Electron transfer non-hemes

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. 

Excellent reviews in this field have recently appeared (12, 34) and general discussions of the biological reactions and chemical nature of these electron-transferring non-heme iron proteins were held at recent symposia (56, 71). This paper is intended to be a review of recent works in our laboratory pertaining to non-heme iron protein serving as an electron transfer intermediate in steroid hydroxylation in mammalian glands. 

There are many questions to be solved in future studies on the electronic state of the iron in electron-transferring non-heme iron protein. Magnetic susceptibility measurement is one hopeful approach to ascertaining the state of the iron. 

It has always been assumed that these simple proteins act as electron-transfer proteins. This is also a fair conclusion if we take in account that different proteins were isolated in which the Fe(RS)4 center is in association with other non-heme, non-iron-sulfur centers. In these proteins the Fe(RS)4 center may serve as electron donor/ac-ceptor to the catalytic site, as in other iron-sulfur proteins where [2Fe-2S], [3Fe-4S], and [4Fe-4S] clusters are proposed to be involved in the intramolecular electron transfer pathway (see the following examples). 

To explain how H+ transfer occurred across the membrane Mitchell suggested the protons were translocated by redox loops with different reducing equivalents in their two arms. The first loop would be associated with flavoprotein/non-heme iron interaction and the second, more controversially, with CoQ. Redox loops required an ordered arrangement of the components of the electron transport system across the inner mitochondrial membrane, which was substantiated from immunochemical studies with submitochondrial particles. Cytochrome c, for example, was located at the intermembranal face of the inner membrane and cytochrome oxidase was transmembranal. The alternative to redox loops, proton pumping, is now known to be a property of cytochrome oxidase. 

Although electron transfers in biological systems are generally expected to be non-adiabatic, it is possible for some intramolecular transfers to be close to the adiabatic limit, particularly in proteins where several redox centers are held in a very compact arrangement. This situation is found for example in cytochromes C3 of sulfate-reducing bacteria which contain four hemes in a 13 kDa molecule [10, 11], or in Escherichia coli sulfite reductase where the distance between the siroheme iron and the closest iron of a 4Fe-4S cluster is only 4.4 A [12]. It is interesting to note that a very fast intramolecular transfer rate of about 10 s was inferred from resonance Raman experiments performed in Desulfovibrio vulgaris Miyazaki cytochrome Cj [13]. 

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... 

It should be noted that non-heme oxygenases can also degrade aromatics such as biphenyls and naphthalene (Scheme 7.23). A naphthalene dioxygenase consists of a catalytic oxygenase component with a mononuclear iron site, an iron-sulfur flavoprotein reductase and an iron-sulfur ferredoxin transferring electrons from... 

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. ... 

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 ... 

A different type of concerted reaction involves the bacterial cytochrome c peroxide, where two hemes are coupled together, so that hydrogen peroxide undergoes a two-electron reduction to water without the formation of radical species. In a number of dioxygenases, dioxygen is reduced to peroxide by concerted electron transfer from [2Fe-2S] and non-heme Fe11 centres. 

Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique to explore the electronic state of iron complexes. EPR spectroscopy of the non-heme iron component in the electron transfer system of mitochondria has been extensively used and discussed by Beinert (9), who showed that this type of iron has a so-called g = 1.94 type signal upon reduction. Consideration of the EPR spectrum of adrenodoxin has been described previously (68). 

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