Hemoglobin metal ligands

Of the ten trace elements known to be essential to human nutrition, seven are transition metals. For the most part, transition metals in biochemical compounds are present as complex ions, chelated by organic ligands. You will recall (Chapter 15) that hemoglobin has such a structure with Fe2+ as the central ion of the complex. The Co3+ ion... 

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. 

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. 

There are many organic dyestuffs but only one class, that of the metal chelates, has been found to be active in electrocatalysis. The first compounds described were the phthalocyanines 1-4), which are similar in structure to the heme in the blood pigment, hemoglobin in both, the metal atom is surrounded by four nitrogen ligands. 

In hemoglobin and myoglobin the heme serves as a carrier with oxygen-heme iron (Fe2+) interacting in a ligand-metal coordination relationship. In the cytochromes heme plays a redox role, with the iron interconverting between Fe2+ and Fe3+. 

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