The intramolecular electron transfer between copper sites of nitrite reductase: a comparison with ascorbate oxidase

Abstract

The intramolecular electron transfer (ET) between the type 1 Cu(I) and the type 2 Cu(II) sites of Alcaligenes xylosoxidans dissimilatory nitrite reductase (AxNiR) has been studied in order to compare it with the analogous process taking place in ascorbate oxidase (AO). This internal process is induced following reduction of the type 1 Cu(II) by radicals produced by pulse radiolysis. The reversible ET reaction proceeds with a rate constant k ET= k 1→2+ k 2→1 of 450±30 s −1 at pH 7.0 and 298 K. The equilibrium constant K was determined to be 0.7 at 298 K from which the individual rate constants for the forward and backward reactions were calculated to be: k 1→2=185±12 s −1 and k 2→1265±18 s −1. The temperature dependence of K allowed us to determine the Δ H° value of the ET equilibrium to be 12.1 kJ mol −1. Measurements of the temperature dependence of the ET process yielded the following activation parameters: forward reaction, Δ H ≠=22.7±3.4 kJ mol −1 and Δ S ≠=−126±11 J K −1 mol −1; backward reaction, Δ H ≠=10.6±1.7 kJ mol −1 and Δ S ≠=−164±15 J K −1 mol −1. X-ray crystallographic studies of NiRs suggest that the most probable ET pathway linking the two copper sites consists of Cys 136, which provides the thiolate ligand to the type 1 copper ion, and the adjacent His 135 residue with its imidazole being one of the ligands to the type 2 Cu ion. This pathway is essentially identical to that operating between the type 1 Cu(I) and the trinuclear copper centre in ascorbate oxidase, and the characteristics of the internal ET processes of these enzymes are compared. The data are consistent with the faster ET observed in nitrite reductase arising from a more advantageous entropy of activation when compared with ascorbate oxidase.