Location of the Heme in Horse Heart Ferricytochrome c by X-Ray Diffraction

Abstract

Abstract A low resolution (4A) electron density map of horse heart cytochrome c crystals has been obtained by x-ray diffraction methods. The molecule is a prolate spheroid, approximately 25 x 25 x 37 A. It has a cleft or crevice along one side into which the heme is inserted, normal to the surface, with only one of the edges of the porphyrin ring exposed to solvent. The ligands occupying coordination positions 5 and 6 of the heme iron extend out from either side of the cleft. The thioether links binding the vinyl side chains of the heme to cysteinyl residues in positions 14 and 17 of the amino acid sequence are visible. One of the iron ligands can be identified from its shape and location as the imidazole side chain of the histidyl residue in position 18. The other is not an imidazole side chain and is probably located in the carboxyl-terminal half of the amino acid sequence. There is little or no α-helix present, the body of the molecule being an extended chain shell around a core of packed hydrophobic side chains.