Iron porphyrin, coordinative groups

This behavior, as well as complementary observations, can be explained on the basis of the reaction mechanism depicted in Scheme 5.3. The main catalytic cycle involves three successive forms of the enzyme in which the iron porphyrin prosthetic group undergoes changes in the iron oxidation state and the coordination sphere. E is a simple iron(III) complex. Upon reaction with hydrogen peroxide, it is converted into a cation radical oxo complex in which iron has a formal oxidation number of 5. This is then reduced by the reduced form of the cosubstrate, here an osmium(II) complex, to give an oxo complex in which iron has a formal oxidation number of 4. 

Common to all hemeproteins is the iron porphyrin prosthetic group (Fig. 2.2). Four of the six coordination positions of the heme iron are occupied by nitrogens from the porph3U in ring. The other two coordination positions on either side of the heme plane are available for further ligands which strongly influence the redox potential and reactivity of the heme protein [20]. 

Hemerythrin is a respiratory protein isolated from sipun-culids (marine worms). All sipunculids examined have, in the coelomic fluid, erythrocytes loaded with the protein which in most species so far examined is octameric, but sometimes tri-meric (18, 19) and in one instance dimeric and tetrameric (20, 21). From the retractor muscle of Themiste zostericola, the protein has been characterized as a monomer (22). The monomer (23) and the subunits of the trimer (24) and octamer (25) are remarkably similar in tertiary structure, having a M.W. of about 13,500 daltons. Each subunit contains one binuclear iron site. There is no porphyrin ring and the irons are coordinated only to amino acids, some of which, as well as probably an oxy group, form the binding atoms (26). 

Figure 4 Molecular structure of O2-bound picket fence iron porphyrin complex. The iron porphyrin has four pivalamide groups forming a hydrophobic cavity for the ligand O2. The axial ligand is 1,2-dimethylimidazole. X-ray crystal structure shows the end-on and bent coordination of O2 to the heme-iron...

Iron porphyrins, the prosthetic group of the heme proteins are of course of great interest. Tetraphenylporphine, (TPPH2) (Fig. 2) is not a naturally occurring porphyrin. But because of its ease of preparation and close similarity to other porphyrins, tetraphenylporphinatoiron (TFPFe) derivatives have been extensively studied. Hemes are Fe(II) compounds such as TPPFe(II), and hemins are rive coordinate Fe(III) derivatives such as TFPFe(III) X (X = halide, pseudohalide, carboxylate, alkoxide, etc.)16). Six coordinate Fe(III) prophyrins are known as hemichromes, such as [TPPFe(III) L2] X (L = pyridine, piperidine, imidazole, etc.)16). Hemichromes are usually low spin, S = 1/2, systems 0 eff 1-9 B.M.) and hemins are of the high spin,... 

Dioxygen combines reversibly with Hb and Mb in the blood and tissues by virtue of a heme [iron(II) porphyrin] prosthetic group. The proteins bind one O2 for each Fe(Il). For this class of respiratory pigments, the Fe(II) is coordinated to the four core N atoms of the protoporphyrin (1). 

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