Heme moieties

For CO-myoglobin a Fe-CO stretch at 502 cm and a Fe-C-O bend at 572 cm has been observed [112]. The drastic increase of the out-of-plane stretch compared to deoxy- and metmyoglobin is due to the strong covalent Fe-CO bond. Raising the temperature from 50 to 110 K led to a broad resonance at around 25 cm which has been assigned to the translational motion of the whole heme moiety. 

Figure 14.1 (S)-Warfarin-binding site and protein-ligand interactions as observed in the pdb entry log5. The ligand is located too far away from the heme moiety to establish direct interactions. However, it is well positioned to offer additional interactions for other molecules...

Fig. 1. Overview of intravascular heme catabolism. Hemoglobin, myoglobin, and other heme proteins are released into the circulation upon cellular destruction, and the heme moiety is oxidized by O2 to the ferric form (e.g., methemoglobin and metmyoglobin). Haptoglobin can bind a substantial amount of hemoglobin, but is readily depleted. Ferric heme dissociates from globin and can be bound by albumin or more avidly by hemopexin. Hemopexin removes heme from the circulation by a receptor-mediated transport mechanism, and once inside the ceU heme is transported to heme oxygenase for catabolism.

The heme moiety provides de novo designed heme proteins with an intrinsic and spectroscopically rich probe. The interaction of the amide bonds of the peptide or protein with the heme macrocycle provides for an induced circular dichroism spectrum indicative of protein-cofactor interactions. The strong optical properties of the heme macrocycle also make it suitable for resonance Raman spectroscopy. Aside from the heme macrocycle, the encapsulated metal ion itself provides a spectroscopic probe into its electronic structure via EPR spectroscopy and electrochemistry. These spectroscopic and electrochemical tools provide a strong quantitative base for the detailed evaluation of the relative successes of de novo heme proteins. 

The rich spectroscopy and electrochemistry of the heme moiety yields a wealth of opportunities for the denovo heme protein design to evaluate the success of the heme binding site design. Combinations of these spectroscopic and electrochemical methods are elucidating the structure and function of de novo heme proteins and illustrating that they serve as excellent bioinorganic model complexes for simple cytochromes. 

Some transition metal complexes are excellent conductors. Thin films of cyto-chrome-C3, which contains four heme moieties coordinated by protein, exhibited a high conductivity with mixed valence state (Fe /Fe ) and showed an increase in conductivity as the temperature was decreased (2 x 10 S cm at 268 K) [68-70]. The temperature dependence of conductivity in the highly conductive region is the opposite of that of semiconductors and may preclude the ionic conduction as a dominant contribution. However, since the high conductivity is realized in the presence of hydrogenase and hydrogen, the system is not strictly a single but rather a multicomponent molecular solid. 

The cytochrome P-450 destroyed may be a specific isoenzyme, as is the case with carbon tetrachloride and allylisopropylacetamide (Table 5.28). Indeed, with carbon tetrachloride the isoenzyme destroyed is the one, which is responsible for the metabolic activation (CYP1A2). With allylisopropylacetamide, it is the phenobarbital-inducible form of the enzyme, which is preferentially destroyed as can be seen from Table 5.25. It seems that it is the heme moiety, which is destroyed by the formation of covalent adducts between the reactive metabolite, such as the trichloromethyl radical formed from carbon tetrachloride (see chap. 7), and the porphyrin ring. 

Figure 7.66 The heme moiety of the hemoglobin molecule showing the binding of the oxygen molecule to the iron atom. As shown in the diagram, carbon monoxide binds at the same site. Abbreviation His, side chain of the amino acid histidine. Source From Ref. 18.

While hemoglobin proteolysis yields needed amino acids, it also releases toxic free heme. Fig. (1) Upon proteolysis, heme is released into the digestive vacuole where the iron of the heme moiety is oxidized from the predominantly ferrous (+2) state to the ferric (+3) state. Estimates suggest... 

The pink cured meat pigment mononitrosylhemochrome is a complex of nitric oxide (NO), ferrous heme iron, and heat-denatured globin protein (Table F3.2.1). The pink nitrosylheme (NO-heme) moiety may be extracted from the protein in aqueous acetone and quantitated by A540 (see Basic Protocol 1). The percent nitrosylation may be determined from measurement of ppm NO-heme relative to ppm total acid hematin (hemin) extracted in acidified acetone (see Basic Protocol 2), since NO-heme is completely oxidized to hemin in acid solution (i.e., 1 ppm NO-heme = 1 ppm hemin). 

Hornsey (1956) also found that extraction with an acidified 80% acetone solution for 1 hr gave a hemin solution, derived from oxidation of the heme moiety of both NO-heme and non-nitrosylated heme pigments. Hemin absorption spectra exhibited distinct peaks at 512 and 640 nm. Hornsey (1956) used the A640 of sample filtrates as a measure of the total heme pigments. Solutions of both hemin and NO-heme in 80% acetone conformed with Beer s... 

The H NMR spectra of ferricytochromes d are typical of high spin iron(III). The iron atom is pentacoordinated, with four ligands provided by the nitrogen atoms of a porphyrin (see Fig. 5.7) and the fifth ligand being a histidine residue exposed to the solvent. Being a cytochrome of c type, the heme moiety is covalently bound to two cysteinyl residues by means of thioether links (Fig. 5.11). 

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