NMR structure of the haem core of a novel tetrahaem cytochrome isolated from Shewanella frigidimarina: identification of the haem-specific axial ligands and order of oxidation

The tetrahaem cytochrome isolated during anaerobic growth of Shewanella fri gidimarina NCIMB400 is a small protein (86 residues) involved in electron t ransfer to Fe(III), which can be used as a terminal respiratory oxidant by this bacterium. A 3D solution structure model of the reduced form of the cy tochrome has been determined using NMR data in order to determine the relat ive orientation of the haems. The haem core architecture of S. frigidimarin a tetrahaem cytochrome differs from that found in all small tetrahaem cytoc hromes c(3) so far isolated from strict anaerobes, but has some similarity to the N-terminal cytochrome domain of flavocytochrome c(3) isolated from t he same bacterium. NMR signals obtained for the four haems of S. frigidimar ina tetrahaem cytochrome at all stages of oxidation were cross-assigned to the solution structure using the complete network of chemical exchange conn ectivities. Thus, the order in which each haem in the structure becomes oxi dised was determined. (C) 2001 Federation of European Biochemical Societies . Published by Elsevier Science B.V. All rights reserved.