X-RAY STRUCTURE OF THE CAMBIALISTIC SUPEROXIDE-DISMUTASE FROM PROPIONIBACTERIUM-SHERMANII ACTIVE WITH FE OR MN

The first structure of a cambialistic superoxide dismutase (SOD) from Propionibacterium shermanii exhibiting similar activity with iron and with manganese was solved at a resolution of 1.6 Angstrom and 1.9 Angs trom respectively. Surprisingly, no obvious differences between the tw o SODs were observable. The protein crystallises as a home dimer in th e asymmetric unit. Because of the crystallographic symmetry, it forms a tetramer. Structures of both the manganese and the ferric form were solved using molecular replacement techniques and multiple isomorphous replacement. The tertiary structure is similar to that of the other s uperoxide dismutases, the metal being fivefold coordinated by three hi stidines, one aspartate and one water molecule. The second shell of re sidues consists of hydrophobic amino acids? histidines and two water m olecules, which are assumed to be involved in both the catalytic activ ity and structural stability of this superoxide dismutase. This shell may also be responsible for the cambialistic behaviour. This work show s that the reason for the metal specificity is not trivial, although m inor alterations in the metal environment might be responsible for thi s behaviour.