Pathways of electron transfer and proton translocation in the action of superoxide dismutase dimer.

Superoxide dismutase, known to gain large rate enhancement on dimerization, forms a homodimer stabilized by hydrogen bonding between a number of internal water molecules and a few amino acid residues at the interface. Within each subunit the β-sheets provide a sequence of delocalized π-electron units of peptide bonds alternating with hydrogen bonds referred as π-H pathway. These pathways in the two subunits in the dimer are interlinked through a chain of four water molecules bridged by hydrogen bonds at the interface.

Connecting CuA with metal centers of heme a, heme a, CuB and Zn by pathways with hydrogen bond as the bridging element in cytochrome c oxidase.

Pathways formed of delocalized π-electron systems and polar groups of polypeptide chains bridged by hydrogen bonds are referred as π-H pathways. Suitable for electron transfer, these pathways in cytochrome c oxidase connect CuA, the source of electrons distributed in cytochrome c oxidase, with the metal centers, heme a, heme a, CuB, the constituents of the catalytic binuclear center. The unusually rapid electron transfer between heme a and heme a would have been facilitated by the link pathway of a long sequence of alternate peptide unit and hydrogen bond spanning Pro336-Val374, referred as suprahelix, between these hemes.

Dioxygen reduction, reduced oxygen species, oxygen toxicity and antioxidants — A commentary.

Molecular oxygen, a diradical, needs intervention of redox metal ions or other radicals to receive electrons for its reduction. The oxygen radicals thus produced are responsible for oxygen toxicity and oxidative stress. But, autoxidation, relevant in ischemia-reperfusion injury, is absent in any discussion on oxygen toxicity.