Nitrous-oxide reductase

nitrous oxide reductase
Identifiers
EC number	1.7.2.4
CAS number	55576-44-8
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / EGO
Search PMC articles PubMed articles NCBI proteins

In enzymology, a nitrous oxide reductase also known as nitrogen:acceptor oxidoreductase (N₂O-forming) is an enzyme that catalyzes the final step in bacterial denitrification, the reduction of nitrous oxide to dinitrogen.^[1][2]

N₂O + 2 reduced cytochome c $\rightleftharpoons$ N₂ + H₂O + 2 cytochrome c

It plays a critical role in preventing release of a potent greenhouse gas into the atmosphere.

Function

N₂O is an inorganic metabolite of the prokaryotic cell during denitrification. Thus, denitrifiers comprise the principal group of N₂O producers, with roles played also by nitrifiers, methanotrophic bacteria, and fungi. Among them, only denitrifying prokaryotes have the ability to convert N₂O to N₂.^[3] Conversion of N₂O into N₂ is the last step of a complete nitrate denitrification process and is an autonomous form of respiration. N₂O is generated in the denitrifying cell by the activity of respiratory NO reductase.^[4]

Structure

Nitrous-oxide reductase is a homodimer that is located in the bacterial periplasm. X-ray structures of the enzymes from Pseudomonas nautica and Paracoccus denitrificans have revealed that each subunit (MW=65 kDa) is organized into two domains.^[5] One cupredoxin-like domain contains a binuclear copper protein known as Cu_A.

The second domain comprises a 7-bladed propeller of β-sheets that contains the catalytic site called Cu_Z, which is a tetranuclear copper-sulfide cluster.^[6] The distance between the Cu_A and Cu_Z centers within a single subunit is greater than 30Å, a distance that precludes physiologically relevant rates of intra-subunit electron transfer. However, the two subunits are orientated "head to tail" such that the Cu_A center in one subunit lies only 10 Å from the Cu_Z center in the second ensuring that pairs of redox centers in opposite subunits form the catalytically competent unit.^[7] The Cu_A center can undergo a one-electron redox change and hence has a function similar to that in the well-known aa₃-type cytochrome c oxidases (EC 1.9.3.1) where it serves to receive an electron from soluble cytochromes c.^[8]

Inhibitors

Acetylene is the most specific inhibitor of nitrous-oxide reductase.^[9] Other inhibitors include azide anion,^[10] thiocyanate, carbon monoxide, iodide, and cyanide.^[11]

References

1. Schneider, Lisa K.; Wüst, Anja; Pomowski, Anja; Zhang, Lin; Einsle, Oliver (2014). "Chapter 8. No Laughing Matter: The Unmaking of the Greenhouse Gas Dinitrogen Monoxide by Nitrous Oxide Reductase". In Peter M.H. Kroneck and Martha E. Sosa Torres. The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Metal Ions in Life Sciences 14. Springer. pp. 177–210. doi:10.1007/978-94-017-9269-1_8.
2. Berks BC, Ferguson SJ, Moir JW, Richardson DJ (December 1995). "Enzymes and associated electron transport systems that catalyse the respiratory reduction of nitrogen oxides and oxyanions". Biochim. Biophys. Acta 1232 (3): 97–173. doi:10.1016/0005-2728(95)00092-5. PMID 8534676.
3. Bothe H (2006). Biology of the Nitrogen Cycle. Elsevier Science. ISBN 978-0-444-52857-5.
4. Zumft WG (January 2005). "Nitric oxide reductases of prokaryotes with emphasis on the respiratory, heme-copper oxidase type". J. Inorg. Biochem. 99 (1): 194–215. doi:10.1016/j.jinorgbio.2004.09.024. PMID 15598502.
5. Haltia T, Brown K, Tegoni M, Cambillau C, Saraste M, Mattila K, Djinovic-Carugo K (January 2003). "Crystal structure of nitrous oxide reductase from Paracoccus denitrificans at 1.6 A resolution". Biochem. J. 369 (Pt 1): 77–88. doi:10.1042/BJ20020782. PMC 1223067. PMID 12356332.
6. Pomowski, A., Zumft, W. G., Kroneck, P. M. H., Einsle, O., "N2O binding at a [lsqb]4Cu:2S copper-sulphur cluster in nitrous oxide reductase", Nature 2011, 477, 234. doi:10.1038/nature10332
7. Rasmussen T, Brittain T, Berks BC, Watmough NJ, Thomson AJ (November 2005). "Formation of a cytochrome c-nitrous oxide reductase complex is obligatory for N2O reduction by Paracoccus pantotrophus". Dalton Trans (21): 3501–6. doi:10.1039/b501846c. PMID 16234931.
8. Hill BC (April 1993). "The sequence of electron carriers in the reaction of cytochrome c oxidase with oxygen". J. Bioenerg. Biomembr. 25 (2): 115–20. doi:10.1007/bf00762853. PMID 8389744.
9. Balderston WL, Sherr B, Payne WJ (April 1976). "Blockage by acetylene of nitrous oxide reduction in Pseudomonas perfectomarinus". Appl. Environ. Microbiol. 31 (4): 504–8. PMC 169812. PMID 1267447.
10. Matsubara, T; Mori T (Dec 1968). "Studies on denitrification. IX. Nitrous oxide, its production and reduction to nitrogen". J Biochem 64 (6): 863–71. PMID 5718551.
11. Kristjansson JK, Hollocher TC (January 1980). "First practical assay for soluble nitrous oxide reductase of denitrifying bacteria and a partial kinetic characterization". J. Biol. Chem. 255 (2): 704–7. PMID 7356639.