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<StructureSection load='1oah' size='350' side='left' caption='CYTOCHROME C NITRITE REDUCTASE FROM DESULFOVIBRIO DESULFURICANS ATCC 27774 (PDB entry [[1oah]])' scene=''>
The dissimilatory '''cytochrome 𝑐 nitrite reductase''' (ccNiR) from Desulfovibrio desulfuricans ATCC 27774 is able to reduce nitrite to ammonia in a six-electron transfer reaction<ref>pmid 14511372</ref> <ref>pmid 12618432</ref> <ref>pmid 8798514</ref> <ref>pmid 17207484</ref> <ref>pmid 20689707</ref> <ref>pmid 17207484</ref>:

NO2− + 8H+ + 6e− → NH4+ + 2H2O

This reaction is involved in a respiratory process that represents an important branch of the biological nitrogen cycle.

The physiological form of the enzyme is believed to be a double trimer of 2 NrfA and 1 NrfH subunits ([[2j7a)]]<ref>pmid 17139260
</ref>. The catalytic subunit NrfA is a pentaheme cytochrome c where the short distances between <scene name='CcNiR/Hemes3/5'>hemes</scene> allow a fast and efficient electron transfer; the active site<scene name='CcNiR/Heme1/2'>heme 1</scene>) has been reported as an unusual lysine-coordinated high-spin heme. NrfH is a small membrane-bound cytochrome comprising four c-type heme groups and it serves a double purpose: on one hand, it anchors the catalytic subunits to the membrane. On the other hand, it serves as a quinol oxidase, transferring electrons from the quinone pool to the catalytic subunits <ref>pmid 17139260</ref>. The overall haem arrangement of the NrfHA complex is rather unusal, comprising 28 hemes in a highly compact arrangement.




</StructureSection>    


Cytochrome c nitrite reductase is a multicenter enzyme that uses a five-coordinated heme to perform the reduction of nitrite to ammonium  in a six-electron transfer reaction. In the sulfate reducing bacterium Desulfovibrio desulfuricans ATCC 27774, the enzyme is purified as a NrfA4NrfH4 complex that houses 28 hemes. 

All but the active centre at the pentahemic subunit NrfA are hexa-coordinated c-type hemes whereas the catalytic heme is bound to a lysine as the fifth coordinate ligand and has the sixth axial position available for substrate binding. NrfA is strongly bound to its physiological electron donor, the smaller hydrophobic polypeptide tetrahemic NrfH, composed of 4 c-types hemes; in vitro, the protein complexes associate each other forming huge aggregates (min. 890 kDa).




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