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Nitric Oxide Signaling Pathway01:28

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Nitric oxide (NO), an inorganic gas, acts as a potent second messenger in most animal and plant tissues. NO diffuses out of the cells that produce it and enters the neighboring cells to generate a downstream response. NO synthase (NOS) catalyzes NO production by the deamination of the amino acid arginine. There are three isoforms of NOS. Endothelial cells have endothelial NOS (eNOS), nerve and muscle cells have neuronal NOS (nNOS), and macrophages produce inducible NOS (iNOS) upon exposure...
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Isomerism in Complexes
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The hemoglobin in the blood, the chlorophyll in green plants, vitamin B-12, and the catalyst used in the manufacture of polyethylene all contain coordination compounds. Ions of the metals, especially the transition metals, are likely to form complexes.
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meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H01:13

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All meta-directing substituents are deactivating groups. These substituents withdraw electrons from the aromatic ring, making the ring less reactive toward electrophilic substitution. For example, the nitration of nitrobenzene is 100,000 times slower than that of benzene because of the deactivating effect of the nitro group. The first step in an electrophilic aromatic substitution is the addition of an electrophile to form a resonance-stabilized carbocation. The energy diagrams for...
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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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Secondary amines react with nitrous acid to form N-nitrosamines, as depicted in Figure 1. Nitrous acid, a weak and unstable acid, is formed in situ from an aqueous solution of sodium nitrite and strong acids, such as hydrochloric acid or sulfuric acid, in cold conditions. In the presence of an acid, the nitrous acid gets protonated. The subsequent loss of water results in the formation of the electrophile known as nitrosonium ion.
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Exploring second coordination sphere effects in nitric oxide synthase.

Ashley B McQuarters1,2, Amy L Speelman1,2, Li Chen3

  • 1Department of Chemistry, University of Michigan, Ann Arbor, MI, 48109, USA.

Journal of Biological Inorganic Chemistry : JBIC : a Publication of the Society of Biological Inorganic Chemistry
|October 1, 2016
PubMed
Summary

Second coordination sphere effects influence heme-thiolate proteins. Low-temperature MCD spectroscopy reveals a conserved S(Cys) → Fe σ CT band, a hallmark of these active sites.

Keywords:
Cytochrome P450Electronic SpectraHemeMagnetic circular dichroismNitric oxide synthaseSecond coordination sphere effects

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Area of Science:

  • Biochemistry
  • Spectroscopy
  • Protein Science

Background:

  • Second coordination sphere (SCS) effects modulate protein active sites via interactions like hydrogen bonding.
  • In cytochrome P450s, hydrogen bonding networks near the heme's proximal cysteinate ligand (Cys pocket) are thought to regulate the Fe-S interaction.
  • Previous studies identified an S(Cys) → Fe σ CT transition in Cyt P450cam using low-temperature MCD spectroscopy.

Purpose of the Study:

  • To investigate the impact of a specific hydrogen bond (from W409) to the heme's axial Cys ligand in neuronal nitric oxide synthase oxygenase (nNOSoxy).
  • To determine if the S(Cys) → Fe σ CT transition is a conserved feature in heme-thiolate active sites.
  • To assess the influence of this hydrogen bond on the Fe-S(Cys) bond strength in the high-spin ferric state.

Main Methods:

  • Low-temperature (LT) MCD spectroscopy was employed.
  • Wild-type nNOSoxy and W409 mutants were analyzed.
  • The intense S(Cys) → Fe σ CT band was monitored.

Main Results:

  • An intense S(Cys) → Fe σ CT band was observed around 27,800 cm⁻¹ in the LT MCD spectrum of nNOSoxy, similar to Cyt P450cam.
  • This spectral feature appears to be a characteristic of heme-thiolate active sites.
  • W409 mutants showed only a minor effect on the Fe-S(Cys) bond strength in the high-spin ferric state.

Conclusions:

  • The S(Cys) → Fe σ CT band is a conserved spectral signature for high-spin ferric heme-thiolate active sites.
  • Low-temperature MCD spectroscopy provides a method for classifying these active sites.
  • The specific hydrogen bond from W409 to the axial Cys ligand has a limited impact on the Fe-S(Cys) bond strength in the studied state.