Heme Iron Proteins

A novel cytochrome b model compound containing two heme binding sites was tested by EPR and Mossbauer spectroscopy. Both sites were found to be occupied in a 1 1 stoichiometry. For one of the sites a normal b-hemichrome EPR signal with rhombic -tensor symmetry was found as expected for a bis-histidine ligation with parallel histidine planes. For the other site unusual EPR features (e.g. axial y-tensor) were obtained which were ascribed to a configuration with perpendicular histidine planes.267 A new type of cytochrome b was described for a preparation from the cytoplasmic fraction of an archaeon, Acidianus ambivalens. This is thus the first soluble cytochrome found in this 

Several studies have dealt with the influence of lipids on conformational equilibria in cytochrome c via hydrophobic and electrostatic interactions. The binding of sodium dodecyl sulfate monomers and micelles was reported to cause a transition of cytochrome c to a state B2 which is of potential physiological relevance. The interplay between heme only state changes and secondary structure changes was analyzed by freeze-quench and stopped-flow experiments.276 The response of the heme spin state to lipid acyl chains in cytochrome c was 

The peroxidase reaction is coupled with the formation of free radicals, either directly at the protein backbone or at the porphyrin moiety or both. This topic is borderline to the scope of this review and we will restrict ourselves to those studies which have at least a strong connection with the heme iron situation. We mention however, some relevant articles which have appeared in the period reviewed. Specific interest was given to a bi-functional enzyme from Mycobacterium tuberculosis which has both catalase and peroxidase activity.286-287,288-289 We also mention that a critical role of cations like Ca2+ and K+ has been described.290-291 

Heme containing proteins are also involved in nitrite reduction. Cytochrome 

The method of cryoreduction was applied to oxy-ferrous hemoproteins. In one study, the monomeric hemoglobin from Glycera dibranchiata was used. This hemoglobin is different from other hemoglobins not only due to its monomeric 

Cytochrome C550 is part of the cyanobacterial photosystem II (PSII) and has an unusually low reduction potential. Attempts have been made to unravel the major parameters determining this value by working with isolated protein from Synechocystis. Both EPR and other spectroscopic measurements of the isolated cytochrome and of PSII fragments showed that the heme group is not very different from that of other cytochromes c in both samples. However, electrochemical experiments led the authors to propose that the degree of solvent 

In photosystem II (PSII) of plants there is an electron-transfer pathway with low quantum yield bringing electrons from cytochrome 559 to the photo-oxidized pigment via chlorophyll Z and a carotenoid. Details of this 

During the reaction of cytochrome P450, the oxyferrous, camphor-bound form associates in a binary complex with putidaredoxin. This interaction, which can be monitored well in the reduced putidaredoxin, has been reviewed and the roles of structural changes observed with regard to the camphor monooxygenation are discussed. This complex was also studied by Resonance Raman and EPR spectroscopies in an oxycytochrome P450 mutant, which is disturbed in the proton delivery path. It was found that the electron transfer from putidaredoxin to P450 is also blocked.  

A review has been written on the topic of synthetic active-site analogues of heme-thiolate proteins. As well as cytochrome P450, models for chloro-peroxidase are dicussed. Alkanethiolate-coordinated iron porphyrins and their dioxygen adducts have been synthesized and characterized as P450 models.  

In a more general context of hemoproteins some further studies appear worth mentioning. A coral allene oxide synthase has been characterized which employs a heme in the conversion of 8R-hydroperoxyeicosatetraenoic acid into the corresponding allene oxide. EPR of the ferric enzyme and its cyanide and azide complexes strongly suggested tyrosinate ligation, as in catalase, but the access of small molecules to the heme as well as the interaction with the protein environ- 

Myoglobin, a single-heme system, and hemoglobin, a multiheme system, both contain an identical heme prosthetic group however, their O2 binding properties are significantly different, as is the second coordination shell of iron. Consequently, this leads these two important proteins to have different 

In the sections that follow we will describe representative spectroelectrochemical investigations of different Hpxs, Mbs and Hbs and we will discuss the observed differences in the various parameters obtained from a redox analysis of these heme proteins. 

The chemistry and biochemistry of Hpx has been reviewed and a crystal structure is available. Hemopexin is present in serum at about 10 pM and its primary function is to transport released heme to its degradation site in the parenchymal cells of the liver via receptor-mediated endocytosis. Encapsulation of a single heme by Hpx occurs via bis-histidyl protein side-chain coordination of the Fe. Spectroelectrochemical investigation of the heme-Hpx assembly gives insight into the role of Hpx in controlling the reduction potential of the heme Fe, the efficiency of electron transfer at the metal centre, the influence of bis-histidyl coordination at the Fe centre, and the possible role of Fe redox in the Hpx-mediated transport and recycling of heme. 

The influence of pH on the formal reduction potential was also examined using the spectroelectrochemical technique. The 1/2 value for MHpx at pH 

2 in the presence of 0.2 M KCl was shifted from 6 mV to 105 mV on dropping the pH to 5.5. Therefore, as the pH is lowered the ease of Fe reduction is increased. The Nernst equation predicts a 59 mV shift in 1/2 for each pH unit change for a redox equilibrium that involves a single proton. Consequently, the observed 100 mV positive shift in 1/2 that occurs with a decrease in pH from 

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W. A. Bulen, J. R. LeComte, R. C. Bums, and J. Hinkson, in A. San Pietro, ed., Non-Heme Iron Proteins Role in Lnerg Conversion Antioch Press, Yellow Springs, Ohio, 1965, p. 261. 

Biochemical aspects of Fe—S linkages in non-aqueous heme iron proteins with special reference to Andrenodoxin. T. Kimura, Struct. Bonding (Berlin), 1968, 5, 2-40 (72). 

Ruettinger RT, GR Griffith, MJ Coon (1977) Characteristics of the u-hydroxylase of Pseudomonas oleovorans as a non-heme iron protein. Arch Biochem Biophys 183 528-537. 

Kimura, T. Biochemical Aspects of Iron Sulfur Linkage in None-Heme Iron Protein, with Special Reference to Adrenodoxin . Vol. 5, pp. 1-40. 

Hydrogenase and other components of the N2 fixing apparatus of bacteria have been shown to be non-heme iron proteins (66). 

The important biochemical role played by this type of (non-heme) iron proteins has stimulated efforts to synthesise Fe4S4 complexes bound to thiolate groups. For instance, Figure 10 shows the voltammetric behaviour of two such complexes, [Fe4S4(SPh)4]2 and [Fe4S4(SBut)4]2-.la... 

Mortenson, L.E. Nitrogen fixation in extracts of Clostridium pasteurianum. In Non-heme Iron Proteins Role in Energy Conversion,... 

To the broader biochemical community the term non-heme iron proteins has frequently suggested a limited group of low-molecular-weight proteins confined to electron transfer between enzymes in a limited number of reactions, such as nitrogen fixation and photosynthesis. ... 

In addition to their varied biological roles, non-heme iron proteins contain a magnificent assortment of iron sites having a multitude of chemical and structural properties. Indeed, the catalog of iron centers is a bit like the taxonomy of insects—a seemingly limitless variation of a few structural themes, yet each new form sufficiently different to define a new species. It is beyond the scope of any review of non-heme iron proteins to be inclusive, and there are excellent recent reviews which detail selected topics. Rather, it is our intention to provide in one chapter an overview of the major classes with an emphasis on proteins for which a crystal structure is available. This review begins with a survey of the types of protein iron structures and a discussion of some methods and problems associated with establishing the iron center type. This should provide an introduction to readers less familiar with the area. Sections II to IV include the current status and recent developments for a limited number of proteins from the major iron classes. These have been chosen in the subjective vein of a limited review the omission of a topic does not indicate its relative importance or interest, only the limitation of space. The purpose of this section is to emphasize the diversity of iron center structures and functions. 

Fig. 1. Iron center types found in non-heme iron proteins.

A representative sampling of non-heme iron proteins is presented in Fig. 3. Evident from this atlas is the diversity of structural folds exhibited by non-heme iron proteins it may be safely concluded that there is no unique structural motif associated with non-heme iron proteins in general, or even for specific types of non-heme iron centers. Protein folds may be generally classified into several categories (i.e., all a, parallel a/)3, or antiparallel /8) on the basis of the types and interactions of secondary structures (a helix and sheet) present (Richardson, 1981). Non-heme iron proteins are found in all three classes (all a myohemerythrin, ribonucleotide reductase, and photosynthetic reaction center parallel a/)8 iron superoxide dismutase, lactoferrin, and aconitase antiparallel )3 protocatechuate dioxygenase, rubredoxins, and ferredoxins). This structural diversity is another reflection of the wide variety of functional roles exhibited by non-heme iron centers. 

Despite the lack of structural similarities among non-heme iron proteins, there are several themes common to the iron center environments in different proteins. These themes should be taken as rough generalizations present in at least several of the characterized non-heme iron proteins, but they certainly are not fundamental laws of nature that are always obeyed. 

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