CRYSTAL-STRUCTURE OF THE 2[4FE-4S] FERREDOXIN FROM CHROMATIUM-VINOSUM- EVOLUTIONARY AND MECHANISTIC INFERENCES FOR [3 4FE-4S] FERREDOXINS/

The crystal structure of the 2[4Fe-4S] ferredoxin from Chromatium vino sum has been solved by molecular replacement using data recorded with synchrotron radiation. The crystals were hexagonal prisms that showed a strong tendency to develop into long tubes. The hexagonal prisms dif fracted to 2.1 Angstrom resolution at best, and a structural model for C. vinosum ferredoxin has been built with a final R of 19.2%. The N-t erminal domain coordinates the two [4Fe-4S] clusters in a fold that is almost identical to that of other known ferredoxins. However, the str ucture has two unique features. One is a six-residue insertion between two ligands of one cluster forming a two-turn external loop; this sho rt loop changes the conformation of the Cys 40 ligand compared to othe r ferredoxins and hampers the building of one NH ... S H-bond to one o f the inorganic sulfurs. The other remarkable structural element is a 3.5-turn alpha-helix at the C-terminus that covers one side of the sam e cluster and is linked to the cluster-binding domain by a six-residue external chain segment The charge distribution is highly asymmetric o ver the molecule. The structure of C. vinosum ferredoxin strongly sugg ests divergent evolution for bacterial [3/4Fe-4S] ferredoxins from a c ommon ancestral cluster-binding core. The unexpected slow intramolecul ar electron transfer rate between the clusters in C. vinosum ferredoxi n, compared to other similar proteins, may be attributed to the unusua l electronic properties of one of the clusters arising from localized changes in its vicinity rather than to a global structural rearrangeme nt.