Conformational component in the coupled transfer of multiple electrons andprotons in a monomeric tetraheme cytochrome

Cell metabolism relies on energy transduction usually performed by complex membrane-spanning proteins that couple different chemical processes, e.g. e lectron and proton transfer in proton-pumps. There is great interest in det ermining at the molecular level the structural details that control these e nergy transduction events, particularly those involving multiple electrons and protons, because tight control is required to avoid the production of d angerous reactive intermediates. Tetraheme cytochrome c(3) is a small solub le and monomeric protein that performs a central step in the bioenergetic, metabolism of sulfate reducing bacteria, termed "proton-thrusting," linking the oxidation of molecular hydrogen with the reduction of sulfate. The mec hanochemical coupling involved in the transfer of multiple electrons and pr otons in cytochrome c. from Desulfovibrio desulfuricans ATCC 27774 is descr ibed using results derived from the microscopic thermodynamic characterizat ion of the redox and acid-base centers involved, crystallographic studies i n the oxidized and reduced states of the cytochrome, and theoretical studie s of the redox and acid-base transitions. This proton-assisted two-electron step involves very small, localized structural changes that are sufficient to generate the complex network of functional cooperativities leading to e nergy transduction, while using molecular mechanisms distinct from those es tablished for other Desulfovibrio sp. cytochromes from the same structural family.