The evolution of the human mitochondrial bc1 complex- adaptation for reduced rate of superoxide production?

The mitochondrial bc1 complex is a major source of mitochondrial superoxide. While bc1-generated superoxide plays a beneficial signaling role, excess production of superoxide lead to aging and degenerative diseases. The catalytic core of bc1 comprises three peptides -cytochrome b, Fe-S protein, and cytochrome c1. All three core peptides exhibit accelerated evolution in anthropoid primates. It has been suggested that the evolution of cytochrome b in anthropoids was driven by a pressure to reduce the production of superoxide. In humans, the bc1 core peptides exhibit anthropoid-specific substitutions that are clustered near functionally critical sites that may affect the production of superoxide. Here we compare the high-resolution structures of bovine, mouse, sheep and human bc1 to identify structural changes that are associated with human-specific substitutions. Several cytochrome b substitutions in humans alter its interactions with other subunits. Most significantly, there is a cluster of seven substitutions, in cytochrome b, the Fe-S protein, and cytochrome c1 that affect the interactions between these proteins at the tether arm of the Fe-S protein and may alter the rate of ubiquinone oxidation and the rate of superoxide production. Another cluster of substitutions near heme bH and the ubiquinone reduction site, Qi, may affect the rate of ubiquinone reduction and thus alter the rate of superoxide production. These results are compatible with the hypothesis that cytochrome b in humans (and other anthropoid primates) evolve to reduce the rate of production of superoxide thus enabling the exceptional longevity and exceptional cognitive ability of humans.