A preliminary analysis of the three-dimensional structure of dimeric di-haem split-Soret cytochrome c from Desulfovibrio desulfuricans ATCC 27774 at 2.5-angstrom resolution using the MAD phasing method: a novel cytochrome fold with a stacked-haem arrangement

Haem-containing proteins are directly involved in electron transfer as well as in enzymatic functions. The ''split-Soret'' cytochrome (SSC) was isolated from the sulfate-and nitrate-reducing bacterium Desulfovibrio desulfuricans ATCC 27774 and has no significant nitrate or nitrite reductase activity, The protein received its name due its unusual spectral properties. It is a dimer containing two identical subunits of 26.3 kDa, each with two haem-c groups. A preliminary model for the three-dimensional structure of this cytochrome was derived using the Multiple Wavelength Anomalous Dispersion (MAD) phasing method. This model shows that SSC is indeed a dimer containing four haems at one end of the molecule. In each monomer the two haems have their edges overlapped within van der Waals contacts with an iron-to-iron distance of 9 Angstrom. The polypeptide chain of each monomer supplies the sixth axial ligand to the haems of the other monomer, This work shows that SSC constitutes a new class of cytochrome. The stacking of the two haems in the monomer within van der Waals distances of each other, and also the short (van der Waals) distances between the two monomers in the dimeric molecule are unprecedented in hemoproteins. This particular haem arrangement is an excellent model for the spectral study (undertaken several years ago) of haem-haem interaction using the aggregated haem undecapeptide derived from mammalian cytochrome c.