engineering

Nitric-oxide reductase

Nitric-oxide reductase is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Nitric-oxide reductase rather than just read about it. In short: Nitric oxide reductase, an enzyme, catalyzes the reduction of nitric oxide (NO) to nitrous oxide (N2O). The enzyme participates in nitrogen metabolism and in the microbial defense against nitric oxide toxicity.

Key takeaways

Nitric-oxide reductase belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Nitric-oxide reductase to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Nitric-oxide reductase from memory before moving on to harder problems.

Reference excerpt

Nitric oxide reductase, an enzyme, catalyzes the reduction of nitric oxide (NO) to nitrous oxide (N2O). The enzyme participates in nitrogen metabolism and in the microbial defense against nitric oxide toxicity. The catalyzed reaction may be dependent on different participating small molecules: Cytochrome c (EC: 1.7.2.5, Nitric oxide reductase (cytochrome c)), NADPH (EC:1.7.1.14), or Menaquinone (EC:1.7.5.2).

Nomenclature Nitric oxide reductase was assigned Enzyme Commission number (EC) 1.7.2.5. Enzyme Commission numbers are the standard naming system used for enzymes. The EC identifies the class, subclass, sub-subclass, and serial number of the enzyme. Nitric oxide reductase is in Class 1, therefore it is an oxidoreductases. Nitric oxide reductase belongs to the family of oxidoreductases, specifically those acting on other nitrogenous compounds as donors with other acceptors. The systematic name of this enzyme class is nitrous-oxide:acceptor oxidoreductase (NO-forming). Other names in common use include nitrogen oxide reductase, and nitrous-oxide:(acceptor) oxidoreductase (NO-forming).

Function Organisms reduce nitrate (NO3−) to nitrogen gas (N2) through the process of denitrification, see Figure 1. Two important intermediates of the reduction pathway are nitric oxide (NO) and nitrous oxide (N2O). The reducing reaction that transforms NO into N2O is catalyzed by nitric oxide reductase (NOR). NO is reduced to N2O also to prevent cellular toxicity. N2O, a potent greenhouse gas, is released.

Reaction In enzymology, a nitric oxide reductase (NOR) catalyzes the chemical reaction:

2 NO + 2 e− + 2 H+ ⇌ {\displaystyle \rightleftharpoons } N2O + H2O The enzyme acts on 2 nitric oxide (substrate). The enzyme converts NO, electrons and protons to products: nitrous oxide, and H2O. Inputs: 2 molecules of NO, 2 electrons, 2 protons Outputs: 1 molecule of N2O, 1 molecule of H2O

Mechanism NOR catalyzes the formation of nitrogen to nitrogen (N--N) bonding. The conformation changes of the active site and attached ligands (ie. Glu211) allows NO to be positioned in the crowded binuclear center and form N--N bonds. The precise mechanism of catalysis is still unknown, although hypotheses have been proposed. Cordas et al. 2013 proposes three options: the trans-mechanism, the cis-FeB and the cis-heme b3 mechanisms. Based on the structure of the enzyme, Shiro 2012 proposes the following mechanism: (1) NO molecules bind at the binuclear center, (2) electrons are transferred from the ferrous irons to the NO, (3) charged NO molecules have the potential to form N to N bonds, and (4) N to O bonds are potentially broken by water, allowing for the N2O and H2O to be released. According to Hino et al. 2010, the changing charge of the active site causes NO to bind, form N2O and leave the enzyme. The NOR active site is positioned near two hydrogen bound glutamic acids (Glu). The Glu groups provide an electron-negative charge to the active site. The electro-negative charge reduces the reaction potential for heme b3 and allows NO to bind to the binuclear activation site. Glu residues also provide protons needed for removal of N2O and production of H2O.

Structure

Subunits NOR is made up of two subunits, NorC (small) and NorB (large), with a binuclear iron centre. The binuclear iron center is the active site. It is composed of two b-type hemes and a non-heme iron (FeB). The ligands are connected through a μ-oxo bridge. Histidine (His) residues are attached to the heme b3 in the small subunit. The hydrophilic region of the larger subunit has His and methionine (Met) ligands. Structure is similar to cytochrome oxidases. The active site is conserved between cNOR and qNOR, although differences (ie. heme type) occur between cNOR and qNOR.

Folding Enzymatic folding produced 13 alpha-helices (12 from NorB, 1 from NorC) located within and through the membrane. The folded metalloenzyme transverses the membrane.

Species distribution Bacteria, archaea and fungi use NOR. qNOR is found in denitrifying bacteria and archaea, as well as pathogenic bacteria not involved in denitrification. Denitrifying fungi reduce NO using P-450nor soluble enzyme.

Types Three types of NOR were identified from bacteria: cNOR, qNOR, and qCuNOR. cNOR was found in denitrifying bacteria: Paracoccus denitrificans, Halomonas halodenitrificans, Pseudomonas nautica, Pseudomonas stutzeri, and Pseudomonas aeruginosa. cNOR was first isolated from P. aeruginosa. qNOR was isolated from Geobacillus stearothermophilus.

References

Worked examples

Example 1 — a first encounter with Nitric-oxide reductase

Start with the simplest possible case. Write down what Nitric-oxide reductase claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Nitric-oxide reductase before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Nitric-oxide reductase ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Nitric-oxide reductase

In research: Nitric-oxide reductase appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Nitric-oxide reductase in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Nitric-oxide reductase is common in secondary-school and first-year university syllabi. It links to neighbouring topics EC 1.7.99, Enzymes of known structure, so understanding it makes those chapters shorter.
In everyday life: Look for Nitric-oxide reductase outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Nitric-oxide reductase in 20 minutes

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Close the page and write down what Nitric-oxide reductase means in your own words.
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Frequently asked questions

What is Nitric-oxide reductase in simple terms?

Why does Nitric-oxide reductase matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Nitric-oxide reductase?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Nitric-oxide reductase.