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.