Using Functional Models to Understand the Four-Electron Reduction of Dioxygen by Cytochrome c Oxidase Under Steady State Conditions

Abstract

A new functional biomimetic model of cytochrome c oxidase is reported. It is synthesized in 32 steps. Single turnover reaction with oxygen leads to the same intermediates as the ones reported in the enzyme: Fe(III)O 2˙-/Cu(I)/PhOH (oxygen complex), and Fe(IV)=O/Cu(II)/PhO˙ (PM). A sister model derivatized with an alkyne was immobilized on azide-functionnalized self-assembled monolayer (SAM) coated gold electrodes by Cu(I)-catalyzed click chemistry. The rate of electron transfer from the electrode to the model was controlled by the length of the linker in the SAM. Steadystate turnover was achieved at biologically relevant electron transfer, where CuB and Tyr244 mimics rapidly deliver their electrons, hence minimizing the release of partially reduced oxygen species (PROS). Using Functional Models to Understand the Four-Electron Reduction of Dioxygen by Cytochrome c Oxidase Under Steady State ConditionsBull. Jpn. Soc. Coord. Chem. Vol.54 (2009) 4 a partially-reduced enzyme (mixed valence) where heme a and CuA are oxidized and the active site is reduced (4 available electrons). The rate at which electrons are transferred one-at-atime by cyt. c to CcO is much slower than the rates of internal electron transfer between the various redox sites in CcO (Fig. 1). Hence, during steady-state turnover (when the enzyme continuously reduces dioxygen), CcO is always starved for electrons which suggests that the mixed valence state must be involved.2 As a result, a supply of electrons from a nearby organic redox-active center becomes important for partially reduced CcO to reduce dioxygen without leaking PROS.Several studies suggested Tyr244 to be the likely source of the fourth electron 9-11 and a proton in the mixed valence enzyme. 12Mimicking the active site of cytochrome c oxidase may be important for two reasons: 1) a model is well defined, hence it could help understand the mechanism of oxygen reduction by CcO, and hence is relevant to health-related matters; 2)CcO is at the heart of the mammalian energy machinery and