Ligand distal atom

Figure 3.12 Active site of a reduced form of the Fe-only hydrogenase from Desulphovibrio desul-phuricans. The Fe atom on the right is defined as the proximal Fe (relative to the neighbouring [Fe-S] cluster), Fep the Fe atom on the left is defined as the distal Fe, FeD. The arrow indicates the potential hydron-binding site on FeD that is occupied by either HzO or an extrinsic CO in the structure of Cp I. Also shown is a close contact between the bridgehead atom X of the exogenous dithiolate ligand and the S atom of cysteine-178. (Reprinted with permission from Parkin et al., 2006. Copyright (2005) American Chemical Society.)...

Fig. 7. The calcium binding sites of HRP C (detail). Ligands to the distal (A) and proximal (B) calcium atoms are shown. The O-donor ligand sets are hsted in Section IV,D.

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. 

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