Iron protoporphyrin structure

Cytochromes were first named and classified on the basis of their absorption spectra (Figure 21.9), which depend upon the structure and environment of their heme groups. The b cytochromes contain iron—protoporphyrin IX (Figure 21.10), the same heme found in hemoglobin and myoglobin. The c cytochromes contain heme c, derived from iron-protoporphyrin IX by the covalent attachment of cysteine residues from the associated protein. UQ-cyt c... 

FIGURE 21.10 The structures of iron protoporphyrin IX, heme c, and heme a. 

Figure 8.1 Heme (iron protoporphyrin IX) structure. The most frequently observed cleavages in LDMS of the intact species are denoted. Heme elemental composition is C34H32N404Fe monoisotopic molecular mass is 616.176 Da average molecular mass is 616.487 Da.

Respiratory pigments similar to the vertebrate haemoglobins have also been identified in many invertebrates. These vary from small proteins with two Fe-porphyrin units to large molecules containing up to 190 Fe-porphyrin units. Myoglobin, the 02 storage protein in muscle tissue, is also a small iron-protoporphyrin protein. The crystal structures of this and a number of other porphyrin proteins are now known (Chapter 20.2, Table 11). 

In 1989, BH4 was found to be a cofactor for nitric oxide synthase (NOS) [ 126, 127]. BH4 is also involved in dimerization of NOS, as NOS is catalytically active in a homodimer structure. Three isoforms of NOS exist neuronal NOS (NOS 1), inducible NOS (NOS 2) and endothelial NOS (NOS 3). BH4 is essential for all NOS isoforms. The NOS isoforms share approximately 50-60% sequence homology. Each NOS polypeptide is comprised of oxygenase and reductase domains. An N-terminal oxygenase domain contains iron protoporphyrin IX (heme), BH4 and an arginine binding site, and a C-terminal reductase domain contains flavin mononucleotide (FMN), and a reduced nicotin-amide adenine dinucleotide phosphate (NADPH) binding site. 

Fig. 5.1 The structure of iron protoporphyrin IX, the prosthetic heme group of the peroxidases except where it is modified by the formation of covalent bonds to the methyl or vinyl groups. The a, (3-, y-, and 5-meso-carbon atoms, defined with respect to the pattern of the ring substituents, are labeled...

Figure 3. Structures of iron protoporphyrin IX (protoheme IX) (left) and iron mesoporphyrin IX (mesoheme IX) (right).

FIGURE 5A. Structure of iron protoporphyrin IX, showing the nomenclature used in this review. 

Figure 1. Structure of protoheme IX (iron protoporphyrin IX, heme b).

Figure 4 Schematic structure of the heme group (iron protoporphyrin IX) found at the active site of Mbs...

Microperoxidase-11, MP-11, heme-undecapeptide, prepared by pepsin digestion of cytochrome c, retains the proximal His ligand and two thioether bonds between iron-protoporphyrin IX and two Cys residues. The peroxidase activities of MPs have been demonstrated in the oxidation of so-called peroxidase s substrates, phenolic compounds, in the presence of H2O2. MPs are unique and simple hemoprotein which are not restricted by consideration of the apoprotein structure. In this study, cytochrome P-450-hke reactivities of MP-11, sulfide oxidation, N-dealkylation, and olefin epoxidation, were... 

Fig. 29.10 The structure of cytochrome P-450 from the bacterium Pseudomonas putida. The structure was determined for cytochrome P-450 complexed with (15)-camphor, but this is omitted from the figure. The protein chain is shown in ribbon representation, with cysteine (Cys) residues highlighted in stick representation. One Cys residue is bound to the Fe(III) centre of the iron protoporphyrin(IX) unit (shown in a space-filling representation with colour code Fe, green O, red C, grey N, blue S, yellow).

The active site in a cytochrome P-450 is a haem unit. An iron protoporphyrin(IX) complex (Fignre 29.7b) is covalently bound to the protein through an Fe cystrinc bond. This has been confirmed crystallographically for cytochrome P-450 complexed with (15)-camphor (Figure 29.10). The active site contains a 5-coordinate Fe(ni) centre, schematically represented by structure 29.16. In its rest state, cytochrome P-450 contains a low-spin Fe(III) centre. Carbon monoxide adducts of cytochromes P-450 absorb at 450 nm and this is the origin of the name of the enzyme. It is proposed that the catalytic cycle for the conversion of RH to ROH follows the sequence of steps ... 

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