EXAFS INVESTIGATION OF THE ACTIVE-SITE OF IRON SUPEROXIDE-DISMUTASE OF ESCHERICHIA-COLI AND PROPIONIBACTERIUM-SHERMANII

The local structure of the iron site in ferric superoxide dismutase fr om P. shermanii was analyzed by X-ray absorption spectroscopy. The met al-ligand cluster of the enzyme is found to be similar to the crystall ographically investigated ferric superoxide dismutase from E. coli. At pH 6.4 the enzyme is five-fold coordinated with three histidines, an aspartate and a water molecule. The average bond lengths between the m etal and the histidines are about 2.10 Angstrom, between metal and asp artate they are about 1.86 Angstrom and between metal and water 1.96 A ngstrom. With an increase in pH a change in the coordination number fr om five to six is observed both in pre-edge peak and EXAFS spectra ana lysis. However, the bond lengths of the ligands do not change dramatic ally, they are conserved for the aspartate and increase slightly to 2. 13 Angstrom for the average metal - histidine distance at pH 9.3. The observation of the increase in coordination number is correlated with a decrease in enzymatic activity which occurs in the high pH range. Th e zinc EXAFS spectra of P. shermanii superoxide dismutase have shown t hat zinc can be incorporated in the active center instead of the iron.