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The structure of ferricytochrome c' from Rhodospirillum molischianum has been
crystallographically refined to 1.67 A resolution using a combination of
reciprocal space and restrained least-squares refinement methods. The final
crystallographic R-factor for 30,533 reflections measured with I greater than
sigma (I) between infinity and 1.67 A is 0.188. The final model incorporates
1944 unique protein atoms (of a total of 1972) together with 194 bound solvent
molecules. The structure has been analysed with respect to its detailed
conformational properties, secondary structural features, temperature factor
behavior, bound solvent sites, and heme geometry. The asymmetric unit of the
cytochrome c' crystal contains a dimer composed of chemically identical
128-residue polypeptide chains. Although the refined structure shows the
monomers to be very similar, examination of the differences that do occur allows
an evaluation of how different lattice contacts affect protein conformation and
solvent binding. In particular, comparison of solvent binding sites in the two
subunits allows identification of a common set that are not altered by lattice
interactions. The preservation of these solvent interactions in different
lattice environments suggests that they play a structural role in protein
stabilization in solution. The refined structure additionally reveals some new
features that relate to the ligand binding properties and unusual mixed-spin
state character of cytochrome c'. Finally, comparison of the heme binding
geometry in cytochrome c' and other structurally unrelated c-type cytochromes
shows that two alternative, but sterically favorable, conformational variants
occur among the seven examples examined.
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