Non-Heme Iron-Containing Proteins

The preceding sections of Chapter 7 have discussed iron-containing proteins and enzymes having a porphyrin ring system. Section 7.9 presents a short introduction to the many non-heme iron-containing proteins and enzymes. Two of these are iron-sulfur proteins (Section 7.9.2) and iron-oxo proteins (Section 7.9.3). 

---

The configuration of the localized iron binding area in adrenal and testis non-heme iron proteins could be extensively studied by measuring the optical rotatory dispersion (ORD) and circular dichroism (CD). The ORD properties of various non-heme iron containing proteins were reported by Vallee and Ulmer (67). 

Some pyridine-containing ligands of this type have been used to mimic the protein environment in non-heme iron metal proteins. The ligands L (10 and 11) tend to bind strongly to five positions of the coordination sphere leaving the sixth position available to bind unidentate ligands X [FeLX]w+. 

One large class of non-heme iron-containing biomolecules involves proteins and enzymes containing iron-sulfur clusters. Iron-sulfur clusters are described in Sections 1.7 (Bioorganometallic Chemistry) and 1.8 (Electron Transfer) as well as in Section 3.6 (Mossbauer Spectroscopy). See especially Table 3.2 and the descriptive examples discussed in Section 3.6.4. Iron-sulfur proteins include rubredoxins, ferrodoxins, and the enzymes aconitase and nitrogenase. The nitrogenase enzyme was the subject of Chapter 6 in the hrst edition of this text—see especially Section 6.3 for a discussion of iron-sulfur clusters. In this... 

Active Site Structure of Rubredoxin There are several non-heme iron-sulphur proteins that are involved in electron transfer. They contain distinct iron-sulphur clusters composed of iron atoms, sulphydryl groups from cysteine residues and inorganic or labile sulphur atoms or sulphide ions. The labile sulphur is readily removed by washing with acid. The cysteine moieties are incorporated within the protein chain and are thus not labile. The simplest type of cluster is bacteria rubredoxin, (Cys-S)4 Fe (often abbreviated FelSO where S stands for inorganic sulphur), and contains only non labile sulphur. It is a bacterial protein of uncertain function with a molecular weight of 6000. The single iron atom is at the centre of a tetrahedron of four cysteine ligands (Fig.). 

There are several non heme iron-sulfur proteins that are involved in electron transfer. They have received considerable attention in the last few years. They contain distinct iron-sulfur clusters composed of iron atoms, sulfhydryl groups from cysteine residues, and inorganic or labile sulfur atoms or sulfide ions- The latter are readily removed by washing with acid ... 

All non-heme iron containing ribonucleotide reductases are also inhibited by hydroxyurea and related hydroxamates, while the adenosyl-cobalamin-dependent reductases are not affected (27, 156). The inhibition by these reagents can be partially reversed by excess Fe+2 or dithiols. Reaction of ribonucleotide reductase of E. coli with [14C]hydro-xyurea inactivated only the B2 subunit and this inactivation was not reversed by removal of the radioactivity (157). Inactivation by hydroxyurea does not affect the iron content of protein B2, but involves the destruction of the stable free radical (66,67). Reactivation can be accomplished by removal of the iron and reconstitution of apoprotein B2 with Fe+2. Hydroxyurea has been demonstrated to be a powerful radical scavenger in another system (158). 

Hemosiderin, a mammalian non-heme iron storage protein with a similar function to ferritin. It contains iron oxyhy-droxide cores similar to those of ferritin, and it has been reported that these cores are present as large, dense, membrane-bound aggregates in vivo. It is assumed that hemosiderin is produced by lysosomal degradation of ferritin or possibly of ferritin polymers. Hemosiderin is deposited in the liver and spleen, especially in diseases such as pernicious anemia or hemochromatosis. The deposits are yellow to brown-red pigments. The iron content of hemosiderin is about 37%. Nonheme iron is also abundantly present in the brain in different forms. In the so-called high-molecular-weight complexes, iron is bound to hemosiderin and ferritin. The total amount of iron may differ in health and disease [F. A. Fischbach et al, J. Ultrastruct. Res. 1971, 37, 495 M. P. Weir, T. J. Peters, Biochem.J. 1984, 223, 31]. 

In the past 15-20 years, several studies have been performed on mononuclear non-heme iron(II) enzymes. Among them, Rieske oxygenases constitute a relevant and paradigmatic example of mononuclear non-heme iron(II) proteins involved in O2 activation reactions. Their efficiency and versatility are even greater than those of the related heme-containing Cyt P450, being able to catalyze stereoselective... 

A component of the ribotide reductase complex of enzymes, protein Ba, has been shown to contain two non-heme iron atoms per mole (77). This enzyme plays a vital, albeit indirect, role in the synthesis of DNA. Curiously, the lactic acid bacteria do not employ iron for the reduction of the 2 hydroxyl group of ribonucleotides. In these organisms this role has been assumed by the cobalt-containing vitamin Bi2 coenzyme (18). The mechanism of the reaction has been studied and has been shown to procede with retention of configuration (19). 

The hemerythrin of Golfingia gouldii consists of eight subunits, each of which contains two iron atoms, in a protein with molecular weight 108,000. Spectral and magnetic data point to an oxo-bridged structure around the non-heme iron atom (99). Protein B2 of ribotide reductase of E. coli has some properties in common with hemerythrin presumably a protein corresponding to that of E. coli reduces ribotides in animal tissues, a conclusion based on probes with inhibitors. 

The state of knowledge on iron-sulfur proteins which contain non-heme iron bonded to sulfur ligands (cysteinyl residues from the protein and inorganic sulfur) has been reviewed by several authors258"264. With magnetic resonance techniques it has been possible to obtain detailed information on the nature of the active site in many of these proteins. The contributions from ENDOR have recently been summarized by Sands265 so that we shall only give an outline of the crucial points. 

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. 

The fourth chapter offers the perspectives of James B. Howard and Douglas C. Rees on non-heme iron protein chemistry. Section I of this chapter presents a particularly broad and accessible summary of iron-containing proteins, and subsection B gives a quite general discussion of experimental methods for characterizing metalloproteins which will be helpful to newcomers to the field. 

---