Heme ligands

Fig. 12. Crystal structure of the complex formed between the heme and FMN domains of cytochrome P450BM-3 133). The FMN domain docks on the proximal surface of the heme domain. The thicker trace in the heme domain highlights residues 387 to the heme ligand, Cys400. This section of polypeptide contacts the FMN and might provide an electron transfer conduit to the heme ligand. The two interacting surfaces are electrostatically complementary, with similar complementarity expected for HO and NOS.

As in peroxidases, globins, and P450s, HO-1 has a helix over the distal surface of the heme (Figs. 16,18). In other heme proteins side chains from the distal helix provide the primary contacts with the heme as well as side chains that interact with heme ligands. In sharp contrast, the distal helix in HO-1 lies much closer to the heme such that backbone atoms form the primary heme contacts. In addition, there is no neighboring residue that could serve the same fimction as the distal His in the globins and peroxidases for interaction with iron-linked ligands. 

The great difficulty in understanding the evolution of a protein can be illustrated by cytochromes c, which are known to have a methionine and a histidine as heme ligands. The position of the methionine, especially, in the sequence of the proteins is very variable. Again cytochromes c have very different molecular weights. If this is true what is the implication for the overall fold The immediate neighbors of the heme, not bound to iron, include a tryptophan, almost invariable in structure but not in sequence position. How has this been evolved There is but one cytochrome c that has no tryptophan. I do not believe that we could follow evolution with confidence from sequences of single enzymes. Cytochrome c3 sequences show this very clearly, for these proteins, all from sulfur bacteria, show an enormous sequence variation [R. P. Ambler, Syst. Zool. 22, 554 (1974)]. There is no evidence that this is related to their evolution. 

Diagram 1. Proposed mechanism ferric Hbl after laser exposure, reduction process could occur by an electron transfer from the orbital a2ub2u to the eg(n ) [11]. A n radical cation is formed in the porphyrin after an electron pass from eg(rt ) to the metal d orbitals. It cannot be excluded the possibility of that some amino acid might take place in the process, Sample photoinduced reduction is a step involved in ferric Hbl processes after photoexcitation, which could or could not led to heme-ligand bond breakage. 

The active site structure of peroxidases (Fig. 16-13) is quite highly conserved. As in myoglobin, an imidazole group is the proximal heme ligand, but it is usually hydrogen bonded to an aspartate carboxylate as a catalytic diad (Fig. 16-13).223 In cytochrome c peroxidase... 

Nam, W., High-valent iron(IV) -oxo complexes of heme and non-heme ligands in oxygenation reactions. Acc. Chem. Res. 2007, 40, 522-531. 

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