Chelates in Living Systems

Ten of the 29 elements known to be necessary for human life are transition metals. (Section 2.7, Chemistry and Life Elements Required by Living Organisms ) These ten elements—V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, and Cd— form complexes with a variety of groups present in biological systems. 

Although our bodies require only small quantities of metals, deficiencies can lead to serious iUness. A deficiency of manganese, for example, can lead to convulsive disorders. Some epilepsy patients have been helped by the addition of manganese to their diets. 

Among the most important chelating agents in nature are those derived from the porphine molecule ( FIGURE 23.13). This molecule can coordinate to a metal via its four nitrogen donor atoms. Once porphine bonds to a metal ion, the two H atoms on the nitrogens are displaced to form complexes called porphyrins. Two important porphyrins are hemes, in which the metal ion is Fe(ll), and chlorophylls, with a Mg(ll) central ion. 

In both myoglobin and hemoglobin, the iron is coordinated to the four nitrogen atoms of a porphyrin and to a nitrogen atom from the protein chain ( FIGURE 23.15). In hemoglobin, the sixth position around the iron is occupied either by O2 (in oxyhemoglobin, the 

Myoglobin. This ribbon diagram does not show most of 

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