Mechanism of Two-/Four-Electron Reduction of Nitroaromatics by Oxygen-Insensitive Nitroreductases: The Role of a Non-Enzymatic Reduction Step
Open Access
Abstract
Oxygen-insensitive NAD(P)H:nitroreductases (NR) reduce nitroaromatics (Ar-NO2) into hydroxylamines (Ar-NHOH) through nitroso (Ar-NO) intermediates. Ar-NO may be reduced both enzymatically and directly by reduced nicotinamide adenine dinucleotide or its phosphate NAD(P)H, however, it is unclear which process is predominant in catalysis of NRs. We found that E. coli NR-A (NfsA) oxidizes 2 mol of NADPH per mol of 2,4,6-trinitrotoluene (TNT) and 4 mol of NADPH per mol of tetryl. Addition of ascorbate, which reduces Ar-NO into Ar-NHOH, changes the stoichiometry NADPH/Ar-NO2 into 1:1 (TNT) and 2:1 (tetryl), and decreases the rate of NADPH oxidation. Ascorbate does not interfere with the oxidation of NADPH during reduction of quinones by NfsA. Our analysis of ascorbate inhibition patterns and both enzymatic and non-enzymatic reduction of nitrosobenzene suggests that direct reduction of Ar-NO by NADPH rather than enzymatic reduction is the predominant mechanism during nitroaromatic reduction.Keywords
Funding Information
- Lietuvos Valstybinis Mokslo ir Studiju Fondas (Measure No. 09.33-LMT-K-712, grant No. DOTSUT-34/09.3.3.-LMT-K712-01-0058/LSS-600000-58)
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