Investigation of structural stability and enzymatic activity of glucose oxidase and its subunits

Glucose oxidase (β-d-glucose:oxygen 1-oxidoreductase, EC 1.1.3.4) catalyzes the oxidation of β-d glucose utilizing molecular oxygen as an electron acceptor to produce d-glucono-1,5-lactoneand hydrogen peroxide, which has applications in food, biotechnology and medical industries. It was known that dimer form was considered to be active and monomer form has inactive conformation. However, there are no evidences at the molecular levels for Glucose oxidase (GOx) inactivation through dissociation. Here, using molecular dynamic simulation, it has been investigated for the first time that why dimer form of the enzyme is active. We have performed a series of molecular dynamics simulations at different forms of GOx (monomer and dimer with and without FAD cofactor). The analysis of tertiary structure showed that monomer is more unstable and has more deviation from the crystal structure. In contrast, dimer has a stable conformation during simulation. These results are in good agreement with experimental data about enzyme inactivation by dissociation. It was also found that when FAD is removed from monomer, it became more unstable in comparison with monomer containing cofactor. This shows essential role of FAD in both activity and stability of the enzyme. According to the MD simulation, enzyme inactivation is associated with changing in secondary structure at the interface. Interestingly, it was found that some secondary structures are destructed while some structures are formed in monomer upon dissociation. The analysis of active site structure during simulation revealed that both dissociation and release of the FAD influence on inactivation of GOx. This study provided novel insight to understand the mechanism of enzyme inactivation upon dissociation, which would be useful for rational enzyme design.