The D13C variant of Bacillus schlegelii 7Fe ferredoxin is an 8Fe ferredoxin as revealed by H-1-NMR spectroscopy

The N-terminal cluster binding motif Cgs(8)XXXXXXXCys(16)....Cys(49) of Bacillus schlegelii 7Fe ferredoxin, which provides the Ligands to the [Fe3S4](+) cluster, was modified by the mutation Asp(13) --> Cys, The mutant D13C is expressed in Escherichia coli as an 8Fe ferredoxin, with NMR properties similar to those of clostridial-type ferredoxins. The full assignment of the hyperfine shifted resonances indicates that Cys(13) serves as ligand to the new fourth iron atom in the N-terminal cluster despite the atypical binding sequence CysXXXXCysXXCgs....Cys. The C alpha-C beta-Fe dihedral angles of all cysteine Ligands to the two [Fe4S4](2+) clusters of the D13C variant are similar to those observed in other 8Fe and 4Fe ferredoxins. (C) 1997 Federation of European Biochemical Societies.