Heme-iron

Iron in the hemoglobin binds O. The chemical reaction equation for this binding is expressed as follows  

In some of these catalytic reactions, Fe is said to take the Fe(IV) oxidation state as in FeO +. When iron is bound with a negatively charged N- or S-ligand(s) in cytochrome P-450 or peroxidases, the higher oxidation state such as Fe(TV) is believed to be stabilized and hence will be realized. 

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Molybdenum. Molybdenum is a component of the metaHoen2ymes xanthine oxidase, aldehyde oxidase, and sulfite oxidase in mammals (130). Two other molybdenum metaHoen2ymes present in nitrifying bacteria have been characteri2ed nitrogenase and nitrate reductase (131). The molybdenum in the oxidases, is involved in redox reactions. The heme iron in sulfite oxidase also is involved in electron transfer (132). 

W. A. Bulen, J. R. LeComte, R. C. Bums, and J. Hinkson, in A. San Pietro, ed., Non-Heme Iron Proteins Role in Lnerg Conversion Antioch Press, Yellow Springs, Ohio, 1965, p. 261. 

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. 

Movement of the Heme Iron hy Less Than 0.04 nm Induces the Conformational Change in Hemoglobin... 

In deoxyhemoglobin, histidine F8 is liganded to the heme iron ion, but steric constraints force the Fe His-N bond to be tilted about 8° from the perpendicular to the plane of the heme. Steric repulsion between histidine F8 and the nitrogen atoms of the porphyrin ring system, combined with electrostatic repulsions between the electrons of Fe and the porphyrin 77-electrons, forces the iron atom to lie out of the porphyrin plane by about 0.06 nm. Changes in... 

FIGURE 15.32 Changes in the position of the heme iron atom upon oxygenation lead to conformational changes in the hemoglobin molecnle. 

Electrons from cytochrome c are transferred to Cu sites and then passed to the heme iron of cytochrome a. Cu is liganded by two cysteines and two histidines (Figure 21.18). The heme of cytochrome a is liganded by imidazole rings of histidine residues (Figure 21.18). The Cu and the Fe of cytochrome a are within 1.5 nm of each other. 

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. 

Tyrosine hydroxylase (TH) is an enzyme that catalyzes the hydroxylation of tyrosine to 3,4-dihydroxypheny-lalanine in the brain and adrenal glands. TH is the rate-limiting enzyme in the biosynthesis of dopamine. This non-heme iron-dependent monoxygenase requires the presence of the cofactor tetrahydrobiopterin to maintain the metal in its ferrous state. 

Biochemical aspects of Fe—S linkages in non-aqueous heme iron proteins with special reference to Andrenodoxin. T. Kimura, Struct. Bonding (Berlin), 1968, 5, 2-40 (72). 

Bioinorganic applications of m.c.d. spectroscopy copper, rare earth ions, cobalt and non-heme iron systems. D. M. Dooley and J. H. Dawson, Coord. Chem. Rev., 1984, 60,1 (176). 

Cyclopentene-l-dithiocarboxylic acid, 2-amino-meta complexes, 2, 800 Cyclophane chlorophylls, 3, 58 Cyclophane hemes iron complexes, 4,1269 Cyclophosphazenes metal complexes, 2, 81 Cyclopropane carbonylation... 

Carbene form binds to heme iron in sixth ligand position... 

Apomyoglobin Provides a Hindered Environment for Heme Iron... 

Figure 6-3. Angles for bonding of oxygen and carbon monoxide to the heme iron of myoglobin. The distal E7 histidine hinders bonding of CO at the preferred (180 degree) angle to the plane of the heme ring.

In methemoglobinemia, the heme iron is ferric rather than ferrous. Methemoglobin thus can neither bind nor transport Oj. Normally, the enzyme methemoglobin... 

Inorganic iron is absorbed only in the (reduced) state, and for that reason the presence of reducing agents will enhance absorption. The most effective compound is vitamin C, and while intakes of 40-60 mg of vitamin C per day are more than adequate to meet requirements, an intake of 25-50 mg per meal will enhance iron absorption, especially when iron salts are used to treat iron deficiency anemia. Ethanol and fructose also enhance iron absorption. Heme iron from meat is absorbed separately and is considerably more available than inorganic iron. However, the absorption of both inorganic and heme iron is impaired by calcium—a glass of milk with a meal significantly reduces availabiUty. 

Ruettinger RT, GR Griffith, MJ Coon (1977) Characteristics of the u-hydroxylase of Pseudomonas oleovorans as a non-heme iron protein. Arch Biochem Biophys 183 528-537. 

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