Acidity of a Cu-bound histidine in the binuclear center of cytochrome c oxidase

Fadda, Elisa, Chakrabarti, Nilmadhab and Pomès, Régis (2005) Acidity of a Cu-bound histidine in the binuclear center of cytochrome c oxidase. Journal of Physical Chemistry B, 109 (47), 22629-22640. (doi:10.1021/jp052734+).

Abstract

Cytochrome c oxidase (CcO) is a crucial enzyme in the respiratory chain. Its function is to couple the reduction of molecular oxygen, which takes place in the Fe_a3-Cu_B binuclear center, to proton translocation across the mitochondrial membrane. Although several high-resolution structures of the enzyme are known, the molecular basis of proton pumping activation and its mechanism remain to be elucidated. We examine a recently proposed scheme (J. Am. Chem. Soc. 2004, 126, 1858; FEBS Lett. 2004, 566, 126) that involves the deprotonation of the Cu_B-bound imidazole ring of a histidine (H291 in mammalian CcO) as a key element in the proton pumping mechanism. The central feature of that proposed mechanism is that the pK_a values of the imidazole vary significantly depending on the redox state of the metals in the binuclear center. We use density functional theory in combination with continuum electrostatics to calculate the pK_a values, successively in bulk water and within the protein, of the Cu-bound imidazole in various Cu- and Cu-Fe complexes. From pK_as in bulk water, we derived a value of -266.34 kcal·mol^-1 for the proton solvation free energy (ΔG_sol^H+). This estimate is in close agreement with the experimental value of -264.61 kcal·mol^-1 (J. Am. Chem. Soc. 2001, 123, 7314), which reinforces the conclusion that ΔC _sol^H+ is more negative than previous values used for pK_a calculations. Our approach, on the basis of the study of increasingly more detailed models of the CcO binuclear center at different stages of the catalysis, allows us to examine successively the effect of each of the two metals' redox states and of solvation on the acidity of imidazole, whose pK_a is approximately 14 in bulk water. This analysis leads to the following conclusions: first, the effect of Cu ligation on the imidazole acidity is negligible regardless of the redox state of the metal. Second, results obtained for Cu-Fe complexes in bulk water indicate that Cu-bound imidazole pK_a values lie within the range of 14.8-16.6 throughout binuclear redox states corresponding to the catalytic cycle, demonstrating that the effect of the Fe oxidation states is also negligible. Finally, the low-dielectric CcO proteic environment shifts the acid-base equilibrium toward a neutral imidazole, further increasing the corresponding pK_a values. These results are inconsistent with the proposed role of the Cu-bound histidine as a key element in the pumping mechanism. Limitations of continuum solvation models in pK_a calculations are discussed.

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Contributors

Author: Elisa Fadda

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