Studies of the solution conformation of Spirulina platensis cytochrome c-553 by 1H-nuclear magnetic resonance and circular dichroism

Abstract

The 1H-NMR lines of heme c and the axial ligands of the heme iron in reduced cytochrome c-553 from the blue-green algae Spirulina platensis were individually assigned and the coordination geometry of the axial ligands was determined. In ferricytochrome c-553, corresponding resonance assignments were to a limited extent also obtained and the chirality of the axial methionine was determined with circular dichroism. The axial methionine chirality and the electronic structure in S. platensis cytochrome c-553 were found to be of the same type as in cytochrome c-552 from Euglena gracilis and in mammalian cytochromes c. Further studies, mainly by nuclear Overhauser enhancement experiments and by chemical determination of the amino-acid sequence for positions 1–44 in S. platensis cytochrome c-553, indicated that the spatial structure homologies between mitochondrial cytochrome c from horse, chloroplast cytochrome c-552 from E. gracilis and cyanobacterial cytochrome c-553 from S. platensis extend also to the polypeptide conformation of the heme pocket outside the first coordination sphere of the heme iron.