Structure of rice ferricytochrome c at 2.0 A resolution.

The crystal structure of ferricytochrome c from rice embryos has been solved by X-ray diffraction to a resolution of 2.0 A, applying a single isomorphous replacement method with anomalous scattering effects. The initial molecular model was built on a graphics display system and was refined by the Hendrickson and Konnert method. The R factor was reduced to 0.25. Rice cytochrome c consists of III amino acid residues. In comparison with animal cytochromes c, the peptide chain extends for eight residues at the N-terminal end, which is characteristic for plant cytochromes c. These additional residues display a collagen-like conformation and an irregular reverse turn, and are located around the C-terminal alpha-helix on the surface or the rear side of the molecule. Two hydrogen bonds between the carbonyl oxygen of the N-terminal acetyl group and O eta of Tyr65, and between the peptide carbonyl oxygen of Pro-1 and O epsilon 1 of Gln89, are involved in holding these eight residues on the molecular surface, where Tyr65 and Gln89 are invariant in plant cytochromes c. Except for the extra eight residues, the main-chain conformations of both rice and tuna cytochromes c are essentially identical, though small local conformational differences are found at residues 24, 25, 56 and 57.