Allosteric cooperativity in proton energy conversion in A1-type cytochrome c oxidase

Cytochrome c oxidase (CcO), the CuA, heme a, heme a3, CuB enzyme of respiratory chain, converts the free energy released by aerobic cytochrome c oxidation into a membrane electrochemical proton gradient (∆µH+). ∆µH+ derives from the membrane anisotropic arrangement of dioxygen reduction to two water molecules and transmembrane proton pumping from a negative (N) space to a positive (P) space separated by the membrane. Spectroscopic, potentiometric analyses and X-ray crystallography, characterize allosteric cooperativity of dioxygen binding and reduction with protonmotive conformational states of CcO. These studies show that allosteric cooperativity stabilizes the favourable conformational state for conversion of redox energy into a transmembrane ∆µH+. (i) Dioxygen binding to Fea3 at BNC selects a CcO state in which the conformation of the subunit I-helix X, next in sequence to the hemes a and a3 histidine axial ligands, opens access of pumped protons from the N space to a Mg2+-storage site at the opposite (P) membrane side. (ii) Sequential electron transfer at CuA and heme a is associated with additional CcO conformational change in which protons are translocated from the Mg2+-site to the propionate(s) of heme a. From there protons are expelled in the P space upon heme a oxidation by electrostatic repulsion along an hydrogen bond network of amino acid residues and/or structured water molecules.