliva contributes to the preservation of honey.

In other insects, depending on the stage of the life cycle, it works in the disinfection of ingested food and in the suppression of the defense systems of the plants (when it comes to phytophagous insects, for example).

For many fungi, this is a crucial enzyme for the formation of hydrogen peroxide that promotes the degradation of lignin. In turn, for other types of fungi it is merely an antibacterial and antifungal defense system.

Functions in the industry

In the industrial field, glucose oxidase has been exploited in many ways, among which we can specify:

- As an additive during food processing, where it works as an antioxidant, preservative and stabilizer of food products.

- In the preservation of dairy derivatives, where it works as an antimicrobial agent.

- It is used during the production of egg powder for the elimination of glucose and the production of hydrogen peroxide that prevents the growth of microorganisms.

- It is also useful in the production of low alcohol wines. This is due to its ability to consume the glucose present in the juices used for fermentation.

- Gluconic acid, one of the secondary products of the reaction catalyzed by glucose oxidase, is also exploited for the dyeing of textiles, the cleaning of metal surfaces, as a food additive, as an additive in detergents and even in drugs and cosmetics.

Glucose sensors

There are various tests to census the glucose concentration under different conditions that are based on the immobilization of the enzyme glucose oxidase on a specific support.

Three types of assays have been designed in the industry that use this enzyme as a biosensor and the differences between them are relative to the glucose and / or oxygen consumption detection system or the production of hydrogen peroxide.

In addition to their utility in the food industry, glucose biosensors are exploited for determining the amount of glucose in body fluids such as blood and urine. These are usually routine tests for the detection of pathological and other physiological conditions.

References

1. Bankar, SB, Bule, M. V, Singhal, RS, & Ananthanarayan, L. (2009). Glucose oxidase - An overview. Biotechnology Advances, 27 (4), 489–501.
2. Haouz, A., Twist, C., Zentz, C., Tauc, P., & Alpert, B. (1998). Dynamic and structural properties of glucose oxidase enzyme. Eur Biophys, 27, 19–25.
3. Raba, J., & Mottola, HA (1995). Glucose Oxidase as an Analytical Reagent. Critical Reviews in Analytical Chemistry, 25 (1), 1–42.
4. Wilson, R., & Turner, A. (1992). Glucose Oxidase: an ideal enzyme. Biosensors & Bioelectronics, 7, 165–185.
5. Wong, CM, Wong, KH, & Chen, XD (2008). Glucose oxidase: natural occurrence, function, properties and industrial applications. Appl Microbiol Biotechnol, 75, 927-938.