Heme iron oxidation

The UV/Vis spectra of heme-containing proteins, such as cytochrome c, contain two major bands—the Soret and a-bands. The most intense of these two is the Soret band, which occurs between 400 and 450 nm, depending on the nature of the heme iron and oxidation state of the protein [181]. Both bands are attributed to the tr tt transitions of the heme and are, therefore, very sensitive indicators of the heme iron oxidation state. The spectra presented, specifically between 400 and 600 nm. 

The automated method differs from the ICSH method chiefly in that oxidation and ligation of heme iron occur after the hemes have been released from globin. Therefore, ferricyanide and cyanide need not diffuse into the hemoglobin and methemoglobin, respectively. Because diffusion is rate-limiting in this reaction sequence, the overall reaction time is reduced from approximately three minutes for the manual method to 3 —15 seconds for the automated method. Reaction sequences in the Coulter S + II and the Technicon H 1 and H 2 are similar. Moreover, similar reactions are used in the other Coulter systems and in the TOA and Unipath instmments. 

Nothing is known about the identity of the iron species responsible for dehydrogenation of the substrate. Iron-oxo species such as FeIV=0 or Fem-OOH are postulated as the oxidants in most heme or non-heme iron oxygenases. It has to be considered that any mechanistic model proposed must account not only for the observed stereochemistry but also for the lack of hydroxylation activity and its inability to convert the olefinic substrate. Furthermore, no HppE sequence homo-logue is to be found in protein databases. Further studies should shed more light on the mechanism with which this unique enzyme operates. 

Hewitson, K.S., Granatino, N., Welford, R.W.D. and Schofield, CJ. (2005). Oxidation by 2-oxog-lutarate oxygenases non heme iron synthesis. Phil. Trans Roy. Soc. A363, 807-829. 

Selective Conversion of Hydrocarbons with H202 Using Biomimetic Non-heme Iron and Manganese Oxidation Catalysts... 

Simultaneous generation of nitric oxide and superoxide by NO synthases results in the formation of peroxynitrite. As the reaction between these free radicals proceeds with a diffusion-controlled rate (Chapter 21), it is surprising that it is possible to detect experimentally both superoxide and NO during NO synthase catalysis. However, Pou et al. [147] pointed out that the reason is the fact that superoxide and nitric oxide are generated consecutively at the same heme iron site. Therefore, after superoxide production NO synthase must cycle twice before NO production. Correspondingly, there is enough time for superoxide to diffuse from the enzyme and react with other biomolecules. 

Feelisch, M., Noack, M., Nitric oxide (NO) formation from nitrovasodilators occurs independently of hemoglobin or non-heme iron. Eur. J. Pharmacol. (1987), 142, p. 465—469... 

Methylhydroxyurea (28, Fig. 7.5) oxidizes oxyHb to metHb and reduces metHb to deoxyHb but neither of these reactions produces HbNO, further supporting the mechanism depicted in Scheme 7.16 for the formation of NO and HbNO from the reactions of hydroxyurea and hemoglobin [115]. The O-methyl group of 27 prevents the association and further reaction of 27 with the heme iron [115]. Scheme 7.16 predicts the redox chemistry observed during the reaction of 28 with hemoglobin and the failure to detect HbNO shows the inability of 28 or any derivative radicals to transfer NO during these reactions [115]. These results indicate that nitric oxide transfer in these reactions of hydroxyurea requires an unsubstituted acylhydroxylamine (-NHOH) group. 

The stepwise oxidation of alkylamine, which leads to N-dealkylation, generates nitrones that form tightly bound complexes with the heme iron [48]. These heme iron complexes give rise to characteristic changes in the UV-Vis spectrum of the CYP. 

Compounds containing a benzodioxole moiety are oxidized by CYPs to generate a carbene that forms a tightly bound complex with the heme iron, easily measurable by UV-Vis spectroscopy. There are numerous examples in the literature paroxetine, MDP alkylamines, MDP benzothiazines, MDP benzothiazolines and MDP piperazines (see Scheme 11.2) [54—60],... 

Cytochrome c can easily be extracted from tissue particles by dilute salt solutions. It was isolated by Keilin and Hartree in 1930 and shown to contain a porphyrin ring structure. In 1933 Zeilen and Reuter established that cytochrome c was a heme (iron-porphyrin) protein. Slightly different forms of cytochrome a were distinguished in yeast and bacteria by Keilin in 1934 and the different properties of cytochrome a and a3 by Tamiya et al. in 1937. The identity of cytochrome 03, the enzyme which activates oxygen with Warburg s atmungsferment, was proposed by Keilin in 1939. Cytochrome a/a3 was renamed cytochrome oxidase by Malcolm Dixon (1939). The oxidation route then offered was ... 

Denaturation of hemoproteins in cooked meats leads to liberation of the heme and oxidation of the porphyrin ring. Nonheme iron is less available nutritionally than heme iron and affects lipid oxidation more. In methemoglo-bin and metmyoglobin solutions heated for one hour at 78°C and 100°C the degradation of heme was about 22 to 26%, while after two hours at 120°C it increased to about 85 to 95% (Oellingrath, 1988). In meat cookery, however, such severe conditions do not apply. 

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