Mechanistic Insights into Enzymatic Nitric Oxide Reduction Revealed By Surface-Enhanced Infrared Absorption Spectroscopy

Abstract

Biological nitric oxide reduction occurs in certain bacteria containing nitric oxide reductases (NORs). The NOR is a transmembrane metalloenzyme and selectively catalyzes the reduction of nitric oxide (NO) to nitrous oxide (N2O) in the microbial denitrification process. The enzymatic NO reduction is catalyzed at a binuclear active center of the non-heme FeB and the heme b 3.[1] The molecular mechanism of the enzymatic NO reduction has been extensively studied using physicochemical techniques including protein film electrochemistry.[2] However, the molecular mechanism still remains under debate. Herein, we report electrochemical and spectroscopic studies on protein film electrochemistry and surface-enhanced infrared absorption (SEIRA) spectroscopy of NOR-modified gold electrodes. The NOR was isolated from P. aeruginosa and then immobilized on SEIRA-active gold electrodes via a self-assembled monolayer of alkyl thiols. The NOR-modified gold electrode showed electrocatalytic NO reduction currents with an onset potential of ca. –0.4 V vs. SHE. Potential-dependent SEIRA spectra of the NOR-modified electrode under CO, which was used as a vibrational probe to identify the electrochemical reduction of iron cofactors, showed a characteristic band at 1972 cm–2 at –0.4 V vs. SHE. This band was associated with the CO binding to heme b 3. Thus, these results suggest that the heme b 3 should be involved in the enzymatic NO reduction.[3] SEIRA spectra of the NOR-modified electrode in the presence of NO are underway to understand the binding mode of NO to iron cofactors in the electrocatalytic condition.