Ferry porphyrins

Cytochromes have been reported at low concentrations in the milk fat globule membrane (Gregory et al., 1976 Jarasch et al., 1977). The powerful pro-oxidant properties of ferri-porphyrin proteins, together with their juxtaposition with lipids in the milk fat globule membrane, suggest that cytochromes may play a role in oxidation. Gregory et al. (1976) concluded that they do exert a role in milk lipid oxidation. 

New nitrosyl complexes of alkyl aryl)ferri-porphyrins with Fe-C o x>nds. 

Oxidation of the ferrous ion of haem gives rise to a ferri-porphyrin which is given the name haematin . An oxygen molecule can bring about this oxidation. 

The ferric ion has 5 unpaired electrons. Ferri-porphyrin also possesses 5 unpaired electrons hence all the bonds in the complex are of an electrostatic character. The complex, in which the ferric ion has a coordination number of 6, can be represented thus ... 

Laverman and coworkers have reported activation parameters for the aqueous solution reactions of NO with the iron(II) and iron(III) complexes of the water soluble porphyrins TPPS andTMPS (21). These studies involved systematic measurements to determine on and kQ as functions of temperature (298—318 K) and hydrostatic pressure (0.1—250 MPa) to determine values of AH, AS and AV for the on and off reactions of the ferri-heme models and for the on reactions of the ferro-heme models (Table II). Figure 2 illustrates hydrostatic pressure effects on kOTL and kQff for Fem(TPPS). 

Fig. 12. Schematic views of bis-histidyl ferri-, ferro-, and CO-ferro-heme-hemopexin. Unlike myoglobin with one open distal site, heme bound to hemopexin is coordinated to two strong field ligands, either of which a priori may be displaced by CO. This may well produce coupled changes in protein conformation like the Perutz mechanism for 02-binding by hemoglobin (143). The environment of heme bound to hemopexin and to the N-domain may be influenced by changes in the interactions of porphyrin-ring orbitals with those of aromatic residues in the heme binding site upon reduction and subsequent CO binding.

As shown in Fig. 4, a FenNO + species has been proposed to be formed following coordination of NO into Fe(III). This reaction appears to be related to the one in Eq. (3), associated with NP formation. However, the electron-transfer step must proceed in an inner-sphere way in the Fe(III) porphyrin system, subsequently to the water-release. As anticipated above, the evidence on the detailed electronic structure in the ferri-hemes is still ambiguous (24,25). Whether the species formed in the reaction of the oxidized form of heme cd1 nitrite reductase (NiR) with NO is best formulated as FenNO + or FeinNO has been qualified as a matter of semantics... 

From these measurements the resonance positions in ferrocytochrome c of the protons corresponding to the hyperfine-shifted lines in ferricytochrome c are obtained. Gupta and Redfield (123) found that one of the ring methyl resonances of heme c is shifted upfield by ca. 1.5 ppm, indicating close proximity to the face of an aromatic amino acid residue in the reduced cytochrome c. The resonance positions in ferrocytochrome c are probably a better approximation of the diamagnetic heme resonance positions in ferri cytochrome c than are the porphyrin-zinc (II) complexes (Fig. 9). Hence experiments of this type could conceivably contribute towards a more accurate determination of the spin density distribution in the heme groups. 

The Compound I species of many peroxidases, as illustrated for HRP, is characterized by an [Fe(IV)=Q] ferry species coupled with a porphyrin radical cation. 

NO reacts with both ferric and ferrous centers in hemoproteins to form the respective iron(II) and iron(III) nitrosyl adducts, whose structural features are similar to those observed for iron (II) and iron(III) porphyrin nitrosyls. These analogies are also reflected in similar chemical reactivity observed for nitrosylated ferri- and ferroproteins and their respective porphyrin models. For example, NO-adducts of Fe(III) undergo reductive nitrosylation in the presence of an excess of NO, and a similar process is commonly observed for synthetic Fe(III) porphyrins. The first step of this reaction involves nucleophilic attack of OH on the nitrosyl ligand coordinated to the iron center, as presented in reaction (13) (33,60) ... 

Porphy s Hemeland. The ferris wheel (Krebs cycle) of the Main Powerhouse connects with Porphy s Hemeland, as the succinyl CoA of the ferris wheel is a precursor of porphyrins and their derivatives, such as heme. 

If you are pow ready for blood and guts adventure, let us visit Parphy s Hemetand. Succinyl CoA, a molecule in the Krebs cycle ferris wheel, is the entry point from the Main Powerhouse to Porphy s Hemeland, where one may find the pon>hyriu pinudieels (porphyrin rings) (C-12). 

The arrangement of four nitrogen atoms in the center of the porphyrin ring enables porphyrins to chelate various metal ions. Protoporphyrin that contains iron is known as heme ferroheme refers specifically to the complex and ferri-heme to Fe. Ferriheme associated with a chloride counter ion is known as hemin, or hematin when the counter ion is hydroxide. 

Bonding and Dissociation Reactions in Nitrosyl Porphyrins How is NO Released from the Ferri-Hemes ... 

The reactions of NO with the high-spin ferri-hemes (Equation 7.3), either in model porphyrin complexes or in proteins (with H20 being eventually replaced by other ligands such as histidine or a thiolate),5 show some similarities with the classical complexes discussed in Section 7.3.1 (cf. Equation 7.1). 

Reduction of compound I by one electron produces compound II, which has the characteristics of a normal ferry 1-porphyrin complex, analogous to 2, i.e., (L)Fe (P)(0). Reaction of compound II with hydrogen peroxide produces compound III, which can also be prepared by reaction of the ferrous enzyme with dioxygen. It is an oxy form, analogous to oxymyoglobin, and does not appear to have a physiological function. The reactions producing these three forms and their proposed formulations are summarized in Reactions (5.85) to (5.88). 

FIG. 17 Reversal of the photocurrent sign upon replacing the electron donor DCMFc (a) by the electron acceptor TCNQ (b) in the presence of the porphyrin heterodimer ZnTMPyP-ZnTPPS at the water-DCE interface. The strong back electron-transfer features in the photoreduction of TCNQ were diminished upon addition of an equimolar ratio of ferri/ferrocyanide acting as supersensitizer in the aqueous phase (b). The mechanism of supersensitization is described in Fig. 11. From the potential relationship between these redox couples (Fig. 4), these phenomena can be regarded as interfacial photosynthetic processes as defined in Fig. 3(b). (Reprinted with permission from Ref. 87. Copyright 1999 American Chemical Society and from Ref. 166 with permission from Elsevier Science.)... 

Despite uncertainties concerning the site of the first one-electron oxidation of Fe P, the second oxidation is thought to form a ferry 1 species, 0 = Fe P, which can oxidize or hydroxylate various organic compounds. In parallel with this chemistry of iron porphyrins, the oxidation of ruthenium and osmium porphyrins was studied by radiation chemical methods. 

The synthesis of a ferric-peroxo porphyrin eomplex, [(Porp)Fe (02)], was first reported by Valentine and co-workers. The ferrie-peroxo speeies was prepared by reacting Fe(TPP)Cl or Fe(OEP)Cl with two equivalents of superoxide in the presenee of erown ether in aprotie solvents sueh as aeetonitrile or toluene. The first equivalent of superoxide reduees the ferric porphyrin to the ferrous porphyrin, and the second superoxide eonverts the ferrous porphyrin to the ferrie-r/ -peroxo porphyrin complex (Equation (14)). The EPR speetrum of [(0EP)Fe (02)] eomplex... 

Heme an iron-porphyrin complex (generic term, used for either ferroheme or ferri-heme). 

It is a beautiful red, iron-porphyrin-containing protein which functions as a link in the chain of the cell-respiration enzymes, the iron atom now taking up and now giving off an electron, and the iron thus alternating valency between the 3-valent ferri and the 2-valent ferro stages. It is a very pleasant substance to work with, not merely because it is lovely to look at, but also because it is uncommonly stable and durable. From 100 kg heart-meat of horse one can produce 3-4 g of pure cytochrome c. The molecule weighs about 12,000 and contains one mol iron porphyrin per mol. 

In view of the Greek origin of the word cytochrome, it seems to us inadvisable to substitute cytichrome for ferricytochrome ferri- and ferro- will therefore be used as prefixes to cytochrome. Hemoproteins will be used for all iron porphyrin proteins, irrespective of whether they contain ferrous or ferric iron. 

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