Carbohydrate oxidases selectively oxidize glucose, galactose, and other sugars, accompanied by H₂O₂ production, oxygen consumption, or electron transfer. Different carbohydrate oxidases exhibit distinct substrate ranges and regioselectivities, acting on positions such as C1, C2, or C6 of the sugar backbone.

Keywords: carbohydrate oxidases; glucose oxidase; pyranose oxidase; galactose oxidase; FAD; hydrogen peroxide; biosensing; biocatalysis

1 Overview of Carbohydrate Oxidases

Carbohydrate oxidases are oxidoreductases that use monosaccharides, oligosaccharides, or carbohydrate derivatives as substrates and catalyze the oxidation of specific C–OH groups or hemiacetal structures. Representative enzymes include glucose oxidase (GOx), pyranose oxidase (POx/P2O), galactose oxidase (GalOx), hexose oxidase (HOx), and glucooligosaccharide oxidase.

GOx, POx, HOx, and certain oligosaccharide oxidases are flavin-dependent enzymes, whereas GalOx belongs to the copper-radical oxidase family.

1.2 Substrate Selectivity and Oxidation Sites

GOx primarily catalyzes oxidation at the C1 position of glucose. POx typically oxidizes the C2 position of various aldopyranoses, whereas GalOx mainly oxidizes the C6 primary alcohol of galactose structures. HOx acts on the reducing end of multiple hexoses, while certain oligosaccharide oxidases recognize both the reducing end and adjacent structural features of oligosaccharides. Oxidation at different positions produces lactones, ketones, or aldehydes.

2 Representative Carbohydrate Oxidases

2.1 Glucose Oxidase

(1) Glucose Recognition and C1 Oxidation

Glucose oxidase (GOx/GOD) uses FAD as its redox cofactor and exhibits high selectivity for β-D-glucose. After glucose enters the active site, its C1 position is oxidized to form D-glucono-δ-lactone, while FAD is reduced to FADH₂. FADH₂ is subsequently reoxidized by O₂, producing H₂O₂. In aqueous solution, D-glucono-δ-lactone can undergo further hydrolysis to form gluconic acid.

(2) Signal-Output Modes

The GOx reaction can be monitored through O₂ consumption or H₂O₂ production. H₂O₂ can be coupled with horseradish peroxidase (HRP) and a chromogenic or fluorogenic substrate to construct a GOD–POD assay. In electrochemical systems, electrons from the reduced enzyme may be transferred to the electrode through an O₂/H₂O₂ pathway or a redox mediator.

GOx preparations from different biological sources may vary in specific activity, stability, and retention of activity after immobilization.

2.2 Pyranose Oxidase

(1) Broad Substrate Recognition and C2-Selective Oxidation

Pyranose oxidase (POx/P2O) is an FAD-dependent oxidoreductase capable of acting on multiple aldopyranoses, including D-glucose, D-xylose, and D-galactose. Typical fungal POx enzymes primarily catalyze oxidation at the C2 position of the sugar molecule. For example, D-glucose is converted into 2-dehydro-D-glucose while O₂ is reduced to form H₂O₂.

GOx positions the C1 atom of glucose near the catalytic center, whereas POx primarily positions the C2 atom close to the flavin cofactor.

(2) Electron-Acceptor Properties

POx usually uses O₂ as the electron acceptor, reoxidizing reduced FAD and generating H₂O₂. It can also use alternative electron acceptors, including quinones and certain complexed metal ions. When an alternative electron acceptor is used, the carbohydrate substrate is still oxidized during the reductive half-reaction, but reoxidation of reduced FAD is no longer entirely dependent on O₂.

2.3 Galactose Oxidase

(1) Copper–Radical Catalytic Mechanism

Galactose oxidase (GalOx) does not use FAD as its central cofactor. Its active site contains a copper ion and a radical cofactor formed through a tyrosine–cysteine crosslink. GalOx catalyzes the two-electron oxidation of the C6 primary alcohol of D-galactose to an aldehyde while reducing O₂ to H₂O₂.

(2) Oxidation of Galactose and Carbohydrate Chains

In addition to free galactose, GalOx can oxidize certain carbohydrate chains containing accessible terminal galactose residues. The aldehyde generated through C6 primary-alcohol oxidation can undergo oxime or hydrazone formation with aminooxy- or hydrazide-containing compounds, allowing new chemical groups to be introduced at the corresponding carbohydrate residue.

2.4 Hexose Oxidase

Hexose oxidase (HOx) accepts multiple hexoses, including glucose and galactose. It primarily catalyzes oxidation at the C1 position of the reducing end to form the corresponding lactone while producing H₂O₂.

Compared with GOx, HOx generally has a broader substrate range. Consequently, several hexoses in a mixed-sugar system may contribute to the final oxidation signal.

2.5 Oligosaccharide and Other Carbohydrate Oxidases

Certain carbohydrate oxidases recognize disaccharides or oligosaccharides. Enzymes such as glucooligosaccharide oxidase primarily oxidize the reducing end of carbohydrate chains. Their substrate reactivity can be affected by chain length, glycosidic-bond type, and the residues adjacent to the reducing end. Substrate recognition therefore involves a structural environment formed by multiple sugar units.

3 Catalytic Mechanisms and Structural Basis

3.1 FAD-Dependent Oxidation Cycle

The catalytic cycle of FAD-dependent carbohydrate oxidases such as GOx and POx consists of reductive and oxidative half-reactions. During the reductive half-reaction, the carbohydrate substrate is oxidized, and FAD accepts reducing equivalents to form FADH₂. During the oxidative half-reaction, FADH₂ transfers electrons to O₂ or another electron acceptor, restoring FAD to its oxidized state. When O₂ serves as the acceptor, H₂O₂ is generally produced.

3.2 Active-Site Positioning and Regioselectivity

Sugar molecules contain several hydroxyl groups with similar chemical properties. The active site fixes the substrate in a specific conformation through hydrogen bonding, steric restriction, and other noncovalent interactions, positioning a particular reaction site close to the catalytic center.

The C1, C2, and C6 selectivities of GOx, POx, and GalOx, respectively, arise from their distinct modes of positioning the sugar backbone within the active site.

3.3 O₂ and Alternative Electron Acceptors

O₂ accepts electrons from the reduced cofactor or catalytic center, allowing the enzyme to return to its oxidized state. In carbohydrate oxidoreductases capable of using alternative electron acceptors, quinones or artificial redox mediators can accept electrons and subsequently transfer them to an electrode in electrochemical systems.

4 Key Limitations on Catalytic Performance

4.1 O₂ Supply and Enzyme Reoxidation

For oxygen-dependent enzymes such as GOx and POx, O₂ directly participates in the oxidative half-reaction. When carbohydrate substrate concentrations are high but local O₂ availability is limited, reoxidation of the reduced enzyme becomes slower.

High substrate concentrations, high enzyme loading, and thick immobilized-enzyme layers can all create O₂ concentration gradients. Alternative electron acceptors or redox mediators can reduce the dependence of certain systems on O₂ mass transfer.

4.2 H₂O₂ Accumulation and Catalase Coupling

H₂O₂ generated during carbohydrate oxidation can serve as an analytical signal, but it may also accumulate during prolonged reactions or at high substrate-conversion rates. Catalase catalyzes the following reaction:

2H₂O₂ → 2H₂O + O₂

When the oxidized carbohydrate is the desired product, catalase can consume H₂O₂ and release O₂. In analytical systems that use H₂O₂ as the reporting signal, however, catalase decreases the detectable H₂O₂ concentration.

4.3 Immobilization and Mass-Transfer Limitations

After an enzyme is immobilized on a membrane, electrode, or porous material, the carbohydrate substrate and O₂ must diffuse into the immobilized region. Carrier pore size, enzyme-layer thickness, and enzyme loading can create internal concentration gradients, causing the observed reaction rate to be controlled by both catalysis and mass transfer.

When substrate or O₂ diffusion becomes rate limiting, increasing enzyme loading does not produce a proportional increase in reaction rate.

5 Bioanalytical Applications of Carbohydrate Oxidases

5.1 Substrate Selectivity and Analytical Specificity

GOx, POx, and GalOx determine which carbohydrate structures produce a response, whereas HRP, chromogenic substrates, fluorogenic substrates, and electrodes determine how the signal is measured.

GOx primarily responds to β-D-glucose. POx can act on several aldopyranoses, including D-glucose, D-xylose, and D-galactose, whereas HOx responds to multiple hexoses. When an oxidase with broad substrate specificity is used, a single H₂O₂ signal may therefore arise from several different carbohydrate substrates.

5.2 Oxidase–Peroxidase Cascades

A GOD–POD system consists of two consecutive enzymatic reactions:

Glucose → GOx → H₂O₂ → HRP → Chromogenic or fluorescent product

GOx controls the conversion of glucose into H₂O₂, while HRP uses H₂O₂ to oxidize the reporter substrate. If HRP activity or the reporter-substrate concentration is insufficient, the downstream reaction limits the final optical signal. GalOx, POx, and other H₂O₂-producing oxidases can likewise be coupled to peroxidase-based reporting reactions.

5.3 Electrochemical Signal Transduction

Electrochemical detection using carbohydrate oxidases primarily involves three electron-transfer pathways:

Sugar → Enzyme → O₂ → H₂O₂ → Electrode

Sugar → Enzyme → Redox mediator → Electrode

Sugar → Enzyme → Electrode

O₂/H₂O₂-based systems are affected by dissolved oxygen concentrations and the potential required for H₂O₂ detection. In mediator-based systems, a redox mediator accepts electrons from the reduced enzyme and transfers them to the electrode.

Direct electron transfer requires effective electronic exchange between the enzyme’s redox center and the electrode. Its efficiency depends on the spatial location of the active center and the orientation of the enzyme on the electrode surface.

5.4 Multienzyme Cascades for Expanded Substrate Detection

Carbohydrates that cannot directly enter the target oxidase reaction can first be converted into detectable monosaccharides using a hydrolase. For example, β-galactosidase hydrolyzes lactose into glucose and galactose, which can subsequently enter GOx- or GalOx-mediated reactions. The final signal is determined jointly by the extent of upstream hydrolysis and downstream monosaccharide oxidation.

6 Selective Biocatalysis Using Carbohydrate Oxidases

6.1 Different Oxidation Sites Produce Different Products

Different carbohydrate oxidases act at distinct positions of the sugar backbone:

GOx: C1 oxidation → D-Glucono-δ-lactone

POx: C2 oxidation → 2-Keto sugar

GalOx: C6 oxidation → Terminal aldehyde

Oxidation at C1, C2, or C6 changes different functional groups and therefore produces oxidized carbohydrates with distinct chemical properties.

6.2 Formation of C2-Oxidized Sugars by POx

POx can oxidize the C2 secondary alcohol of D-glucose to form 2-dehydro-D-glucose. This reaction introduces a carbonyl group at C2 while leaving other positions of the carbohydrate backbone unoxidized under appropriate regioselective conditions. Subsequent reactions targeting the C2 carbonyl can further modify the structure or stereochemistry at this position.

6.3 Formation of Carbohydrate-Chain Aldehydes by GalOx

GalOx can oxidize the accessible C6 primary alcohol of a terminal galactose residue into an aldehyde:

Gal-C6-OH → GalOx → Gal-C6-CHO

The resulting aldehyde can be selectively coupled with aminooxy- or hydrazide-containing reagents. For glycoproteins and other glycoconjugates containing terminal galactose, the actual degree of oxidation also depends on the accessibility of the galactose residues.

6.4 Reaction Matching in Multienzyme Cascades

When a carbohydrate oxidase is coupled with other enzymes, the oxidation product from the preceding step must be recognized by the subsequent catalyst. In addition, the generated H₂O₂ must not substantially inhibit downstream enzymes. Catalase can be introduced when H₂O₂ removal is required.

7 Products

7.1 Representative Carbohydrate Oxidases

7.2 Carbohydrate Substrates and Flavin Cofactor

7.3 H₂O₂ Coupling and Peroxidase Products

7.4 H₂O₂ Removal and Cascade-Regulation Products

7.5 Carbohydrate Oxidation and Analytical Assay Kits

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For more related articles, please see below:

[1] Functional Systems of Oxidases in Microbial and Yeast Metabolic Networks

[2] Amino Acid Oxidases: Oxygen-Dependent Oxidative Deamination Systems and Their Analytical and Applied Uses

[3] Glucose Oxidase: A Versatile Biocatalyst Across Multiple Fields