Uptake transport distribution

Plants show a taxonomic distinction in the way they mobilize iron from the rhizosphere and transport it across the plasmalemma (Brown and Jolley, 1989; Romheld and Marschner 1986a,b; Marschner et al. 1986a). Iron deficiency induces two distinct and mutually exclusive mechanisms or strategies for iron acquisition (Bienfait, 1988; Marschner and Romheld, 1994); one by the dicotyledons and monocotyledons except the graminaceae (Strategy I); and the other by the members of the graminaceae (Strategy II). Strategy I plants use a reduction-based uptake mechanism. Roots of plants belonging to this strategy reduce external Fe3+ to Fe2+ and transport iron across the plasmalemma only in the reduced form (Fe2+). Strategy II plants lack this mechanism or express it at a very low level (Zacharieva and Romheld, 2001). They mobilize iron from the rhizosphere by producing and releasing ferric (Fe3+) solubilizing compounds termed phytosiderophores (Takagi, 1976; Romheld and Marschner, 1986). The phytosiderophores, exemplified by mugineic acids, are non-proteinogenic amino acids produced in response to iron deficiency. Strategy II plants take up iron in the form of Fe3+ chelates of mugineic acids.

Strategy 1 Plants

Strategy I plants, adapted to reduction-based uptake of iron, respond to iron deficiency stress by inducing increased reduction of soluble Fe3+ to Fe2+. Several morphological and physiological changes in roots have been suggested to contribute to the increased reduction of iron. The more important of these are:

(a) Changes in root morphology, such as formation of transfer cells (Kramer et al. 1980; Romheld and Marschner, 1981b; Landsberg, 1982, 1986; Romheld and Kramer, 1983).

(b) Exudation of organic acids (Tiffin, 1966).

(c) Enhanced secretion of reducing substances (Brown, 1978; Hether et al. 1984).

(d) Enhanced proton extrusion (Brown, 1978; Landsberg, 1989; Alcantara et al. 1991; Rabotti and Zocchi, 1994; Rabotti et al. 1995).

(e) Reduction of Fe3+ to Fe2+ at the plasmalemma (Chaney et al. 1972; Bienfait et al. 1983; Briiggemann et al. 1990; Alcantara et al. 1991).

Bienfait (1985) first showed that reduction of extracellular Fe (III) chelate to Fe (II) chelate is catalyzed by the Fe (III) chelate reductase or a 'turbo' reductase, induced in the plasmalemma of root epidermal cells in response to iron deficiency stress. Reduction of iron by Fe3+ chelate reductase has since been established as the first step (Marschner and Romheld, 1994) and the prime factor essential for uptake of iron by strategy I plants. (Moog and Bruggemann, 1995; Yi and Guerinot, 1996). Yi and Guerinot (1996) presented genetic evidence to show that induction of root Fe (III) chelate reductase activity is essential for iron uptake under conditions of iron deficiency. Arabidopsis mutants frd-l,frd-2 and frd-3 (Ferric Reductase Defective 1,2 and 3) that lack the ability to induce Fe3+ chelate reductase activity show up the defect in uptake of iron (Yi and Guerinot, 1996). Similarity between iron uptake mechanisms in yeast, which are fairly well understood, and higher plants paved the way for characterization of several putative iron transporters in plants. Taking the advantage of sequence similarity to yeast Fe3+ chelate reductases FRE1 and FRE2, Robinson et al. (1999) made the first characterizatiqn of Fe3+ chelate reductase gene from Arabidopsis. This was named FR02 (Ferric Reductase Oxidase 2) and, on the basis of sequence similarity with the human phagocyte NADPH gp 91-phox oxidoreductase, identified as a member of the flavocytochrome superfamily. FR02 encodes the integral membrane Fe3+ chelate reductase, which transfers electrons from the cytosolic NADPH to the enzyme bound FAD through the haem groups to Fe3+ on the opposite (extracellular) side of plasmalemma resulting in reduction of Fe3+ to Fe2+. It has been shown that At FR02 is expressed in roots and its mRNA levels are upregulated by iron deficiency (Robinson et al. 1999). Loss of function mutations of FR02 show a decrease in Fe3+ chelate reductase activity, produce chlorosis and inhibit plant growth under iron deficiency; FR02 expression in iron-uptake defective Arabidopsis mutantfrd-1-1 restores Fe3+ chelate reductase activity. Another Fe3+ chelate reductase gene FROl has been cloned from roots of iron deficient pea (cv. Sparkle) (Waters et al. 2002). This gene Ps FROl shows close sequence similarity to At FRO2 and its mRNA levels are correlated with Fe3+ chelate reductase activity. Even though most abundant in roots, more so in the outer epidermal cells, Ps FROl is also expressed in leaves and root nodules suggesting its possible role in whole plant distribution of iron. (Waters et al. 2002).

The contributions of the morphological and physiological changes associated with enhanced reduction of iron in strategy I plants to iron uptake have been a subject of discussion (Jolley et al. 1996; Ma and Nomoto, 1996; Pearson and Rengel, 1997; Rengel, 1999; Schmidt, 1999; Curie and Briat, 2003). In a recent study, Zheng et al. (2003) reported induction of root Fe (III) chelate reductase in iron efficient red clover (Trifolium pretense L) cultivar Keenland within 24 h of exposure to iron deficiency, within which period the roots did not show any enhancement in H+ extrusion. This suggested that proton extrusion is not a pre-condition for reduction of iron. It is, however, certain that the external pH plays an important role in solubilizing the sparingly soluble Fe3+ in the rhizosphere. Maximum reduction of Fe3+ occurs around pH 5.0. High pH of the external solution, including the apoplasmic fluid, inhibits Fe3+ reduction (Toulon et al. 1992; Susin et al. 1999; Manthey et al. 1996). Some iron deficiency responses are possibly regulated by hormones such as ethylene. Romera et al. (1999) reported an increase in ethylene production in response to iron deficiency and showed that it induced changes in root morphology and Fe (III) reductase activity, contributing to enhanced uptake of iron. Schmidt et al. (2000) observed enhanced production of ethylene and alterations in root morphology of iron-deficient plants of Arabidopsis, which favoured enhanced uptake of iron, but these changes had little effect on Fe (III) reductase activity.

Recently, Zaid et al. (2003) have also reported ethylene-induced changes in root morphology, such as the formation of cluster roots, that contribute to increased uptake of iron in response to iron deficiency.

Transport of iron, reduced at the external surface of the plasmalemma of strategy I plants, into the cytosol involves a high-affinity transport system. In the recent past, concerted efforts have been made to identify and characterize possible plasmalemma-bound Fe2+ transport proteins. Making use of advancements in molecular biology techniques and functional complementation of iron-uptake deficient yeast mutants, several transporter genes, belonging to different gene families, have been identified. The first iron regulated transporter IRT1 (Iron Regulator Transporter 1), belonging to the ZIP (ZRT - 1RT like Proteins) family was identified from Arabidopsis (Eide et al. 1996). The At IRT1 was shown to complement the iron uptake deficiency yeast double mutant fet3fet4. Iron deficiency has been shown to cause accumulation of At IRT1 transcript and recovery from this deficiency leads to a decline in the level of the transcript (Connolly et al. 2002). Vert et al. (2002) have shown that IRT1 mutant line of Arabidopsis is not viable unless supplemented with high iron supply. The knock-out (T-insertion) mutant of At IRT1 (irtl) develops foliar symptoms of iron deficiency and developmental defects in different plant parts (Henriques et al. 2002; Vert et al. 2002). Grotz and Guerinot (2000) have isolated another putative iron transporter gene, IRT2 from Arabidopsis and showed that it is expressed in root epidermal cells in response to iron deficiency. Eckhardt et al. (2001) have characterized two cDNAs from a library constructed from roots of iron deficient tomato plants and designated these Le IRT1 and Le IRT2. It has been shown that both Le IRT1 and Le IRT2 complement iron-uptake mutants of yeast and are predominantly expressed in roots; but only Le IRT1 is upregulated under iron deficiency. An IRT ortholog (RIT1) isolated from pea seeds has been shown to encode a protein that is 63% identical to At IRT1 and is upregulated by iron deficiency (Cohen et al. 1998). IRT1 is now considered as the main transporter involved in high-affinity iron uptake by roots of dicotyledonous (Strategy I) plants under conditions of iron deficiency (Connolly et al. 2002; Vert et al. 2002).

Some Nramp family transporters have also been suggested to be involved in uptake of iron (Belauchi et al. 1997; Curie et al. 2000; Thomine et al. 2000). It has been shown that Arabidopsis Nramps 1, 3 and 4 complement iron-transport defect in yeast mutants and are upregulated under iron deficiency (Thomine et al. 2000). Nramp 3 and Nramp 1 have recently been shown to be involved in vascular transport of iron and other co-substrates (Thomine et al. 2003; Bereczky et al. 2003). Thomine et al. (2003) showed that the Nramp 3 gene is expressed in vascular bundles of roots, stems and leaves and further suggested that the protein encoded by it plays a role in long-distance transport of iron and other co-substrates (manganese and cadmium). Bereczky et al. (2003) cloned a Nramp 1 gene from tomato (Le Nramp 1) and showed it to be specifically expressed in roots and upregulated in response to iron deficiency. Bereczky et al. (2003) showed the Le Nrampl to be localized in the vesicular parenchyma of the root of iron deficient plants and suggested that the transporter is possibly involved in the mobilization of iron in vascular tissues of plants.

While it is unequivocally accepted that the strategy 1 response is activated by limitation in iron availability, it is debatable as to whether the deficiency of iron is sensed and transformed into a signal-regulating iron uptake by the root or the shoot (Curie and Briat, 2003). Early experiments involving reciprocal grafting of the chlorotic tomato mutant T 3238 fer lacking in the ability to activate the strategy I response to the wild type showed that the fer gene evoking the iron-deficiency response is required in root and not in shoot (Brown et al. 1971; Brown and Ambler, 1973). Studies with pea mutant brz also supported this (Kneen et al. 1990). In consonance with these findings, Bienfait et al. (1987) reported that irrespective of whether potato tubers were sprouted or not, their roots showed the same iron-deficiency response. Ling et al. (1996,2002) provided genetic and molecular evidences for the root being the organ involved in perceiving deficiency of iron and activating the deficiency response. Ling et al. (1996) showed that the fer gene was required exclusively in the root. Later, they isolated the fer gene by map-bound cloning from roots of iron-deficient tomato plants and demonstrated that it encodes a regulatory protein (transcription factor) containing a bHLH domain. When the fer gene mutant, lacking in ability to activate the iron-deficiency response, was complemented with the fer gene, the ability to activate the iron-deficiency responses such as upregulation of Fe3 chelate reductase and induction of Le IRT1 expression, were restored. Ling et al. (2002) suggested that the fer gene is expressed in a cell-specific pattern at the root tip, where it senses the availability of iron and activates the iron-deficiency response through transcriptional control. Rogers and Guerinot (2002) have characterized another gene FRD3 from roots of iron-deficient Arabidopsis. The FRD3 gene belongs to the multidrug and toxin efflux (MATE) family and is detectably expressed in the roots only. It has been shown that FRD3 functions in regulation of iron uptake and homeostasis and that the FRD 3 mutants are defective in iron deficiency signalling and iron distribution (Rogers and Guerinot, 2002). In another recent study, Bereczky et al. (2003) have demonstrated that the tomato iron transporter genes Le nramp 1 and LeIRT 1 are expressed specifically in the roots and that both are downregulated in roots of the fer mutant.

Contrary to the above information, several workers (Maas et al. 1988; Romera et al. 1992; Grusak, 1995, Grusak and Pezeshgi, 1996) have presented evidence for a role of shoot in activating the iron-deficiency response. Romera et al. (1992) showed that exposure of half of the roots to an iron-deficient nutrient solution in a split-root experiment induced iron-deficiency response by the other half of the roots, suggesting signal transport through the shoot. Reciprocal grafting experiments by Grusak and Pezeshgi (1996) showed that shoot, not the root, determined the phenotypic response of the iron-accumulating pea mutants brz and dgl. These mutants possess a constitutive Fe3+ chelate reductase in roots, which can be downregulated under iron sufficient condition in response to a shoot-derived signal.

Waters et al. (2002) have shown that the pea Fe3+ chelate reductase gene FROl is differentially regulated in root and shoot of the wild type and the iron-accumulating mutants brz and dgl. In both the wild type and the mutants, FROl expression and Fe3+ chelate reductase activity in shoot exhibit the iron-deficiency response. It is the same in the roots of the wild type (Sparkle), but in the iron-accumulating mutants brz and dgl, both FROl expression and Fe3+ chelate reductase activity are constitutive. These findings are in accord with the results of the reciprocal grafting experiments of Grusak and Pezeshgi (1996). The expression of FROl in roots and shoots is affected by different signals or iron-sensing mechanisms. The FROl expression in roots of the wild type is regulated by a shoot derived signal generated by the shoot iron concentration (Waters et al. 2002).

Recent findings of Zheng et al (2003) show temporal differences in the root- and shoot-generated signals regulating the iron-deficiency response. Deficiency of iron is first perceived by the root, which activates rapid (within 24 h) induction of Fe3+ chelate reductase activity associated with proton extrusion. Subsequently, the Fe3+ chelate reductase activity is upregulated in response to a shoot-derived signal, possibly transmitted from the shoot to the root through the iron concentration in the phloem, as was conceived earlier by Maas et al. (1988). There is also evidence to show the involvement of separate signals for activation of different iron-deficiency responses. Schikora and Schmidt (2001) showed that in pea mutants brz and dgl, Fe3+ chelate reductase activity is high regardless of iron supply but transfer cell formation is induced in response to iron deficiency, which suggests that separate signals may be required for induction of Fe3+ chelate reductase activity and transfer cell formation.

Strategy II Plants

Strategy II plants take up iron as ferric complexes of the mugineic acid family phytosiderophores (Fe3+-PS). The first step in the uptake of iron involves the synthesis of mugineic acids capable of chelating Fe3+- (Mori et al. 1990; Ma and Nomoto, 1996). On the basis of short-term experiments with iron-deficient barley and various chelating substances, including different metals and mugineic acids (MA), Ma et al. (1993) demonstrated that formation of MA-Fe3+- complex is a prerequisite for its recognition by the specific Fe3+- transport system in barley roots. The mugineic acids have been shown to chelate Fe3+ with their amino and carboxyl groups (Ma and Nomoto, 1996).

Mugineic acids are synthesized from methionine ( Mori and Nishizawa, 1987; Shojima et al. 1990; Ma et al. 1995) (Fig. 2.1). The first two reactions of the MA biosynthesis pathway involve the synthesis of NA from Met and are ubiquitous, in no way confined to strategy II plants. In the first reaction of the pathway, Met is converted to S-adenyl-L-methionine (SAM) by SAM synthase (ATP: methionine S-adenyl transferase). The second reaction involves the isomerization of the three molecules of SAM to one molecule of NA, and is catalyzed by nicotianamine synthase (NA synthase). The genes encoding NA synthase have been cloned and characterized from both graminaceous as well as dicotyledonous plants (Shojima et al. 1990; Higuchi et al. 1994,1999; Herbick et al. 1999; Ling et al. 1999). Further steps for the synthesis of MAs from NA are confined to graminaceous (strategy II) plants and the genes encoding the enzymes catalyzing these reactions have been cloned and characterized from plants adapted to this strategy (Okumura et al. 1994; Higuchi et al. 1999; Takahashi et al. 1999; Kobayashi et al. 2001). In a reaction catalyzed by nicotianamine aminotransferase (NAAT), the graminaceous plants convert NA to an unstable intermediate (NA-3'-oxoacid), which is rapidly reduced to 2-deoxymugineic acid (2'-DMA) involving catalysis by DMA synthase. The barley NAAT is encoded by two genes, Naat A and Naat B (Takahashi et al. 1999). Introduction of the barley NAAT genes into rice enhances the secretion of phytosiderophores and contributes to resistance of the transgenics against chlorosis in alkaline soils (Takahashi et al. 2001). Other mugineic family phytosiderophores, viz., -3-epihydroxy mugineic acid (epi-HMA) and 3-epihydroxy, 2-deoxymugineic acid (epi-HDMA) are synthesized by hydroxylation of DMA. The hydroxylation reaction is catalyzed by 2-oxoglutarate-dependent dioxygenases encoded by two genes Ids3 and Ids2. These genes have since been isolated from the roots of iron- deficient barley (Okumura et al. 1994; Nakanishi et al. 2000; Kabayashi et al. 2001).

All graminaceous species secrete MA-phytosiderophores under conditions of iron deficiency but differ in the secretion of the specific MAs (Rengel, 1999). Wheat, rice, corn and soybean genotypes are known to release only 2-DMA.Barley, oats and rye; on the other hand, they do secrete additional MAs such as 3-hydroxy mugineic acid (HMA) and 3-epihydroxy mugineic acid (epi-HMA). These MAs, being more effective in mobilizing iron than 2-DMA, contribute to greater tolerance of barley and rye to iron chlorosis. It has been suggested that young rice plants are prone to iron chlorosis because they secrete only 2-DMA, and this also ceases early, resulting in iron deficiency and consequential damage to roots (Mori et al. 1991). The release of phytosiderophores by the graminaceous plants under conditions of iron deficiency has been shown to follow a distinct diurnal rhythm (Marschner et al. 1986).

L-methionine

S-adenosyl methionine synthase

S-adenosyl methionine

Nicotianamine synthase

Nicotianamine

Nicotianamine aminotransferase

3'-oxo intermediate

Deoxymugineic acid synthase

2'-deoxy mugineic acid

Dioxygenases

3-epihydroxy mugineic acid 3-epihydroxy 2-deoxy mugineic acid

Fig. 2.1. Biosynthetic pathway of mugineic acid phytosiderophores (After Takahashi et al. 2003)

Information about the mechanism involved in secretion of MA-phytosiderophores into the rhizosphere, wherein they solubilize the sparingly soluble Fe3+ by chelation, is rather limited. According to Sakaguchi et al. (1999), the MA-phytosiderophores having high affinity for Fe3+ are secreted through the plasmalemma via anion channels involving Na+/MA symport. Blocking of anion channels in barley root plasma membrane has been shown to result in a sharp decline in secretion of MAs. Based on DNA micro-array of gene expression in rice, Negishi et al. (2002) have suggested the possible involvement of vesicles derived from endoplasmic reticulum in secretion of phytosiderophores under conditions of iron- deficiency. Microbial siderophores in the rhizosphere are also reported to contribute to Fe3+ mobilization by plants, particularly when iron supply is limiting (Zhang et al. 1991c; Masalha et al. 2000).

Uptake of ferric-phytosiderophore complex (Fe3+-PS), in which form iron is transported across the plasmalemma in graminaceous (Strategy II) plants, also involves a high-affinity transport system (Marschner and Romheld, 1994; Curie and Briat, 2003). Initial information about Fe3+-PS transporters came from a study of the iron-deficiency response of iron-uptake defective maize mutant ysl. The iron uptake defect in ysl was caused by a defect in the uptake of Fe3+-PS (Von Wiren et al. 1994). This implied that the Fe3+-PS transporter was encoded by YS1. Proof of this came from cloning of YS1 gene (Curie et al. 2001). Molecular characterization of YS1 showed it to belong to a subclass of oligopeptide transporter (opt) family. It was shown that YS1 is expressed both in roots and shoots and that its expression is upregulated by iron deficiency. Enhanced accumulation of the YS1 transcript in shoots led Curie et al. (2001) to propose its role in distribution of iron to the whole plant. Yamaguchi et al. (2002) have identified an iron transporter gene ID17, localized to the tonoplast in roots of iron-deficient barley plants. It has been shown that the transcript levels of ID17 in roots are strongly correlated with iron nutritional status and its expression in roots (only) is upregulated by iron-deficiency. Yamaguchi et al. (2002) have suggested ID17 involvement in iron transport across the tonoplast.

As a rule, the graminaceous plants lack in reduction-based mechanism for uptake of iron or this is expressed at a very low level (Zacharie and Romheld, 2001), but this may have exceptions. Iron-deficient plants of rice, a typical strategy II plant, has been recently shown to express a functional Fe (II) transporter system, as is characteristic of strategy I plants (Bughio et al. 2002). Bughio et al. (2002) isolated a cDNA clone from roots of iron-deficient rice plants, and showed that its product functions as an Fe2+ transporter. The cDNA clone, named Os IRT1, showed high homology to At IRT1. Its expression in iron-uptake and ferric-reductase defective yeast strain YH003 reversed the growth defect of the strain. It has also been shown that the Os IRT1 gene is predominantly expressed in roots and induced by deficiency of iron (and copper) (Bughio et al. 2002).

While there is a clear distinction between the induction of mechanisms for increased mobilization and uptake of iron in response to iron deficiency, plants adapted to the two strategies show little distinction in the uptake mechanism when iron supply is sufficiently met (adequate). Under conditions of adequate iron supply, even roots of the strategy II plants (graminaceae) can carry out Fe3+ reduction, possibly involving the same reductive system as in strategy I plants (Bagnaresi et al. 1997).

Iron transport from roots to the aerial parts takes place through the xylem, along the transpiration stream. Presence of ferric complexes with organic compounds, particularly citrate, in the xylem exudates suggested that iron is transported as ferric citrate (Tiffin, 1996; Cataldo et al. 1988). This implies that the iron taken up in the reduced form (Fe2+) is reoxidized in the cytoplasm prior to its long-distance transport (Brown and Jolley, 1989). Phloem transport of iron also takes place in an organically bound form. Maas et al. (1988) showed that phloem transport of iron takes place in the form of Fe3 complexes. The main carrier of iron in the phloem is nicotianamine (Pich et al. 1991; Becker et al. 1992; Stephan and Scholz. 1993; Stephan et al. 1996). Nicotianamine (NA), a non proteinogenic tripeptide derived from methane, chelates both Fe3+ and Fe2+ but it is only as the Fe3+ -NA chelate that iron is transported through the phloem.

Evidence based on physiological characterization of the maize mutant YS1 suggests that Fe3+ - NA transport through the phloem is mediated via a high-affinity transporter expressed in response to perception of iron-deficiency signal (Curie and Briat, 2003, references therein). In addition to a role in phloem transport of iron, nicotianamide (NA) is involved in iron homeostasis (Pich et al. 2001; Hell and Stephan, 2003). Immunochemical determination of NA in both cellular and extracellular components of leaves and root elongation zones in pea and tomato grown with varying levels of iron supply, as also in their iron-defective mutants, provide evidence to show chelation of excess iron accumulated in the vacuoles by NA (Pich et al. 2001).

Using castor bean seedlings model, Kruger et al. (2002) showed that phloem transport of iron occurs as a protein-iron complex. Essentially, all the 55Fe fed to the seedlings was found to be associated with a protein fraction of the phloem. Purification of the iron transport protein (ITP) to homogeneity and cloning of the corresponding cDNA from the phloem exudates showed sequence similarity between ITP and the stress-related late embryogenesis-abundant protein family. It was also shown that the ITP binds preferentially to Fe3+ and not Fe2+ (Kruger et al. 2002).

Mobilization and translocation of iron in strategy I plants is influenced by iron accumulation in the apoplasmic fluid. Plants adapted to this strategy show substantial accumulation of iron in the apoplasm (apoplasmic pool). When plants are subjected to iron deficiency, the apoplastic iron is mobilized and translocated to the shoot (Bienfait et al. 1985; Longnecker and Welch, 1990). In a recent report, Graziano et al. 2002) suggested a role of nitric oxide (NO) in the internal distribution of iron and its delivery to active sites of metabolism. Supply of NO to iron-deficient plants prevented the development of chlorosis and inhibitors of NO accentuated chlorosis.

2.3 ROLE IN PLANTS 2.3.1 Enzyme Action

Iron enzymes and electron carrier proteins form integral components of mitochondrial and photosynthetic electron transport systems. A large number of iron enzymes, with a wide range of substrates, catalyze an array of reactions that are essential to primary metabolism and biosynthesis of secondary metabolites. They are involved in the biosynthesis of gibberellins, ethylene and jasmonic acid, which play important roles in regulation of developmental processes and also as signalling molecules. Iron may bind to proteins either in ionic form (non-haem iron enzymes) or in an organically bound form, as iron-protoporphyrin or haem prosthetic group (haem enzymes). In the non-haem enzymes, the iron ion forms a coordinate bond with the protein amino acids. A group of membrane-bound iron enzymes has recently been found to contain a coupled binuclear iron centre (Berthold and Stenmark, 2003). In these enzymes, two iron atoms forming an oxobridge, are bound to four carboxylate and two histidine residues (Fig. 2.2). An important example of the diiron enzymes is the mitochondrial alternative oxidase.

In a large group of non-haem iron enzymes, the iron cofactor is present in the form of an iron-sulphur (Fe-S) cluster, in which iron is coordinated to sulphide ions. Three main variants of iron-sulphur clusters are shown in Fig 2.3. Different aspects of Fe-S clusters have recently been reviewed by Johnson et al. (2005). Ease of inter-conversion between the two oxidation states of iron (Fe2+ Fe3+) in the Fe-S cluster enables the Fe-S proteins to function as efficient acceptors or donors of electrons.

Iron-sulphur Clusters

Iron forms coordinate bonds with sulphide ions to forms three main type of iron-sulphur complexes (clusters):

(a) Fe-S: A single iron ion is tetrahedrally coordinated to the sulphydril groups of four cysteine residues of a protein.

(b) 2Fe-2S: Two iron ions are coordinated to two inorganic sulphide ions.

(c) 4Fe-4S: Contains four iron ions, four inorganic sulphide ions and four cysteine residues.

The Fe-S (single) clusters occur only in bacteria. Lack of one iron ion in a 4Fe-4S cluster forms 3Fe-4S clusters (succinate dehydrogenase). One of the iron ions of the iron-sulphur cluster undergoes a change in its oxidation state (Fe3t ^ Fe2"}. This enables the Fe-S cluster protein to function as acceptors or donors of electrons. Some non-haem iron enzymes, containing iron-sulphur clusters also contain an additional flavin (FMN or FAD) cofactor. Enzymes containing iron-sulphur and flavin prosthetic groups form key components of mitochondrial electron transport complexes.

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