A cytochrome C oxidase model catalyzes oxygen to water reduction under rate-limiting electron flux

Abstract

We studied the selectivity of a functional model of cytochrome c oxidase's active site that mimics the coordination environment and relative locations of Fe(a3), Cu(B), and Tyr(244). To control electron flux, we covalently attached this model and analogs lacking copper and phenol onto self-assembled monolayer-coated gold electrodes. When the electron transfer rate was made rate limiting, both copper and phenol were required to enhance selective reduction of oxygen to water. This finding supports the hypothesis that, during steady-state turnover, the primary role of these redox centers is to rapidly provide all the electrons needed to reduce oxygen by four electrons, thus preventing the release of toxic partially reduced oxygen species.

Fig. 1

(A) Crystal structure of the active site of CcO from the bovine heart (13). (B) Model 1a reproduces the key elements of the active site of CcO. (C) Model 2a, in which the phenol is masked as a methyl ether. Model 2a can be treated with dilute acid to yield the iron-only model (2b), which is not shown.

Fig. 2

ESR spectrum of the product formed by O₂ reacting with the fully reduced catalyst 1a.

Fig. 3

Slow SAM S1 functionalized with model 1a (left) and fast SAM S2 functionalized with model 1a (right).

Fig. 4

Percentage of electrons released as PROS for catalysts 1a, 2a, and 2b on either (A) SAM S1 or (B) SAM S2. Data taken at 0.1 V versus NHE in air-saturated electrolyte buffered at pH = 7. The catalyst coverage on either SAM was 4 × 10⁻¹¹ moles cm⁻². Ring-disk assembly was rotated at 300 revolutions per minute. Error bars indicate standard deviation from the mean.

Fig. 5

Possible intermediates during the reduction of oxygen to the redox level of water. (A) A monometallic heme, such as catalyst 2b, requires electrons to be delivered externally in order to reduce oxygen by four electrons. If those electrons are delivered slowly, the lifetimes of PROS-releasing intermediates increase. (B) Catalyst 1a equipped with both copper and phenol can bind oxygen and rapidly reduce it to the redox level of water without requiring external electrons. PROS release is minimized.