Iron proteins non-heme

As already mentioned above, the class of non-heme iron proteins is inhomogeneous and therefore often divided into subclasses. A distinction can be made based on the ligands that coordinate to the iron center and concomitantly on the types of potential iron cofactors. In most catalytically active non-heme iron proteins, the metal ions are coordinated via nitrogen and oxygen ligands. These types of proteins will be discussed in this chapter. 

Due to the choice of nitrogen- and oxygen-containing ligands, the metal ions are found mostly in an electronic high-spin state in all of the proteins. In the following, a few classes of non-heme iron proteins will be discussed on the basis of selected examples of representative enzymes. Where appropriate, mechanistic details obtained from model structures will also be reviewed. 

Recent years have witnessed increasing interest in the biology, chemistry, and physics of electron-transferring non-heme iron proteins. This class of protein serves as an oxidation-reduction component in various biological functions involved in anaerobic fermentative metabolism, photosynthesis, and hydroxylation reactions. 

The electron transferring non-heme iron proteins can be strictly differentiated from those non-heme iron proteins and polypeptides such as ferritin and ferrichrome which act in biological transfer and storage of iron. They can be distinguished also from iron-flavoproteins, such as succinic dehydrogenase, which contain flavin in addition to the iron constituent. Nevertheless, in many chemical and physical aspects, the non-heme iron moiety of the iron-flavoproteins exhibits behavior similar to that of electron-transferring non-heme iron proteins. 

Bacteria Ferredoxin (Clostridia), High potential iron protein  

Plant Chloroplast ferredoxin or PPNR (green plants, algae, 

Animal Adrenodoxin (adrenal cortex, testis, ovary) 

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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. 

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. 

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. 

Spectroscopic Methods to Characterize Metal Centers in Non-heme Iron Proteins... 

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