THE STRUCTURE OF THE AZIDE COORDINATED SUPEROXIDE-DISMUTASE OF PROPIONIBACTERIUM-SHERMANII INVESTIGATED BY X-RAY STRUCTURE-ANALYSIS, EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE, MOSSBAUER AND ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY

Azide is bound to the cambialistic Fe-SOD (SOD = superoxide dismutase) of Propionibacterium shermanii in the range pH 6 up to pH 9.5. We des cribe experiments at pH 6.15 and pH 7.8. At low ionic strength all the iron centres are complexed by azide. The symmetry of the coordination sphere of the iron is hexacoordinated approximately octahedrally. The electron paramagnetic resonance (EPR) and Mossbauer spectra can be de scribed within this symmetry by a consistent set of crystal field para meters of only one iron species. The models of the azide coordinated S OD from extended X-ray absorption fine structure (EXAFS) at high pH an d the X-ray structure analysis at low pH are compatible. At pH 6.15 th e SOD is pentacoordinated. If azide is added it occupies the sixth coo rdination site. At pH 7.8 the SOD is partly hexacoordinated with an H2 O or an OH-. Azide replaces this solvent molecule and occupies this po sition in addition in the pentacoordinated molecules. The decrease in the activity is explained by a competitive inhibition by the molecule at the sixth coordination site. This work shows that in the presence o f azide the central iron of the SOD is hexacoordinated in the entire p H range. (C) 1998 Elsevier Science S.A. All rights reserved.