O

Heme b

Chlorophyll a

Figure 2 Heme iron in hemoglobin. Heme is a porphyrin ring with iron in the center. Four heme b groups are present in hemoglobin, the iron protein in erythrocytes. A similar structural arrangement is seen with magnesium in chlorophyll a from plants.

S2"

Fe Vy Fe

S2"

Figure 3 Iron-sulfur clusters. Both the Fe2S2 and Fe4S4 clusters are bound to the protein via cysteine residues. The iron in these complexes either engages a substrate or holds and passes electrons.

Figure 3 Iron-sulfur clusters. Both the Fe2S2 and Fe4S4 clusters are bound to the protein via cysteine residues. The iron in these complexes either engages a substrate or holds and passes electrons.

systems. As noted above, catalase and peroxidase, two heme enzymes, use iron to engage dangerous oxidants. Both enzymes are located in the cytosol and in peroxisomes where harmful oxidation reactions occur during the course of normal metabolic events. Perhaps the most familiar iron-containing

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