Ferredoxins from nitrogen-fixing bacteria. Physical and chemical characterisation of two ferredoxins from Mycobacterium flavum 301.
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Amino Acids↗
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Characterization of two soluble ferredoxins as distinct from bound iron-sulfur proteins in the photosynthetic bacterium Rhodospirillum rubrum.
In an earlier investigation (Shanmugam, K. T., Buchanan, B. B., and Arnon, D. I. (1972) Biochim. Biophys. Acta 256, 477-486) the extraction of ferredoxin from Rhodospirillum rubrum cells with the aid of a detergent (Triton X-100) and acetone revealed the existence of two types of ferredoxin (I and II) and led to the conclusion that both are membrane-bound. In the present investigation, ferredoxin and acid-labile sulfur analyses of photosynthetic membranes (chromatophores) and soluble protein extracts of the photosynthetic bacteria R. rubrum and Rhodopseudomonas spheroides showed that ferredoxins I and II are primarily components of the soluble protein fraction. After their removal, washed R. rubrum chromatophores were found to contain a considerable amount of tightly bound iron-sulfur protein(s), as evidenced by acid-labile sulfur and electron paramagnetic resonance analyses. Thus, like all other photosynthetic cells examined to date, R. rubrum cells contain both soluble ferredoxins and iron-sulfur proteins tightly bound to photosynthetic membranes. The molecular weights of ferredoxins I and II from photosynthetically grown R. rubrum cells were found to be 8,800 and 14,500, respectively. Using these molecular weights, the molar extinction coefficients at 390 nm for ferredoxins I and II were determined to be 30.3 and 17.2 mM-1 CM-1, respectively. Ferredoxin I contains 8 non-heme iron and 8 acid-labile sulfur atoms per molecule; ferredoxin II contains 4 non-heme iron and 4 acid-labile sulfur atoms per molecule. Ferredoxin I was found only in photosynthetically grown cells whereas ferredoxin II was present in both light- and dark-grown cells. Ferredoxin II from both light- and dark-grown cells has the same molecular weight (14,500) and absorption spectrum and has 4 iron and 4 acid-labile sulfur atoms per molecule. Low temperature electron paramagnetic resonance spectra of oxidized and photoreduced ferredoxins I and II from R. rubrum were recorded. The EPR spectrum of oxidized ferredoxin II exhibited a single resonance line at g = 2.012. Oxidized ferredoxin I, however, exhibited a spectrum that may arise from the superimposition of two resonance lines near g = 2.012. Photoreduced ferredoxin II displayed a rhombic EPR spectrum with a g value of 1.94. Photoreduced ferredoxin I exhibited a similar EPR spectrum at a temperature of 16 K, but when the temperature was lowered to 4.5 K the spectrum of ferredoxin I changed. This temperature-dependent spectrum may result from a weak spin-spin interaction between two iron-sulfur clusters. These results are consistent with the conclusion that R. rubrum ferredoxins I and II are, respectively, 8 iron/8 sulfur and 4 iron/4sulfur proteins.
Bacterial Chromatophores↗
Comparative studies on two ferredoxins from the cyanobacterium Nostoc strain MAC.
Two ferredoxins were isolated from the cyanobacterium Nostoc strain MAC grown autotrophically in the light or heterotrophically in the dark. In either case approximately three times as much ferredoxin I as ferredoxin II was obtained. Both ferredoxins had absorption maxima at 276, 282 (shoulder), 330, 423 and 465 nm in the oxidized state, and each possessed a single 2 Fe-2S active centre. Their isoelectric points were approx. 3.2. The midpoint redox potentials of the ferredoxins differed markedly; that of ferredoxin I was --350mV and that of ferredoxin II was --445mV, at pH 8.0. The midpoint potential of ferredoxin II was unusual in being pH dependent. Ferredoxin I was most active in supporting NADP+ photoreduction by chloroplasts, whereas ferredoxin II was somewhat more active in pyruvate decarboxylation by the phosphoroclastic system of Clostridum pasteurianum. Though the molecular weights of the ferredoxins determined by ultracentrifugation were the same within experimetnal error, the amino acid compositions showed marked differences. The N-terminal amino acid sequences of ferredoxins I and II were determined by means of an automatic sequencer. There are 11--12 differences between the sequences of the first 32 residues. It appears that the two ferredoxins have evolved separately to fulfil different roles in the organism.
Amino Acid Sequence↗
Amino acid sequences of two ferredoxins from pokeweed, Phytolacca americana.
The amino acid sequences of two ferredoxins isolated from pokeweed, Phytolacca americana, were determined. Tryptic peptides of maleyl-carboxymethyl-ferredoxin I and carboxymethyl-ferredoxin II were prepared and analyzed. The large peptides were further digested with staphylococcal protease and chymotrypsin. Ferredoxins I and II were composed of 96 and 98 amino acid residues, respectively. Though ferredoxin I lacks tryptophan and methionine, ferredoxin II contains both of them. In a comparison of the amino acid sequences with those of other higher plant ferredoxins, ferredoxin I is one residue shorter than others at the carboxyl-terminus and ferredoxin II one longer than others at the amino-terminus. Ferredoxins I and II differ in 23 sites from each other and in 27 to 37 sites from other higher plant ferredoxins. This suggests that duplication of the ferredoxin gene occurred after the divergence of pokeweed from other higher plants. A phylogenetic tree including all other ferredoxins was constructed.
Amino Acid Sequence↗
Apparent oxidation-reduction potential of Clostridium acidi-urici ferredoxin. Effect of pH, ionic strength, and amino acid replacements.
The effects of pH and ionic strength on the midpoint reduction potential (Emp) of Clostridium acidi-urici ferredoxin were determined using hydrogen gas and hydrogenase. The Emp of native ferredoxin at 24-25 degrees in 0.1 M Tris-chloride buffer, pH 7.0, is--0.434 V. In the pH range examined, the Emp becomes approximately 13 mv more negative per each pH unit increase. A plot of the log of ionic strength versus the apparent Emp of ferredoxin in 0.1 M Tris-chloride buffer, pH 7.5, Was linear over the range of 1.0 to 0.01 ionic strength with Emp values of--0.414 and--0.475 V, respectively, at these extremes. This effect is the same with sodium chloride, sodium bromide, or ammonium sulfate. Potassium phosphate buffer caused a similar change, but the absolute values of Emp differed from those obtained in the presence of the other salts. This effect of pH and ionic strength on Emp may be general for clostridial-type (Fe4S4)2-ferredoxins, since the apparent Emp of Clostridium pasteurianum ferredoxin is affected in a similar manner by these two variables. The Emp of this ferredoxin in 0.1 M Tris-chloride buffer pH 7.0, is--0.405 V. Since the NH2-terminal amino acid residue, Ala1, and Tyr2 of C. acidi urici ferredoxin are near an (Fe4S4)2-cluster in the protein, the apparent Emp of derivatives that contained amino acid replacements in these two positions were determined. Under similar conditions, the Emp of most of the 13 derivatives examined, including those of [Leu2]- and[3-NH2-Tyr30]ferredoxin, is approximately the same as that of native ferredoxin. However, the Emp of [His2]ferredoxin is approximately 15 mv more positive, whereas that of [Trp2]ferredoxin is 22 mv more negative than that of native C. acidi-urici ferredoxin. Variations in sodium chloride concentration and pH also affected the apparent Emp of the derivatives. It is suggested that the changes observed in the Emp of C. acidi-urici ferredoxin are caused by protein conformational changes.
Amino Acids↗
Isolation and characterization of a rubredoxin and two ferredoxins from Desulfovibrio africanus.
Rubredoxin and two distinct ferredoxins have been purified from Desulfovibrio africanus. The rubredoxin has a molecular weight of 6000 while the ferredoxins appear to be dimers of identical subunits of approximately 6000 to 7000 molecular weight. Rubredoxin contains one iron atom, no acid-labile sulfide and four cysteine residues per molecule. Its absorbance ratio A278/A490 is 2.23 and its amino acid composition is characterized by the absence of leucine and a preponderance of acidic amino acids. The two ferredoxins, designated I and II, are readily separated on DEAE-cellulose. The amino acid compositions of ferredoxins I and II show them to be different protein species; the greater number of acidic amino acid residues in ferredoxin I than in ferredoxin II appears to account for separation based on electronic charge. Both ferredoxins contain four iron atoms, four acid-labile residues per molecule. Spectra of the two ferredoxins differ from those of ferredoxins of other Desulfovibrio species by exhibiting a pronounced absorption peak at 283 nm consistent with an unusual high content of aromatic residues. The A385/A283 absorbance ratio of ferredoxins I and II are 0.56 and 0.62, respectively. The N-terminal sequencing data of the two ferredoxins clearly indicate that ferredoxins I and II are different protein species. However, the two proteins exhibit a high degree of homology.
Amino Acid Sequence↗
Purification and characterization of the ferredoxin component of 25-hydroxycholecalciferol 1 alpha-hydroxylase.
The chick kidney mitochondrial iron--sulphur protein (ferredoxin), a component of the NADPH--cytochrome P-450 reductase functional in the 1 alpha-hydroxylation of 25-hydroxycholecalciferol, was purified to homogeneity by chromatography on DEAE-cellulose, gel filtration on Sephadex G-100 and preparative electrophoresis on polyacrylamide gel. A novel NADPH--cytochrome c reductase assay utilizing crude renal NADPH--ferredoxin reductase was used for the detection of the ferredoxin. A mol. wt. of 53 000 was determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and by Sephadex G-100 gel filtration of the 125I-labelled ferredoxin. The ferredoxin has a sedimentation constant (S 20, w) of 2.66S, an A411/A280 of 0.4, and a molar absorptivity of 7300 cm-1 . M-1. The electron-paramagnetic-resonance spectrum after reduction with Methyl Viologen and dithionite was characteristic of ferredoxins with signals at g = 1.956 and 2.025. Two iron and two labile sulphur atoms per molecule of ferredoxin were released by acid. Ouchterlony immunodiffusion tests by using goat anti-(bovine adrenal ferredoxin) antiserum showed precipitin reactions with the bovine adrenal ferredoxin and the chick renal ferredoxin as antigens, suggesting that the renal ferredoxin shares antigenic determinants(s) with the natural adrenal antigen. Amino acid analysis showed that of the total number of residues per molecule of ferredoxin, glutamic acid and aspartic acid are the most abundant residues, comprising 17 and 15% respectively.
25-Hydroxyvitamin D3 1-alpha-Hydroxylase↗
Structure and function of chloroplast-type ferredoxins.
Comparison of various chloroplast-type ferredoxin sequences, chemical and enzymic modifications, reconstitution experiments, and fluorescence measurement of chloroplast-type ferredoxins have led to the following conclusions. 1. Tyrosine, histidine, and tryptophan residues are not directly involved in the oxidation-reduction mechanism of ferredoxins. The four indispensible cysteine residues in spinach ferredoxin which constitutes a part of the iron-sulfur cluster are located at residues 39, 44. 47 and 77. Two out of six cysteine residues in Spirulina ferredoxin could be easily modified with vinylpyridine without the loss of reconstitutive ability i.e. the apoferredoxin could be converted to the holoform by the addition of iron and sulfide. 2. Spinach ferredoxin was digested with carboxypeptidase A and the terminal alanine could be removed without loss of the spectral properties of native ferredoxin. However, the removal of the terminal three residues gave rise to the loss of reconstitutive ability. 3. The amino groups of spinach ferredoxin were modified by acetic anhydride and four residues were acetylated. The acetylated preparation of ferredoxin had an unique spectrum. Upon the addition of high concentration of ions the spectrum of this derivative resembled the spectrum of native ferredoxin. Acetylferredoxin did not combine with ferredoxin-NADP reductase, but upon the addition of moderate concentrations of cations, it did bind to this enzyme.
Amino Acid Sequence↗
Regulation of the NADH and NADPH-ferredoxin oxidoreductases in clostridia of the butyric group.
NADH and NADPH-ferredoxin oxidoreductases have been studied in Clostridium acetobutylicum, Cl. tyrobutyricum and Cl. pasteurianum. The study of the distribution and regulation of these enzymatic activities in well-defined culture conditions, reveals that the essential function of NADPH-ferredoxin oxidoreductase is to produce NADPH, while NADH-ferredoxin oxidoreductase can, depending on cellular conditions, produce or oxidize NADH. When these Clostridia use glycolysis, regulation of the NADH-ferredoxin oxidoreductase by acetyl-CoA (obligatory activator of NADH-ferroxin reductase activity) and by NADH (competitive inhibitor of ferredoxin-NAD+ reductase activity) allow the enzymes to function correlatively with glyceraldehyde-3-phosphate dehydrogenase and thus control the levels of NAD+ and NADH in the cell. In Cl. tyrobutyricum and Cl. pasteurianum, the ferredoxin-NADP+ reductase activities are regulated by NAD+ and NADH in accordance with the intracellular concentrations of these coenzymes. In Cl. tyrobutyricum growing on pyruvate/acetate, NADH and NADPH-ferredoxin reductase activities cannot be detected; only the ferredoxin-NAD+ and ferredoxin-NADP+ reductase activities are found. In this Clostridium, regulation of the ferredoxin-NADP+ reductase activity is the same whether it is grown on glucose or pyruvate. Contrary to this, the ferredoxin-NAD+ reductase activity undergoes a drastic change, since NADH no longer controls the enzymatic activity. In this case regulation is no longer necessary, since glyceraldehyde-3-phosphate dehydrogenase does not function.
Butyrates↗
Horsetail (Equisetum arvense) ferredoxins I and II Amino acid sequences and gene duplication.
Amino acid sequences of two ferredoxins isolated from Equisetum arvense were determined by conventional procedures. Ferredoxins I and II of E. arvense had 95 and 93 residues, respectively, and nearly identical sequences each with only one amino acid difference from ferredoxins. I and II of E. telmateia (1). The overall structural characteristics of these two ferredoxins were therefore very similar to those of E. telmateia ferredoxins. Ferredoxins I and II from E. arvense differ in 31 sites and those from E. telmateia in 29 sites from each other. These facts suggested that duplication of the ferredoxin gene in one organism occurred at an early evolutionary stage long before the divergence of the two horsetail species. The number of differences in amino acids between horsetail ferredoxins and other chloroplast-type ferredoxins indicated that the duplication occurred after divergence of horsetails from other plants. Comparing green plant ferredoxins, it was estimated that this gene duplication occurred about 250 million years ago. Some comments on the unique amino acid substitutions in horsetail ferredoxins are also presented.
Amino Acid Sequence↗
Direct assignment of the cysteinyl, the slowly exchangeable, and the aromatic ring 1H nuclear magnetic resonances in clostridial-type ferredoxins.
We have directly assigned the 1H NMR corresponding to the cysteinyl protons, the slowly exchangeable protons, and the aromatic ring protons in the 1H NMR spectrum of Clostridium acidi-urici ferredoxin by isotopic labeling and 13C NMR decoupling techniques. We also show that the resonance pattern in the 8- to 20-ppm (from 2,2-dimethyl-2-sialapentanesulfonic acid) region of the 1H NMR spectra of oxidized Clostridium acidi-urici, Clostridium pasteurianum, Clostridium perfringens, and Peptococcus aerogenes ferredoxins are very similar, and we assign the resonances in this region by analogy with the spectrum of C. acidi-urici ferredoxin. The 1H NMR spectra of the beta protons of the cysteinyl residues of these ferredoxins differ, however, from the 1H NMR spectra of equivalent beta protons of the methylene carbon atoms bonded via a sulfur atom to [4Fe-4S] clusters in synthetic inorganic analogues. In the spectra of the synthetic compounds, the beta protons appear as a single resonance shifted 10 ppm from its unbonded reference position. In the spectra of oxidized clostridial ferredoxins, the cysteinyl beta protons appear as a series of at least eight resolved resonances with shifts that range from 6 to 14 ppm, relative to the free amino acid resonance position. This difference in the spectra of the protein and the synthetic compounds probably results from the fact that the equivalent beta protons of the synthetic compounds are not constrained and are free to rotate and thus assume the same average orientation with respect to the [4Fe-4S] cluster. The shift pattern in the 9- to 14-ppm region is identical in three different clostridial ferredoxins. This suggests that the molecular environments of the corresponding cysteinyl residues are identical. Significant differences in the resonance positions occur, however, in the 14- to 18-ppm region, suggesting that the physical environments of these cysteinyl residues differ. This may reflect differences in the orientation of the corresponding cysteinyl residues relative to the [4Fe-4S] clusters or differences in charge density at the cysteinyl beta protons or both. The slowly exchangeable protons were identified by comparing the 1H NMR spectra of ferredoxins reconstituted in H2O and 2H2O. The remaining resonances in the 8- to 20-ppm region were assigned to each of the 2 tyrosyl residues in C. acidi-urici ferredoxin. This was done by comparing the 1H NMR spectra of C. acidi-urici [(3',5'-2H2)Tyr]ferredoxin and C. acidi-urici [PHE2]ferredoxin with that of C. acidi-urici native ferredoxin.
Amino Acids↗
Physicochemical characterization of the four-iron-four-sulphide ferredoxin from Bacillus stearothermophilus.
1. A stable ferredoxin was prepared from Bacillus stearothermophilus and purified by chromatography on DEAE-cellulose and by electrophoresis. 2. The minimum molecular weight determined from the amino acid composition was about 7900 and this was in reasonable agreement with a value of 8500 determined by polyacrylamide-gel electrophoresis. The ferredoxin contained four iron atoms and four labile sulphide groups per molecule. 3. The optical absorption, optical-rotatory-dispersion and circular-dichroism spectra are typical of ferredoxins containing 4Fe-4S clusters. 4. Oxidation-reduction titrations, combined with electron-paramagnetic-resonance (e.p.r.) spectroscopy, showed that the protein has a mid-point potential, at pH8, of -280 +/- 10mV, and that only one electron-accepting paramagnetic species is present. 5. The e.p.r. spectrum of the reduced ferredoxin is more readily saturated with microwave power at low temperatures than those of the eight-iron ferredoxins, indicating that there is another mechanism of electron-spin relaxation in the latter. 6. Mossbauer spectra of both redox states were observed over a range of temperatures and in magnetic fields. At high temperatures (77 degrees K and above) both redox states appear as quadrupole-split doublets; in the reduced state two resolved doublets are seen, suggesting appreciable localization of the additional reducing electron. 7. The average chemical shift indicates formal valences of two Fe3+ and two Fe2+ in the oxidized state and three Fe2+ and one Fe3+ in the reduced state. However, the spectra indicate that there are differing degrees of electron delocalization over the iron atoms. 8. At low temperatures (4.2 degrees K) the oxidized form shows no hyperfine magnetic interaction, even in an applied magnetic field, evidence that the oxidized ferredoxin is in a non-magnetic state as a result of antiferromagnetic coupling between the iron atoms. 9. At 4.2 degrees K the reduced form shows a broad asymmetric pattern resulting from magnetic hyperfine interaction. This contrasts with the reduced ferredoxin of Clostridium pasteurianum, which shows a doublet, suggesting that in the latter there may be interaction between the two 4Fe-4S centres. 10. In large applied magnetic fields, positive and negative hyperfine fields are seen in the Mossbauer spectra of the reduced ferredoxin, evidence for antiferromagnetic coupling between the iron atoms in the 4Fe-4S centre. The high-field spectra of the reduced ferredoxin of B. stearothermophilus are similar to those of the reduced ferredoxin of C. pasteurianum.
Amino Acids↗
Amino acid sequence of a four-iron-four-sulphur ferredoxin isolated from Bacillus stearothermophilus.
1. The primary structure of a 4Fe-4S ferredoxin from Bacillus stearothermophilus was determined and shown to consist of a single polypeptide chain of 81 amino acid residues. The molecular weight of the holoprotein is about 9120. 2. There are only four cysteine residues in the molecule; three of these are located near the N-terminus as a Cys-X-X-Cys-X-X-Cys segment, and the fourth cysteine residue is followed by a proline and located in the C-terminal half. 3. The Fe-S chromophore in B. stearothermophilus ferredoxin was previously well characterized and was shown to consist of a single 4Fe-4S cluster. This ferredoxin sequence establishes for the first time the relative location of the four cysteine residues necessary to bind the 4Fe-4S cluster of a 4Fe ferredoxin, and is in agreement with the criteria for the relative positions of the cysteines proposed from X-ray-crystallographic studies on an 8Fe (two 4Fe-4S clusters) ferredoxin. 4. The sequence of B. stearothermophilus ferredoxin is homologous in many segments to that of other bacterial ferredoxins, the degree of homology being greater towards ferredoxins from Desulfovibrio gigas and photosynthetic bacteria than to Clostridial ferredoxins. 5. The presence of a relatively higher number of glutamic acid and lower number of cysteine residues in the molecule may explain the greater thermal stability and oxygen-insenstivity of this ferredoxin.
Amino Acid Sequence↗
Two plant-type ferredoxins from a blue-green alga, Nostoc verrucosum.
Two plant-type ferredoxins were isolated and purified from a blue-green alga, Nostoc verrucosum. They were separable by chromatography on a DEAE-cellulose column. The slow-moving band was designated ferredoxin I (Fd I) and the fast-moving band was ferredoxin II (Fd II). The ratio of the yield of ferredoxins I and II was about 1 : 0.84. Both ferredoxins had absorption spectra similar to those of plant-type ferredoxins. Two atoms of non-heme iron and two of labile sulfur were found per mol of both ferredoxin I and ferredoxin II. Their molecular weights were identical and estimated to be about 18 000 by a gel filtration method. The biochemical activities of these Nostoc ferredoxins were studied: the NADP photoreduction activity on one hand and the NADP-cytochrome c reductase activity on the other.
Chloroplasts↗
Partial purification of ferredoxin from Ruminococcus albus and its role in pyruvate metabolism and reduction of nicotinamide adenine dinucleotide by H2.
Extracts of Ruminococcus albus were not able to convert pyruvate to acetyl phosphate, CO2, and H2 after passage through a diethylaminoethyl (DEAE)-cellulose column. Activity was restored by a brown protein fraction eluted from the column with 0.4 M Cl-. The protein was partially purified and shown to have the spectral and biological characteristics of ferredoxin. R. albus ferredoxin, Clostridium pasteurianum ferredoxin, and methyl viologen restored activity for pyruvate decomposition by DEAE-cellulose-treated R. albus extracts. R. albus or C. pasteurianum ferredoxin restored the ability of DEAE-cellulose-treated C. pasteurianum extracts to form H2 and acetyl phosphate from pyruvate. Ferredoxin-free extracts of R. albus reduced nicotinamide adenine dinucleotide (NAD) when supplemented with R. albus or C. pasteurianum ferredoxin or with methyl viologen. These extracts reduced NADP with H2 poorly unless both ferredoxin and NAD were added, which indicates the presence of an NADH:NADP transhydrogenase. Flavin mononucleotide and flavin adenine dinucleotide were rapidly reduced by H2 by ferredoxin-free extracts in the absence of ferredoxin.
Ferredoxins↗
Ferredoxin and formyltetrahydrofolate synthetase: comparative studies with Clostridium acidiurici, Clostridium cylindrosporum, and newly isolated anaerobic uric acid-fermenting strains.
Six strains of Clostridium acidiurici and three strains of C. cylindrosporum were isolated from soil samples by enrichment culture with uric acid as the source of carbon, nitrogen, and energy. The newly isolated strains were characterized by their spore morphology and the amounts of glycine and formate formed by the fermentation of uric acid. The strains were easily identified as belonging to one species or the other on the basis of spore morphology and formate production. The crystal properties and spectra of the native ferredoxins of all the strains isolated and the amino acid composition and partial carboxy-terminal sequence of all their apoferredoxins were determined. All the ferredoxins were tested for cross-reactivity with antiserum to C. acidiurici ferredoxin by microcomplement fixation. Five of the six C. acidiurici strains, which had ferredoxins with amino acid compositions identical to that from C. acidiurici, also showed immunological identity (immunological distance = 0.0). These results suggest sequence identity. The one strain with a different amino acid composition failed to show complete cross-reactivity. Two of the three C. cylindrosporum strains have ferredoxin amino acid compositions identical to that from C. cylindrosporum. The third strain had a minimum of five differences in sequence. All C. cylindrosporum strains had ferredoxins that differed considerably from C. acidiurici strains (minimum of eight to nine differences), and none of these ferredoxins cross-reacted with antisera to C. acidiurici ferredoxin. Antisera were prepared to formyltetrahydrofolate synthetase from C. acidiurici and C. cylindrosporum, and all possible comparisons were made by using immunodiffusion and microcomplement fixation. There is more intraspecies variation in the synthetases than in the ferredoxins; however, the results suggest considerable interspecies differences in both proteins. These results suggest a low degree of genomic relatedness between the two species, which contrasts sharply with their apparent high degree of phenotypic similarity.
Amino Acid Sequence↗
[Effects of specific reagents and urea on the reactivity of non-heme iron and thiol groups of pea and corn ferredoxins].
The reactivities of the SH-groups of pea and corn ferredoxins were found to be different. One or two SH-groups in the molecule of pea ferredoxin and one SH-group in the molecule of corn ferredoxin are readily available for the thyol group specific reagents. Four SH-groups of both ferredoxins are completely masked, i. e. available for the thyol reagents only after protein denaturation in the presence of urea. The rates of SH-group interaction with the sulfhydryl reagents in corn ferredoxin are lower than those in pea ferredoxin. The non-haem iron of pea ferredoxin interacts with the complex formers far more rapidly as compared to corn ferredoxin. The ferredoxins tested differ in the amount of iron atoms. The latter require the presence of oxygen for their complete interaction with the complex formers.
Ferredoxins↗
Midpoint redox potentials of plant and algal ferredoxins.
Midpoint potentials of plant-type ferredoxins from a range of sources were measured by redox titrations combined with electron-paramagnetic-resonance spectroscopy. For ferredoxins from higher plants, green algae and most red algae, the midpoint potentials (at pH 8.0) were between --390 and --425 mV. Values for the major ferredoxin fractions from blue-green algae were less negative (between --325 and --390 mV). In addition, Spirulina maxima and Nostoc strain MAC contain second minor ferredoxin components with a different potential, --305 mV (the highest so far measured for a plant-algal ferrodoxin) for Spirulina ferrodoxin II, and --455 mV (the lowest so far measured for a plant-algal ferredoxin) for Nostoc strain MAC ferredoxin II. However, two ferredoxins extracted from a variety of the higher plant Pisum sativum (pea) had midpoint potentials that were only slightly different from each other. These values are discussed in terms of possible roles for the ferredoxins in addition to their involvement in photosynthetic electron transport.
Electron Spin Resonance Spectroscopy↗