Electrocatalytic Nitrite Reduction by a Monomeric NrfA: Commonality in Ammonification Mechanisms.

Area of Science:

• Biochemistry
• Enzymology
• Nitrogen Cycle

Background:

• Cytochrome c nitrite reductase (NrfA) is a pentaheme enzyme crucial for the nitrogen cycle.
• NrfA catalyzes the six-electron reduction of nitrite to ammonia.
• Previously, NrfA was thought to be homodimeric, but recent studies show Geobacter lovleyi (Gl) NrfA is monomeric in solution.

Purpose of the Study:

• To investigate the catalytic properties of monomeric Gl NrfA using protein film electrochemistry.
• To determine if monomeric NrfA exhibits features previously associated with dimeric NrfA enzymes.
• To elucidate the roles of different heme environments in the electron transfer pathways of NrfA.

Main Methods:

• Protein film electrochemistry was employed to study Gl NrfA.
• Site-directed mutagenesis was used to create His to Met heme ligand mutants in distinct electron transfer pathways.

Main Results:

• Monomeric Gl NrfA displayed catalytic activity characteristics previously attributed to dimeric enzymes.
• Mutational analysis indicated that both branches of the electron transfer network are essential for native-like catalysis.
• The results challenge the long-held assumption of NrfA homodimeric structure.

Conclusions:

• The monomeric structure of Gl NrfA is sufficient to support complex catalytic features.
• Specific heme environments and their associated electron transfer pathways are critical for NrfA function.
• This study refines our understanding of NrfA structure-function relationships and its role in the nitrogen cycle.