Histidine hemoglobin ligand

Each heme unit in myoglobin and hemoglobin contains one ion bound to four nitrogen donor atoms in a square planar arrangement. This leaves the metal with two axial coordination sites to bind other ligands. One of these sites is bound to a histidine side chain that holds the heme in the pocket of the protein. The other axial position is where reversible binding of molecular oxygen takes place. 

Although the same transition is expected for the 5-coordinate site in the Ni globins as for 6-coordinate models, ligand release is not observed, presumably, due to rapid geminate recombination in the protein. However, excitation of the 4-coordinate site of Ni hemoglobin does result in transient acquisition of a fifth axial ligand—most likely the proximal histidine. 

Figure 2-4. Structure of hemoglobin and its oxygen-binding site. An expanded view of the heme ring within the hydrophobic crevice is shown to the right. The polypeptide backbone of the nearby F helix is indicated by the ribbon with the imidazole ring of the F8 histidine residue projecting out as one of the ligands of the heme iron atom.

The metalloporphyrin complexes and their derivatives have been studied from the standpoint of model compounds of hemoglobin. The polymer-metalloporphyrin complexes are also formed by the reaction in Scheme 8, and a few qualitative investigations have been made with poly(L-lysine)9,10, poly(L-histidine)11, and poly(vinylimidazole)12 as the polymer ligand. Blauer9 has studied the complex formation of heme with poly(L-lysine) and has discussed the effects of the molecular weight and secondary structure of poly(L-lysine) on complex formation. 

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