Iron protoporphyrin IX

HEMOPROTEINS. These proteins are actually a subclass of metalloproteins because their prosthetic group is heme, the name given to iron protoporphyrin IX (Figure 5.15). Because heme-containing proteins enjoy so many prominent biological functions, they are considered a class by themselves. 

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.

The second ligninolytic enzyme, MnP, has been identified (U), purified and characterized (13-16). MnP is an H O -dependent heme glycoprotein (M -46,000) with an iron protoporphyrin iX prosthetic group. MnP catalyzes the Mn -dependent oxidation of a variety of phenols and phenolic lignin model compounds (6,8.13-15,17). Electronic absorption... 

Iron dioxygen complexes, O NMR spectroscopy, 185 Iron-protoporphyrin IX... 

The cytochromes P-450 monooxygenase system is actually a collection of isoenzymes, all of which possess an iron protoporphyrin IX as the prosthetic group. The monomer of the enzyme has a molecular weight of 45,000 to 55,000. The enzyme is membrane bound within the endoplasmic reticulum. Cytochromes P-450 are closely associated with another vital component of the system, NADPH cytochrome P-450 reductase. This is a flavoprotein, which has 1 mol of FAD and 1 mol of FMN per mol of apoprotein. The monomeric molecular weight of the enzyme is 78,000. The enzyme transfers two electrons to cytochromes P-450, but one at a time. There only seems to be one reductase, which serves a group of isoenzymes of cytochromes P-450, and consequently, its concentration is 1/10 to 1/30 that of cytochromes P-450. 

Iron protoporphyrin IX (heme) is found in the b-type cytochromes and in hemoglobin and myoglobin. In heme c, cysteine residues of the protein (R) are attached covalently by thioether links to the two vinyl (—CH=CH2) groups of protoporphyrin IX. Heme c is found in the c cytochromes. In heme A, which is found in the a cytochromes, a 15-carbon isoprenoid side chain is attached to one of the vinyls, and a formyl group replaces one of the methyls. 

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. 

Figure 3 Formula of the hemes present in cytochromes a and b. Left iron-protoporphyrin IX of cytochromes b. Right heme of cytochromes a (C17H290 =...

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...

LiP has been purified to electrophoretic homogeneity by a combination of anion exchange chromatography and gel filtration (26-28). The enzyme is present as a series of Isozymes (28,29) with pi ranging from 3.2 to 4.0. LIP contains one mole of iron protoporphyrin IX per mole of enzyme and is a glycoprotein of molecular mass 41,000. 

Although most catalases contain the iron-protoporphyrin IX prosthetic group, it has been known for over 30 years that some bacteria are able... 

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

In many of the haemoproteins we shall be discussing, the protoporphyrin IX group is held to the polypeptide chain only by hydrogen bonding, Van der Wools forces and iron-protein bonds. In several other cases, notably in cytochrome-c and its related compounds, the haem is covalently linked to the protein via substituents at the pyrrole carbon atoms. Cyto-chrome-c can be regarded as an iron protoporphyrin IX group with the addition of a protein cysteine side-chain across the vinyl double bonds giving two thio-ether links (Fig. 3). 

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

Figure 4 Iron protoporphyrin IX (heme b), the prosthetic group...

The catalases catalyze the disproportionation of hydrogen peroxide (equations). Most catalases contain the iron-protoporphyrin IX prosthetic group see Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases). However, some bacteria are able to synthesize catalases that are not inhibited even by millimolar concentrations of azide and cyanide, suggesting that some catalases are nonheme enzymes it is now known that these enzymes possess a dinuclear Mn active site. 

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