Effect of periodate oxidation on the structure and properties of glucose oxidase

Abstract

In order to elucidate the molecular structure of glucose oxidase (β- d-glucose: oxygen 1-oxidoreductase, EC 1.1.3.4) and the roles of its carbohydrate moiety, chemical, physiochemical and immunological experiments were performed with enzyme samples before and after periodate oxidation. Hydrodynamic parameters indicated that the native enzyme was a globular protein with values of 1.21 for the frictional ratio and 43 Å for the Stokes radius. The enzyme contained about 12% carbohydrate by weight, of which the main component was mannose. The periodate treatment decreased the carbohydrate content to about 40% of its original value. Slight modifications were detected in the absorbance spectrum and the content of arginyl residue. However, no significant alteration was brought about by this treatment in the catalytic parameters, immunological reactivities or the gross structure, nor in the secondary and quaternary structures of the protein moiety. Thermal denaturation temperature (about 72.5°C) and the enthalpy of denaturation (about 450 kcal/mol) were common to the native and the periodate-oxidized enzymes. The native enzyme was found to be quite resistant to sodium dodecyl sulfate and fairly stable to urea and heating. The periodate-oxidized enzyme was also stable to heat treatment, but it showed a diminished stability when denaturing agents were present. Kinetic analyses of the thermal inactivation processes showed that the entropy of activation was greatly decreased by the denaturing agents, especially in the case of the periodate-oxidized enzyme. It is concluded that the carbohydrate moiety of the enzyme plays a role in increasing the stability of the protein moiety, but does not directly participate in the catalytic activity, in the immunological reactivity, or in maintaining the conformation of the enzyme protein.