D-amino acid oxidase (DAAO, DAO, EC 1.4.3.3) is a typical flavoproteinase class that uses flavin adenine (FAD) as a cofactor and can oxidize the amino groups of D-amino acids to produce corresponding ketone acids and ammonia. D-amino acid oxidase (DAAO, DAO, EC 1.4.3.3) is a typical flavin protease that uses flavin adenine (FAD) as a cofactor, oxidizing the amino groups of D-amino acids to produce corresponding ketone acids and ammonia. The catalytic reaction is as follows: The D-amino acid oxidase binds to the coenzyme FAD and undergoes a reduction reaction with the D-amino acid. The D-amino acid dehydrogenates to the corresponding sub amino acid and binds to the reducing FAD. This step is the only enzymatic reaction in the entire reaction; Under the presence of molecular oxygen, the reducing FAD is re oxidized and oxygen is converted into hydrogen peroxide; Amino acids are hydrolyzed into A-keto acids and ammonia, but when hydrogen peroxide is present in the body, A-keto acids undergo decarboxylation to produce corresponding acids. D-amino acid oxidase has high stereoisomeric selectivity and broad-spectrum selectivity for catalytic reaction substrates, and can be widely used for qualitative and quantitative analysis of D-amino acids, biosensors, and production of L-amino acids and A-ketoacids.