THE ORIENTATION OF THE IRON AXIAL LIGANDS IN THE LOW-POTENTIAL CYTOCHROME C(549) 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 ass ign proton resonances from the haem in the reduced form and correspond ing signals in the oxidised form were located by saturation transfer e xperiments with a partially oxidised sample. Further assignments of H- 1 and C-13 resonances from the harm and its axial ligands were obtaine d from NOESY, TOCSY and HMQC spectra of the ferricytochrome. Character istic signals from two His residues confirm that the haem has bis-hist idinyl axial ligation. Analysis of the paramagnetic shifts of the carb on nuclei positioned cu to the haem shows that the two imidazoles are nearly parallel and that they are oriented such that the projections o f the normals to the rings onto the haem plane lie close to a line dra wn between haem methyl 18CH(3) and thioether methine 8CH. (C) 1998 Els evier Science S.A. All rights reserved.