PMID: 30077971

Authors:

Shimada A, Hatano K, Tadehara H, Yano N, Shinzawa-Itoh K, Yamashita E, Muramoto K, Tsukihara T, Yoshikawa S

Title:

X-ray structural analyses of azide-bound cytochrome c oxidases reveal that the H-pathway is critically important for the proton-pumping activity.

Journal:

J Biol Chem. 2018 Aug 3. pii: RA118.003123. doi: 10.1074/jbc.RA118.003123.

Abstract:

Cytochrome c oxidase (CcO) is the terminal oxidase of cellular respiration, reducing O2 to water and pumping protons. X-ray structural features have suggested that CcO pumps protons via a mechanism involving electrostatic repulsions between pumping protons in the hydrogen bond network of a proton-conducting pathway (the H-pathway) and net positive charges created upon oxidation of an iron site, heme alpha (Fe alpha(2+)), for reduction of O2 at another iron site, heme alpha3 (Fe alpha3(2+)). The protons for pumping are transferred to the hydrogen bond network from the N-side via the water channel of the H-pathway. Back-leakage of protons to the N-side is thought to be blocked by closure of the water channel. To experimentally test this, we examined X-ray structures of the azide-bound, oxidized bovine CcO and found that an azide derivative (N3(-)-Fe alpha3(3+), CuB(2+)-N3(-)) induces a translational movement of the heme alpha3 plane. This was accompanied by opening of the water channel, revealing that Fe alpha3 and the H-pathway are tightly coupled. The channel opening in the oxidized state is likely to induce back-leakage of pumping protons which lowers the proton level in the hydrogen bond network during enzymatic turnover. The proton level decrease weakens the electron affinity of Fe alpha , if Fe alpha electrostatically interacts with protons in the hydrogen bond network. The previously reported azide-induced redox-potential decrease in Fe alpha supports existence of the electrostatic interaction. In summary, our results indicate that the H-pathway is critical for CcO's proton-pumping function.