A model for the solution structure of horse heart ferrocytochrome c has been determined by nuclear magnetic resonance spectroscopy combined with hybrid distance geometry-simulated annealing calculations. Forty-four highly refined structures were obtained using a total of 1940 distance constraints based on the observed magnitude of nuclear Overhauser effects and 85 torsional angle restraints based on the magnitude of determined J-coupling constants. The all-residue root mean square deviation about the average structure is 0.47 +/- 0.09 A for the backbone N, C alpha, and C' atoms and 0.91 +/- 0.07 A for all heavy atoms. The overall topology of the model for solution structure is very similar to that seen in previously reported models for crystal structures of homologous c-type cytochromes. However, a detailed comparison between the model for the solution structure and the available model for the crystal structure of tuna ferrocytochrome c indicates significant differences in a number of secondary and tertiary structural features. For example, two of the three main helices display 3(10) to alpha-helical transitions resulting in bifurcation of main-chain hydrogen bond acceptor carbonyls. The N- and C-terminal helices are tightly packed and display several interhelical interactions not seen in previously reported models. The geometry of heme ligation is well-defined and completely consistent with the crystal structures of homologous cytochromes c as are the locations of four of six structural water molecules. Though the total solvent-accessible surface area of the protoporphyrin ring is similar to that seen in crystal studies of tuna ferrocytochrome c, the distribution is somewhat different. This is mainly due to a difference in packing of residues Phe-82 and Ile-81 such that Ile-81 crosses the edge of the heme in the solution structure. These and other observations help to explain a range of physical and biological data spanning the redox properties, folding, molecular recognition, and stability of the protein.