Protein of the Month

May 2006






Molecule of the Month: Glucose Oxidase




ExPASy Protein Spotlight


Glucose Oxidase and Biosensors

By Jennifer McDowall


            Glucose oxidase is a secreted enzyme produced predominantly by the fungi Aspergillus and Penicillium species.  It catalyses the oxidation of the sugar b-D-glucose to form D-glucono-1,5-lactone and hydrogen peroxide:


Glucose Oxidase Reaction


Glucose oxidase can oxidise b-D-glucose using other oxidising substrates besides molecular oxygen, including quinines and one-electron acceptors.  D-glucono-1,5-lactone can then hydrolyse spontaneously to produce gluconic acid.

            Glucose oxidase is a dimeric protein composed of two identical subunits.  Each subunit, or monomer, folds into two domains: one domain binds to the substrate, b-D-glucose, while the other domain binds non-covalently to a cofactor, flavin adenine dinucleotide (FAD), which it uses as a powerful oxidising agent.  FAD is a common component in biological oxidation-reduction (redox) reactions, in which there is a gain or loss of electrons from a molecule.  In glucose oxidase, FAD acts as an electron acceptor, which causes it to be reduced to FADH2; the FADH2 is then oxidised by the final electron acceptor, molecular oxygen, with the oxygen being reduced to hydrogen peroxide (H2O2).  The active site of glucose oxidase contains three important amino acids involved in catalysis: His516, and Glu412, which is hydrogen-bonded to His559. 

Glucose oxidase usually occurs as a glycoprotein, with a mannose-type carbohydrate content of around 16%.  However, different forms of the enzyme have been isolated, including a non-glycosylated enzyme from the fungus Phanerochaete chrysosporium.  Glucose oxidase is secreted by the fungus, and is distributed between the extracellular fluid surrounding the mould, the cell wall, and in the slime mucilage.  The synthesis of glucose oxidase can be induced by various substances, including molecular oxygen, which induces the transcription of the enzyme.


Glucose Oxidase, a Commercial Goldmine


            Glucose oxidase may seem like an ordinary enzyme, but it has become commercially important in the last few years, gaining a multitude of different uses in the chemical, pharmaceutical, food, beverage, and other industries.  In addition, gluconic acid, which is produced from the hydrolysis of D-glucono-1,5-lactone, has its own important industrial uses.


Next:  From Biosensor to Food Preservative