Structures of the FeSI and FeSII (Shethna) proteins of Azotobacter vinelandii   

B. V. Kabasakal, C. A. R. Cotton, L. Lieber, J. W. Murray

Department of Life Sciences, Imperial College London, Exhibition Road, London, SW7 2AZ, England

burak.kabasakal11@imperial.ac.uk

Nitrogen fixing Azotobacter vinelandii has two [2Fe-2S] proteins, FeSI and FeSII, Shethna protein I and II. In this study, we present the 2.11 Å, and 2.34 Å resolution X-ray crystal structures of FeSI and FeSII, respectively. FeSII has been shown as a protective enzyme for nitrogenase against oxygen-mediated inactivation. However, there is no known relation of A. vinelandii FeSI with nitrogenase, even though it is homologous to the [2Fe-2S] ferredoxin from Clostridium pasteurianum, which has been shown to interact with the nitrogenase MoFe protein. FeSI reveals a homodimer with [2Fe-2S] cluster coordinated by the side chains of surrounding conserved cysteine residues. It is highly similar to the structure of [2Fe-2S] protein from Aquifex aeolicus, including the positions of [2Fe-2S] clusters and conserved cysteine residues. On the other hand, dimeric FeSII reveals five monomers per asymmetric unit. All four copies are in an “open” conformation which may allow the [2Fe-2S] cluster to interact with nitrogenase, whereas one copy is in a “close” conformation which has a less accessible [2Fe-2S] cluster. The conformational changes may be related to oxidation and reduction states of the [2Fe-2S] cluster. These structures will provide crucial information for understanding the nitrogenase oxygen protection mechanism, and elucidating of the relations of iron-sulphur proteins in both structural and evolutionary aspects.