Location of the iron-sulfur cluster in Spirulina platensis ferredoxin by x-ray analysis.

A chlorplast-type ferredoxin containing two non-heme iron and two labile sulfur atoms per molecule was prepared from Spirulina platensis. The protein crystallized in the orthorhombic system with cell dimension a=62.32, b=28.51, and c=108.09A. The space group is C222, and one asymmetric unit contains one molecule. The electron density maps at 5A and 3.5A resolutions were synthesized utilizing the best phase angles calculated by the single isomorphous method coupled with the anomalous dispersion method. The difference Fourier synthesis with the anomalous scattering difference of the native data showed the location of the iron atoms clearly. Comparing the location of the iron atoms with the best phase angle electron density map, it was concluded that the active center of the present molecule is close to the surface of the molecule.