Ferric Heme

Certain other plasma proteins bind heme but not hemoglobin. Hemopexin is a Pj-globuhn that binds free heme. Albumin wiU bind some metheme (ferric heme) to form methemalbumin, which then ttansfets the metheme to hemopexin. 

For example the X-band spectra of high-spin ferric hemes (S = 5/2 g 2 E/D < 0.1) show a single transition described by... 

An illustrative example is given in Figure 6.2 the spectrum (Arendsen et al. 1993) of a high-spin ferric heme with its typical very wide pattern of an intense feature around geff 6 and a weak negative peak near g 2 (Equations 5.34 and 5.35). 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

Compound I will accept a further electron from azurin and decays to a species known as compound II with a bimolecular rate constant of 6 X 10 M s (84). It is anticipated that compound II is a form of CCP containing two ferric hemes, but possibly not identical to the structurally defined fully oxidized enzyme as isolated. This is because during turnover at room temperature there is no obvious reason for the histidine ligand displaced from the peroxidatic heme iron to return. Consequently, it might be assumed that compound II is structurally related to the MV conformer rather than the resting enzyme. 

In vivo heme is released into the plasma by erythrocyte lysis in the form of hemoglobin and by tissue trauma in the form of myoglobin, and both heme proteins are quickly oxidized to their ferric heme forms (methemoglobin and metmyoglobin) at the oxygen tension found in tissue capillary beds. 

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.

Fig. 8. Mossbauer spectra of a sample containing 9 mM [OEP PeCl] and 30 mM Al-methylimidazole in jyjV-dimethylacetamide, into which NO gas was bubbled for 5 minutes, taken at 4.2 K in a magnetic field of (a) 5.34 T parallel and (b) 20 mT perpendicular to the 7-beam. The dotted line corresponds to the spectrum of the low-spin ferric heme complex [OEP Fe(NMelm)2]+Cl (39% relative contribution) and the dashed line to the heme-NO complex [OEP Fe(NMelm)(NO)]+Cl (61% relative contribution). Reproduced with permission from Ref. (86).

The NO in this structure appears to be bound equally in two orientations, a component that is linear (Fe-N distance of 1.6 A and Fe-N-0 angle of 170°, Fig. 17), and a second component that is bent (Fe-N 2.0 A, Fe-N-0 110°). To account for this, the NO molecule has been refined as a mixture of both orientations. The roughly linear orientation is indicative of a ferric (Fe 0 NO complex (2, 65), indicating that the NP4-NO structure represents the first ferric heme-NO complex for any protein. However, the bent orientation is similar in geometry to a ferrous heme-NO complex (2, 65). The bend directs the NO toward a... 

The energetics of peptide-porphyrin interactions and peptide ligand-metal binding have also been observed in another self-assembly system constructed by Huffman et al. (125). Using monomeric helices binding to iron(III) coproporphyrin I, a fourfold symmetric tetracarboxylate porphyrin, these authors demonstrate a correlation between the hydropho-bicity of the peptide and the affinity for heme as well as the reduction potential of the encapsulated ferric ion, as shown in Fig. 12. These data clearly demonstrate that heme macrocycle-peptide hydrophobic interactions are important for both the stability of ferric heme proteins and the resultant electrochemistry. 

In this paper, we present the results of the calculations of the excited-state energies and characters for nitrosyl ferrous and ferric heme complexes as a function of iron-nitrosyl distance. The calculated energy profiles are then used to identify the photoactive states for each complex. Similar calculations are also underway to characterize the photoactive states of Mn(II)N0 complexes (47). 

Table I shows the atomic orbital composition and energies of the ground-state molecular orbitals of the nitrosyl ferric heme complex at three Fe-NO distances of 1.743 (bonding), 1.943, and 2.143 A.

The main effect or lengthening the Fe-NO distance in nitrosyl ferric heme is an increase in the d contribution to virtual orbitals... 

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