Individual 1H-NMR assignments for the heme groups and the axially bound amino acids and determination of the coordination geometry at the heme iron in a mixture of two isocytochromes c-551 from Rhodopseudomonas gelatinosa.

This paper describes chemical and physicochemical studies of two small isocytochromes c-551 (approx. 9000 dalton) from Rhodopseudomonas gelatinosa. In spite of numerous amino acid substitutions in the N-terminal half of the sequence the two isoproteins could not be separated by the procedures used, presumably because they have identical size, charge and isoelectric points. Individual assignments of the 1H-NMR lines of heme c and the axial ligands to the heme iron were therefore obtained by nuclear Overhauser enhancement measurements and saturation transfer experiments in a mixed solution of the two isocytochromes c-551. The conformation of the coordination sphere was investigated by additional 1H-NMR and circular dichroism studies. For both isoproteins the electronic structure of the heme and the chirality of the methionine attachment to the iron were found to coincide with those in Pseudomonas cytochromes c-551, i.e., S chirality was observed for the axial methionine. The Rps. gelatinosa cytochromes c-551 thus differ from mammalian, yeast, Euglena gracilis and Rhodospirillum rubrum cytochromes c, which all have R chirality at the axial methionine and concomitantly a characteristically different electronic heme structure. This is the first observation of S chirality of the axially bound methionine in a species outside the Pseudomonas family. The redox potentials of the two isocytochromes c-551 of Rps. gelatinosa differ by approx. 120 mV, and there is no cross-exchange of electrons between the two species. The two isoproteins could thus function in two different, parallel electron-transfer chains or at two different locations in a single transfer sequence.