A PRELIMINARY-ANALYSIS OF THE 3-DIMENSIONAL STRUCTURE OF DIMERIC DI-HEME 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-HEME 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 Desulfovi brio desulfuricans ATCC 27774 and has no significant nitrate or nitrit e reductase activity, The protein received its name due its unusual sp ectral properties. It is a dimer containing two identical subunits of 26.3 kDa, each with two haem-c groups. A preliminary model for the thr ee-dimensional structure of this cytochrome was derived using the Mult iple Wavelength Anomalous Dispersion (MAD) phasing method. This model shows that SSC is indeed a dimer containing four haems at one end of t he molecule. In each monomer the two haems have their edges overlapped within van der Waals contacts with an iron-to-iron distance of 9 Angs trom. The polypeptide chain of each monomer supplies the sixth axial l igand to the haems of the other monomer, This work shows that SSC cons titutes a new class of cytochrome. The stacking of the two haems in th e monomer within van der Waals distances of each other, and also the s hort (van der Waals) distances between the two monomers in the dimeric molecule are unprecedented in hemoproteins. This particular haem arra ngement is an excellent model for the spectral study (undertaken sever al years ago) of haem-haem interaction using the aggregated haem undec apeptide derived from mammalian cytochrome c.