Magnetic interactions in milk xanthine oxidase

Area of Science:

• Biophysics
• Enzyme kinetics
• Spectroscopy

Background:

• Milk xanthine oxidase is a complex enzyme containing multiple electron-carrying prosthetic groups.
• Understanding the spatial arrangement of these groups is crucial for elucidating enzyme function and electron transfer mechanisms.

Purpose of the Study:

• To investigate magnetic interactions between the paramagnetic centers (MoV, FAD semiquinone, Fe/S I, and Fe/S II) in milk xanthine oxidase.
• To determine the distances between these electron carriers using Electron Paramagnetic Resonance (EPR) spectroscopy.

Main Methods:

• Electron Paramagnetic Resonance (EPR) spectroscopy was employed to measure relaxation behaviors of EPR signals.
• Paramagnetic states of prosthetic groups were achieved by controlling redox potentials and pH.
• Microwave power saturation studies and spectral splitting were analyzed to detect magnetic interactions.

Main Results:

• Significant magnetic interactions were observed between Mo--Fe/S I, FAD--Fe/S I, FAD--Fe/S II, and Fe/S I--Fe/S II.
• The Mo--Fe/S I interaction was also evidenced by EPR spectral splitting.
• No magnetic interactions were detected between FAD and Mo, or between Mo and Fe/S II.

Conclusions:

• The study provides estimated distances between the electron carriers: Mo--Fe/S I (11±3 Å), Fe/S I--Fe/S II (15±4 Å), FAD--Fe/S I (16±4 Å), and FAD--Fe/S II (16±4 Å).
• A model for the spatial arrangement of these prosthetic groups within the milk xanthine oxidase molecule is proposed based on the interaction data.