The iron-containing superoxide dismutase from Propionibacterium shermanii shows, in contrast with other iron superoxide dismutases, only a minor inhibition by azide or fluoride (10-100 mM) of up to 23% at pH 7.8. The activity of the protein with Mn bound to the active site was not diminished under the same conditions. The binding constant between azide and the Fe3+ ion was determined as approx. 2 mM and for fluoride approx. 2.3 mM; they are so far comparable to those known for other iron superoxide dismutases. This seems to be a discrepancy because all other iron superoxide dismutases so far known are described as being inhibited by 50-70% by 10 mM azide. However, towards lower pH there was a drastically increased inhibition by both anions. At pH 6.8 about 80% inhibition was exhibited by azide or fluoride at a concentration of 10 mM or higher. In contrast, on increasing the pH, azide or fluoride still bound to the Fe3+ at the active site but their inhibition capacity decreased. This observation implies that both anions bind to the metal at a position that is empty at low pH, whereas at higher pH water or a negatively charged hydroxyl anion is bound. It is likely that the superoxide anion binds to the same position and has to replace the sixth ligand, leading to a diminished catalytic activity of the superoxide dismutase owing to steric and/or electrostatic inhibition of the ligand.

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The local structure of the iron site in ferric superoxide dismutase from P. shermanii was analyzed by X-ray absorption spectroscopy. The metal-ligand cluster of the enzyme is found to be similar to the crystallographically 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 metal and the histidines are about 2.10 A, between metal and aspartate they are about 1.86 A and between metal and water 1.96 A. With an increase in pH a change in the coordination number from five to six is observed both in pre-edge peak and EXAFS spectra analysis. However, the bond lengths of the ligands do not change dramatically, they are conserved for the aspartate and increase slightly to 2.13 A 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. The zinc EXAFS spectra of P. shermanii superoxide dismutase have shown that zinc can be incorporated in the active center instead of the iron.

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