Interaction of nitric oxide with a functional model of cytochrome c oxidase

Abstract

Cytochrome c oxidase (CcO) is a multimetallic enzyme that carries out the reduction of O2 to H2O and is essential to respiration, providing the energy that powers all aerobic organisms by generating heat and forming ATP. The oxygen-binding heme a(3) should be subject to fatal inhibition by chemicals that could compete with O2 binding. Near the CcO active site is another enzyme, NO synthase, which produces the gaseous hormone NO. NO can strongly bind to heme a(3), thus inhibiting respiration. However, this disaster does not occur. Using functional models for the CcO active site, we show how NO inhibition is avoided; in fact, it is found that NO can protect the respiratory enzyme from other inhibitors such as cyanide, a classic poison.

Fig. 1.

The CcO active site along with the electron-transfer heme a site and model complexes 1 and 2.

Scheme 1.

Proposed reaction mechanism of an NO-inhibited CcO model (2-NO) with O₂ and Fe-only NO-bound ferrous porphyrin (1-NO) with O₂⁻. The ligand superstructures are not included for clarity.

Fig. 2.

Absorption spectrum of the NO derivatives of 1 (A) and 2 (B) and their reactions with O₂ and O₂⁻.

Fig. 3.

EPR spectra of the NO derivatives of 1 (A) and 2 (B) and their reactions with O₂ and O₂⁻. Frequency, 9.38 MHz; power, 10 mW; gain, 5 × 10³; modulation amplitude, 10 G; temperature, 77 K.

Fig. 4.

Absorption (A) and EPR (B) spectra of CO and CN derivatives of 1 and 2 and their reactions with NO and AmN, respectively.

Scheme 2.

Proposed mechanism for recovery from inhibition via in situ NO generation from AmN. The ligand superstructure is not included for clarity.

Fig. 5.

UV-Vis spectrum of CN⁻ derivative of 1-OX and its subsequent reaction with O₂⁻.

Scheme 3.

Proposed mechanism for recovery from CN⁻ inhibition of ferric CcO. The ligand superstructure is not included for clarity.