The orientation of the iron axial ligands in the low-potential cytochrome c549 from Synechocystis sp. PCC 6803 studied by NMR

NMR spectra of ferro-and ferricytochrome c(549) from Synechocystis sp. PCC 6803 have been analysed. Two-dimensional spectra were used to assign proton resonances from the haem in the reduced form and corresponding signals in the oxidised form were located by saturation transfer experiments with a partially oxidised sample. Further assignments of H-1 and C-13 resonances from the harm and its axial ligands were obtained from NOESY, TOCSY and HMQC spectra of the ferricytochrome. Characteristic signals from two His residues confirm that the haem has bis-histidinyl axial ligation. Analysis of the paramagnetic shifts of the carbon nuclei positioned cu to the haem shows that the two imidazoles are nearly parallel and that they are oriented such that the projections of the normals to the rings onto the haem plane lie close to a line drawn between haem methyl 18CH(3) and thioether methine 8CH. (C) 1998 Elsevier Science S.A. All rights reserved.