Octaheme nitrite reductase: The mechanism of intramolecular electron transfer and kinetics of nitrite bioelectroreduction

Kinetic Analysis of Nitrite Reduction Reactions by Nitrite Reductase Derived from Spinach in the Presence of One-Electron Reduced Riboflavin

The development of methods for converting nitrogen oxides in water into valuable resources such as ammonia and hydrazine has been given some attention. By utilizing the nitrite-reducing catalytic activity of nitrite reductase (NiR), nitrite in water can be converted into ammonium. However, there are few reports in the research that synthesized ammonium from nitrite using nitrite reductase. Therefore, we aimed to investigate the effect of temperature on the nitrite-reducing catalytic activity of NiR from spinach in the presence of one-electron reduced riboflavin by kinetic analysis to find the optimum temperature conditions. The results of this study showed that the reaction temperature does not need to be higher than 296.15 K in order to improve the efficiency of ammonium production from nitrite using NiR.

Nature's nitrite-to-ammonia expressway, with no stop at dinitrogen

Since the characterization of cytochrome c₅₅₂ as a multiheme nitrite reductase, research on this enzyme has gained major interest. Today, it is known as pentaheme cytochrome c nitrite reductase (NrfA). Part of the NH₄⁺ produced from NO₂^- is released as NH₃ leading to nitrogen loss, similar to denitrification which generates NO, N₂O, and N₂. NH₄⁺ can also be used for assimilatory purposes, thus NrfA contributes to nitrogen retention. It catalyses the six-electron reduction of NO₂^- to NH₄⁺, hosting four His/His ligated c-type hemes for electron transfer and one structurally differentiated active site heme. Catalysis occurs at the distal side of a Fe(III) heme c proximally coordinated by lysine of a unique CXXCK motif (Sulfurospirillum deleyianum, Wolinella succinogenes) or, presumably, by the canonical histidine in Campylobacter jejeuni. Replacement of Lys by His in NrfA of W. succinogenes led to a significant loss of enzyme activity. NrfA forms homodimers as shown by high resolution X-ray crystallography, and there exist at least two distinct electron transfer systems to the enzyme. In γ-proteobacteria (Escherichia coli) NrfA is linked to the menaquinol pool in the cytoplasmic membrane through a pentaheme electron carrier (NrfB), in δ- and ε-proteobacteria (S. deleyianum, W. succinogenes), the NrfA dimer interacts with a tetraheme cytochrome c (NrfH). Both form a membrane-associated respiratory complex on the extracellular side of the cytoplasmic membrane to optimize electron transfer efficiency. This minireview traces important steps in understanding the nature of pentaheme cytochrome c nitrite reductases, and discusses their structural and functional features.