an enzyme-bound c-type cytochrome with a non-canonical CX2CK motif. CcmI, i.e. cytochrome c maturation system I, an apocytochrome c chaperone, is important and essential for maturation of c-type cytochromes with the non-canonical heme binding motif (HBM) CX2CK, presumably by ensuring that heme attachment at canonical HBMs occurs before apoprotein degradation. Both CcmISo-1 and CcmISo-2 are required for maturation of NrfA. The periplasmic portion of CcmI, CcmI-2, interacts with C-terminus of enzyme NrfA. Heme attachment to the apoprotein is achieved stereochemically by linking of the 2-vinyl and 4-vinyl of heme b via thioether bonds to the N-terminal and C-terminal cysteines, respectively, within heme binding motif. NrfASo is unstable unless heme attachment is timely accomplished, the failure of heme ligation in NrfASo results in rapid degradation, NrfA is rapidly degraded unless properly maturated
10 heme centres range in reduction potential from -30 to 320 mV. The heme oxidation state has a profound effect on the interactions with substrate molecules, nitrite and hydroxylamine
crystal structure of penta-heme NrfA. Four of the NrfA hemes have bis-histidine axial heme-Fe ligation. The catalytic heme-Fe (heme 1) has a lysine distal ligand and an oxygen atom proximal ligand
NrfA contains five hemes and NrfH four hemes. NrfA contains the high-spin catalytic site (-80 mV) as well as a quite unusual high reduction potential (+ 150 mV)/low-spin bis-His coordinated heme, considered to be the site where electrons enter. The four NrfH hemes are all in a low-spin state