Abstract
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.Graphical abstract
Highlights
The focus of this minireview is on multiheme c-type cytochromes, written in honor of Isabel Moura and former SBIC president José Moura (2010–2012) from the Universidade Nova de Lisboa in Portugal on the occasion of their 70th birthday
It catalyses the six-electron reduction of nitrite (NO2−) to ammonium (NH4+) (Eq 1), as part of the dissimilatory nitrate reduction to ammonium (DNRA) process, that competes with denitrification [47–57]
The six-electron reductions of nitrite to ammonia/ ammonium and sulfite to hydrogen sulfide/sulfide (Eqs. 1, 2) are fundamental to early and contemporary life. These multi-electron, multi-proton transfer processes are catalysed by a group of diverse nitrite and sulfite reductases that provide a unique prosthetic group assembly in their active centers with structural features that are key for the catalytic mechanism [39, 46, 188–192]
Summary
The focus of this minireview is on multiheme c-type cytochromes, written in honor of Isabel Moura and former SBIC president José Moura (2010–2012) from the Universidade Nova de Lisboa in Portugal on the occasion of their 70th birthday. When asking myself how I, a coordination chemist by training, got into the field of metal-dependent proteins and enzymes in aerobic and anaerobic microorganisms, clearly all my excursions into microbiology are linked, from the early experiments on sulfur respiration to cytochrome c nitrite reductase and multiheme c-type cytochromes, with Norbert Pfennig.
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