Protoheme complex

The ferrous complex of octaethyl porphyrin in SDS micelles has been characterized as four coordinated (S = 1) ferrous heme species and is similar to that observed for the ferrous protoheme complex in CTAB. It is noted that ferrous complexes of natural porphyrins cannot be stabilized in aqueous SDS micelles, and much larger aqueous micelles like CTAB were needed to stabilize various ferrous protohemes. This indicates that the environment around the octaethyl porphyrin complex in aqueous SDS is more hydrophobic than that of the analogous natural heme species, suggesting that the OEP moiety is embedded much deeper inside the micellar hydrophobic cavity than the protoporphyrin analogue. 

Fig. 4. Correlation between the shifts, delta (NMR), of the peripheral methyl protons (2 Q, 1 O, 1 9b) in low spin protoheme-Fe(III)-bis-pyridine (substituted) — complexes, (Hill and Morattee) and the splitting of the g-values gi—gz (EPR) of the corresponding pyridine-methylthioglycollate-protoheme-complexes...

Peroxidases are found in milk and in leukocytes, platelets, and other tissues involved in eicosanoid metabolism (Chapter 23). The prosthetic group is protoheme. In the reaction catalyzed by peroxidase, hydrogen peroxide is reduced at the expense of several substances that will act as electron acceptors, such as ascorbate, quinones, and cytochrome c. The reaction catalyzed by peroxidase is complex, but the overall reaction is as follows ... 

The biphasic reaction with CO points to the existence of multiple heme-hemopexin conformers, and this is borne out by spectral analyses. The absorbance spectra of rabbit ferri-, ferro-, and CO-ferro-mesoheme-hemopexin are entirely analogous to those of other bis-histidyl heme proteins such as cytochrome 65 142), but the CD spectra exhibit unusual features (Fig. 11). Of particular interest are the weak signal of the ferro complex and the bisignate signal of the CO-ferro complex (also seen in the NO-ferro-mesoheme-hemopexin complex (140) and in human ferri-protoheme—hemopexin (139)). 

Since C has been assigned to a triplet deoxy state in which the axial ligand has been dissociated. As can be seen in Table 111, in most cases lifetimes are found to compare favorably between the carbon monoxide and oxygen forms of the synthetic complexes. In all cases the rate constants used had an accuracy of only one significant figure, resulting in an accuracy no better for the lifetimes or these states. One noticeable discrepancy in occurs between the chelated protoheme 1 -CO and the oxygen form of this compound, i 02. 

B) Coproporphyrin III. Note that a different tautomeric form is pictured in B than in A. Tautomerism of this kind occurs within all of the porphyrins. (C) Protoheme, the Fe2+ complex of protoprophyrin IX, present in hemoglobin, cytochromes b, and other proteins. 

Ferrochelatase (protoheme ferro-lyase)401 403 inserts Fe2+ into protoporphyrin IX to form heme. The enzyme is found firmly bound to the inner membrane of mitochondria of animal cells, chloroplasts of plants, and chromatophores of bacteria. While Fe2+ is apparently the only metallic ion ordinarily inserted into a porphyrin, the Zn2+ protoporphyrin chelate accumulates in substantial amounts in yeast, and Cu2+-heme complexes are known (p. 843). Ferrochelatase, whose activity is stimulated by Ca2+, appears to be inhibited by lead ions, a fact that may account for some of the acute toxicity of lead.404... 

The cytochrome o from Azotobacter vinelandii is reported to consist of one polypeptide of molecular weight 28 000 with two identical heme components. It has also been isolated from the thermophile PS3,1319 Escherichia coli, Vitreoscilla, Pseudomonas aeruginosa, and Rhodopseudomonas spp. The enzyme from Vitreoscilla consists of two identical polypeptides of molecular weight 13 000 and two moles of protoheme IX. A cytochrome b562-o complex from E. coli contains two peptides and, strangely, copper.1320... 

Fig. 2. Structure of some porphyrin-iron complexes. Protoheme IX (Proto) R = —CH=CH2 Deuteroheme IX (Deut) R = —H Mesoheme IX (Meso) R =...

Early attempts at preparing P-450 from membranes were thwarted by the loss of absorption in the 450 nm region (CO complex) and the appearance in its place of absorption at 420 nm (7, 2, 5, 7). The P-420 pigment preserves all the characteristics of a heme-protein and the only heme present is protoheme. The spectra of the P-420 complex are given in Table 1 as they provide a basis for the discussion of P-450. The P-420 spectra are in all respects those of normal hemochromogens i. e. simple six-coordinate Fe(II) and Fe(III) hemo-proteins such as cytochrome b. 

Recombination rates range from 10 to 10 M s, with similar values obtained by nanosecond measurements, whereas photodissociation yields are low and range from 0.03 -0.08. These low yields have been interpreted in terms of rapid relaxation of the photoexcited six-coordinate complex competing with ligand release. Other five-coordinate complexes are also formed on picosecond photolysis for example, a protoheme dimethyl ester bis(t-butyl isocyanide) in pure t-butyl isocyanide or in toluene. This photolysis results in the loss of one isocyanide ligand to produce an electronically excited five-coordinate complex, which decays to the ground state within 40 ps. The recombination rate for the formation... 

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