Divalent cation induced changes in structural properties of the dimeric enzyme glucose oxidase: dual effect of dimer stabilization and dissociation with loss of cooperative interactions in enzyme monomer.

Glucose oxidase (GOD) from Aspergillus niger is a dimeric enzyme having high localization of negative charges on the enzyme surface and at the dimer interface. The monovalent cations induce compaction of the native conformation of GOD and enhance stability against thermal and urea denaturation [Ahmad et al. (2001) Biochemistry 40, 1947-1955]. In this paper we report the effect of the divalent cations Ca2+ and Mg2+ on the structural and stability properties of GOD. A divalent cation concentration dependent change in native conformation and subunit assembly of GOD was observed. Low concentration (up to 1 M) of CaCl2 or MgCl2 induced compaction of the native conformation of GOD, and the enzyme showed higher stability as compared to the native enzyme against urea denaturation. However, higher concentration (> or =2.0 M) of CaCl2 or MgCl2 induced dissociation of the native dimeric enzyme, resulting in stabilization of the enzyme monomer. An interesting observation was that the 3 M CaCl2-stabilized monomer of GOD retained about 70% secondary structure present in the native GOD dimer; however, there was a complete loss of cooperative interactions between these secondary structural elements present in the enzyme. Regarding the mechanism of divalent cation induced structural changes in GOD, the studies suggest that organization of water molecules by divalent cation results in stabilization of enzyme at low divalent cation concentration, whereas direct binding of these cations to the enzyme, at higher divalent cation concentration, results in dissociation and partial unfolding of the dimeric enzyme molecule.