Structure of the three-haem core of cytochrome C-551.5 determined by H-1 NMR

The trihaem cytochrome C-551.5, formerly known as cytochrome C-7, from the organism Desulfuromonas acetoxidans, has been studied in the reduced state by 2D proton NMR. The haem proton resonances were assigned, and several nuclear Overhauser enhancements (NOEs) between resonances arising from different haems were detected and assigned. The relative orientations of the three haems were calculated by fitting both the intensities of the interhaem NOEs and the magnitudes of the ring current shifts of the haem resonances, following the strategy previously used by the authors to reassess the X-ray structure of the haem core in tetrahaem cytochrome c(3) from Desulfumicrobium baculatum. It is concluded that, although the comparison of the protein sequence with those of the tetrahaem cytochromes c(3) shows that in cytochrome C-551.5 about 40% of the sequence is deleted, including the region involved in the attachment of the second of the four haems, this does not induce any significant rearrangement of the remaining three haems other than a slight decrease in the iron-iron distance between two of the haems, namely those corresponding to haems I and IV of cytochrome C-3.