The structure of ferrocytochrome b5 at 2.8 A resolution.

Crystals of cytochrome b5 reduced by sodium dithionite are isomorphous with the oxidized form. An electron density difference map between the two forms was calculated at 2.8 A resolution. There are no changes in main chain conformation or internal side chain orientation upon reduction. However, an ion becomes attached at the entrance of the heme crevice causing displacement of a surface lysine side chain on an adjacent molecule. The ion, identified as a cation by the nature of its coordinating ligands, appears to neutralize one of the heme propionate groups which is partially buried. It is proposed that the negatively charged propionate serves to neutralize the net formal positive charge on the heme iron in the oxidized cytochrome and that the neutralization of the heme iron upon reduction then leads to binding of a cation to the propionate.