Nitrite reductase
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'''Nitrite reductase''' refers to any of several classes of [[enzyme]]s that catalyze the reduction of [[nitrite]]. There are two classes of NIR's. A mutlti [[haem]] enzyme reduces NO<sub>2</sub> to a variety of products. [[Copper]] containing enzymes carry out a single electron transfer to produce [[nitric oxide]]<ref>1</ref>.
==Iron Based Enzymes==
There are several types of iron based enzymes. Cytochrome cd<sub>1</sub>, or ''pseudomonas'' cytochrome oxidase contains two c and two d type [[hemes]] with two [[polypeptide]] chains. Different forms of this reductase catalyze the formation of [[nitric oxide]] or [[nitrous oxide]]<ref>2</ref>. A version of this compound was originally called [Ferrocytochrome c-551:[[oxidoreductase]]]. It was initially considered an oxidase. It catalyzes the reduction of NO<sub>2</sub> to NO. This tetraheme enzyme has two [[subunits]], each containing a c-type and a d-type heme. The reduced d hemes bind nitrite and convert it to product.<ref>3</ref>
Cytochrome C Nitrite Reductase (ccNIR) is a multiheme enzyme that converts nitrite to ammonia on each active site. The active site iron is bound to a [[protoporphyrin]] IX ring that is covalently linked to the enzyme's proteins.
===Proposed Mechanism of Cytochrome C Nitrite Reductase<ref>2</ref>===
The ccNIR protein uses six electrons and seven hydrogens to reduce nitrite to ammonia. The active site of the enzyme contains an iron in a +2 [[oxidation state]]. The oxidation level allows nitrite to bond more strongly than to the +3 state due to increased [[pi backbonding]]. This electronic effect transfers [[electron density]] into the nitrite [[antibonding orbital]] between [[nitrogen]] and [[oxygen]]. The occupation of the [[LUMO]] decreases the strength of the N-O bond. A second electronic effect is the [[hydrogen bonding]] of both oxygens to nearby [[amino acids]]. These acids are often [[Arginine]] and [[Histidine]]. The interactions lengthen the N-O bonds and facilitate cleavage of an oxygen from nitrogen.
The Fe-NO bond is linear and has six shared [[valence elctrons]]. This is not a stable state for an Fe-NO bond. However, a bent seven electron configuration is too stable to undergo further reaction without considerable energy input. To compensate for this barrier, two rapid, consecutive, single electron reductions form an eight electron complex. The electron transfer occurs before a shift in geometry from a [[linear configuration]] to [[bent geometry]].
Two [[protonation]]s of the nitrogen lead to an increased N-O bond distance. The resulting intermediate is a [[hydroxylamine]]. further protonation of the hydroxylamine leads to the breakage of the N-O bond to form water. The oxidation of iron from Fe<sup>(II)</sup> to Fe<sup>(III)</sup>, coupled with a further protonation of nitrogen leads to the release of ammonia.
==Copper Based Enzymes==
To date, there have been several types of Copper Nitrite Reductases discovered. These CuNIR are found in many different plants and bacteria; for example, the bacterial genera [[Pseudomonas]], [[Bordatella]], [[Alcaligines]], and [[Achromobacter]] all contain CuNIR.<ref>5</ref> What is common to all CuNIR is the presence of at least one type 1 copper center in the protein. These centers are similar to [[Azurin]] in their bonding structure. Each type 1 Cu is strongly bonded to a [[thiolate]] sulfur from a [[Cysteine]] and to two [[imidazole]] nitrogens from different Histidines. This induces a distorted [[trigonal planar]] [[molecular geometry]] that is stabilized by two weaker interactions with a [[Methionine]] sulfur and a [[peptide]] oxygen.<ref>2</ref>.
The Cysteine ligated to the type 1 Cu center is located directly next to a Histidine in the [[primary structure]] of the amino acids. This Histidine is bound to the Type 2 Cu center responsible for binding and reducing nitrite. This Cys-His bridge plays an important role in facilitating rapid electron transfer from the type 1 center to the type 2.<ref>6</ref>
===Proposed Mechanism of Multicopper Nitrite Reductase===
The type 2 copper center of a copper nitrite reductase is the [[active site]] of the enzyme. The Cu is bound by nitrogens of two Histidines from one [[monomer]], and bound by one Histidine from another monomer; the Cys-His bridge to the type 1 Cu. This gives the molecule a distorted tetrahedral geometry. In the resting state, the Cu is also binding a water molecule that is displaced by nitrite.<ref>7</ref>
As nitrite dispaces water, Cu is bound by both oxygens in a bidentate fashion. A nearby [[Asparagine]] residue hydrogen bonds to one of the newly formed oxygen [[ligand]]s. An incomming electron reduces the Cu from an [[oxidation state]] (II) to state (I). This change in Cu facilitates a shift in nitrite binding so that the nitrogen is bound to Cu, and one oxygen has an extended bond length due to hydrogen bonding. A second hydrogen bond forms from Histidine and leads to the cleavage of the N-O bond. The Cu is now a five [[coordination]] compound bonded to [[nitric oxide]] and water. Nitric oxide is released as Cu is oxidized to state (II) and returns to the resting configuration.<ref>8</ref>
==Assimilatory nitrate reductase==
Assimilatory nitrate reductase is an enzyme of the assimilative [[metabolism]] involved in [[Redox|reduction]] of [[nitrate]] to [[nitrite]]. The nitrite is immediately reduced to [[ammonia]] (probably via [[hydroxylamine]]) by the activity of nitrite reductase.
The term assimilatory refers to the fact that the product of the enzymatic activity remains in the organism. In this case, the product is ammonia which has an inhibitive effect on assimilatory nitrate reductase, thus ensuring that the organism produces the ammonia according to its requirements.
==See also==
*[[Nitrite oxidoreductase]]
==References ==
* Brock,T.D. et al, Biology of Microorganisms, 6th Ed. Prentice Hall
1. Atkins, Peter; Overton, Tina; Rourke, Jonathan; Weller, Mark; Armstrong, Fraser. ''Inorganic Chemistry'' 4<sup>th</sup> ed. "Biological Inorganic Chemistry" W.H. Freeman & Company; NY. 2006; pg. 755
2. Kroneck, Peter M.H.; Beuerle, Joachim; Schumacher, Wolfram. "Metal Dependent Conversion of Inorganic Nitrogen and Sulfur Compounds". Sigel, Helmut; Sigel, Astrid. Ed. ''Metal Ions in biological Systems'' Vol. 28. Marcel Dekker, Inc; NY. 1992; pg464-465
3. Payne, W.J. "Diversity in the Nitrogen Cycle". Han L. Golterman, Ed. ''Denitrification in the Nitrogen Cycle''. Plenum Press; NY. 1985; pg 56
4. Einsle, Oliver; Messerschmidt, Albrecht; Huber, Rober; Kroneck, Peter M.H.; Neese, Frank. "Mechanism of the Six-Electron Reduction of Nitrite to Ammonia by Cytochrome c Nitrite Reductase". ''J. Am. Chem. Soc.'' 2002; 124; 11737-11745.
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5. Adman, Elinor T. "Structure and Functions of Small Blue Copper Proteins". Harrison, Pauline M. Ed. ''Metalloproteins Part 1: Metal Proteins with Redox Roles''. Verlag Chemie GmbH; Weinheim. 1985; pg 5.
6. Lee, Way-Zen; Tolman, William B. "Toward Synthetic Analogs of Linked Redox and Catalytic Multimetal Sites in Proteins: A Model of the Histidine-Cysteine Bridged Dicopper Array". ''Inorg. Chem.'';(communication); 2002; 41(22); 5656-5658.
7. Suzuki, Shinnichiro; Kataoka, Kunishige; Yamaguchi, Kazuya. "Metal Coordination and Mechanism of Multicopper Nitrite Reductase". ''Acc. Chem. Res.''; 2002; 33; 728-735.
8. Sundararajan, Mahesh; Hiller, Ian H.; Burton, Neil A. "Mechanism of Nitrite Reduction at T<sub>2</sub> Cu Centers: Electronic Structure Calculations of Catalysis by Copper Nitrite Reductase and by Synthetic Model Compounds". ''J. Phys. Chem. B.'' 2007; 111; 5511-5517.
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[[Category:Oxidoreductases]]