New insights into xanthine oxidase behavior upon heating using spectroscopy and in silico approach

• Fluorescence spectroscopy is an useful tool for conformational changes estimation.
• Two distinct species specific to all-or none transition process were identified.
• Protein structure changes were associated to molecular aggregation and dissociation.

Thermal dependent conformational changes of xanthine oxidase (XOD) were studied using sensitive and non-destructive methods like fluorescence spectroscopy and molecular modeling in the temperature range of 25–85 °C. Intrinsic fluorescence studies showed that the microenvironment of tryptophan and tyrosine residues becomes more exposed to solvent as the temperature increased up to 85 °C, whereas in case of flavin cofactor is rather conserved. At higher temperatures, the flavin adenine dinucleotide is displaced from the core of the protein, but is not fully released as shown by the Stern Volmer quenching constant and accessible fraction of the cofactor. Anyway, no significant changes in the structure of XOD monomer were identified after running molecular dynamics simulations at temperatures 25 °C, 65 °C and 85 °C. Therefore, we can conclude that the most important changes in the protein structure at thermal treatment mainly consist on molecular aggregation and dissociation events.