Solution structure of cytochrome b sub(5) mutant (E44/48/56A/D60A) and its interaction with cytochrome c

Using 1617 meaningful NOEs with 188 pseudocontact shifts, a family of 35 conformers of oxidized bovine microsomal cytochrome b sub(5) mutant (E44/48/56A/D60A) has been obtained and is characterized by good resolution (rmsd to the mean structure are 0.047 plus or minus 0.007 nm and 0.095 plus or minus 0.008 nm for backbone and heavy atoms, respectively). The solution structure of the mutant, when compared with the X-ray structure of wild-type cytochrome b sub(5), has no significant changes in the whole folding and secondary structure. The binding between cytochrome b sub(5) and cytochrome c shows that the association constant of the mutant-cytochrome c complex is much lower than the one for wild-type complex (2.2 x 10 super(4) M super(-1) vs. 5.1 x 10 super(3) M super(-1)). The result suggests the four acidic residues have substantial effects on the formation of the complex between cytochrome b sub(5) and cytochrome c, and therefore it is concluded reasonably that the electrostatic interaction plays an important role in maintaining the stability and specificity of the complex formed. The competition between the ferricytochrome b sub(5) mutant and [Cr(oxalate) sub(3)] super(3-) for ferricytochrome c shows that site III of cytochrome c, which is a strong binding site to wild-type cytochrome b sub(5), still binds to the mutant with relatively weaker strength. Our results indicate that certain bonding geometries do occur in the interaction between the present mutant and cytochrome c and these geometries, which should be quite different from the ones of the Salemme and Northrup models.