Reactions of Dioxygen and Its Reduced Forms with Heme Proteins and Model Porphyrin Complexes

Among the many kinds of biological complexes of transition metals that interact with dioxygen or its variously reduced forms, heme proteins are the most common and the most studied. With only a few exceptions, heme proteins are globular proteins which sequester and spatially isolate the active porphyrin-iron complex, protoheme (the iron(II) form) or protohemin (the iron(III) form; Figure 3-1). The iron porphyrin is tightly bound to the protein through numerous hydrophobic interactions and by a single coordinate bond between a base from a so-called proximal amino acid residue, such as an imidazole nitrogen from histidine or phenolate from tyrosine, and the iron atom. This is shown in Figure 3-2, where B represents the proximal base. Dioxygen or other small molecules bind to the opposite (distal) side of the porphyrin.