Spectroscopic characterization and intramolecular electron transfer processes of native and type 2 Cu-depleted nitrite reductases

Abstract

 Native nitrite reductases (NIRs) containing both type 1 and 2 Cu ions and type 2 Cu-depleted (T2D) NIRs from three denitrifying bacteria (Achromobacter cycloclastes IAM 1013, Alcaligenes xylosoxidans NCIB 11015, and Alcaligenes xylosoxidans GIFU 1051) have been characterized by electronic absorption, circular dichroism, and electron paramagnetic resonance spectra. The characteristic visible absorption spectra of these NIRs are due to the type 1 Cu centers, while the type 2 Cu centers hardly contribute in the same region. The intramolecular electron transfer (ET) process from the type 1 Cu to the type 2 Cu in native NIRs has been observed as the reoxidation of the type 1 Cu(I) center by pulse radiolysis, whereas no type 1 Cu in T2D NIRs exhibits the same reoxidation. The ET process obeys first-order kinetics, and observed rate constants are 1400–1900 s^–1 (t_1/2 = ca. 0.5 ms) at pH 7.0. In the presence of nitrite, the ET process also obeys first-order kinetics, with rate constants decreased by factors of 1/12–1/2 at the same pH. The redox potential of the type 2 Cu site is estimated to be +0.24 - +0.28 V, close to that of the type 1 Cu site. Nitrate and azide ions bound to the type 2 Cu site change the redox potential. Nitrite also would shift the redox potential of the type 2 Cu by coordination, and hence the intramolecular ET rate constant is decreased. Pulse radiolysis experiments on T2D NIRs in the presence of nitrite demonstrate that the type 1 Cu(I) site is slowly oxidized with a first-order rate constant of 0.03 s^–1 at pH 7.0, suggesting that nitrite bound to the protein accepts an electron from the type 1 Cu. This result is in accord with the finding that T2D NIRs show enzymatic activities, although they are lower than those of the native enzymes.