It was generally accepted that plants, algae, and phototrophic bacteria use adenosine 5-phosphosulfate (APS) for assimilatory sulfate reduction, whereas bacteria and fungi use phosphoadenosine 5-phosphosulfate (PAPS). The corresponding enzymes, APS and PAPS reductase, share 25-30% identical amino acids. Phylogenetic analysis of APS and PAPS reductase amino acid sequences from different organisms, which were retrieved from the GenBank TM , revealed two clusters. The first cluster comprised known PAPS reductases from enteric bacteria, cyanobacteria, and yeast. On the other hand, plant APS reductase sequences were clustered together with many bacterial ones, including those from Pseudomonas and Rhizobium. The gene for APS reductase cloned from the APS-reducing cyanobacterium Plectonema also clustered together with the plant sequences, confirming that the two classes of sequences represent PAPS and APS reductases, respectively. Compared with the PAPS reductase, all sequences of the APS reductase cluster contained two additional cysteine pairs homologous to the cysteine residues involved in binding an iron-sulfur cluster in plants. Mö ssbauer analysis revealed that the recombinant APS reductase from Pseudomonas aeruginosa contains a [4Fe-4S] cluster with the same characteristics as the plant enzyme. We conclude, therefore, that the presence of an iron-sulfur cluster determines the APS specificity of the sulfatereducing enzymes and thus separates the APS-and PAPSdependent assimilatory sulfate reduction pathways.For all living organisms, sulfur is an essential element with many different functions. It is found in reduced form in amino acids, peptides, and proteins and in iron-sulfur clusters, lipoic acid, and other cofactors and in oxidized form as sulfonate group-modifying proteins, polysaccharides, and lipids. Reduced sulfur compounds, such as hydrogen sulfide, serve as electron donors for chemotrophic or phototrophic growth in a large and diverse group of Archae and bacteria, including purple and green sulfur bacteria (1). On the other hand, oxidized sulfur compounds such as sulfate can function as a terminal electron acceptor in respiration to support the growth of sulfate-reducing bacteria (2).The majority of sulfur in living organisms is present in the reduced form of organic thiols. For their synthesis, inorganic sulfate is reduced and incorporated into bioorganic compounds in a pathway named assimilatory sulfate reduction. Before reduction, sulfate is activated with ATP to adenosine 5Ј-phosphosulfate (APS), 1 which can subsequently be converted into phosphoadenosine 5Ј-phosphosulfate (PAPS) using a second ATP. Either form of activated sulfate can be reduced to sulfite and reduced further to sulfide by sulfite reductase. Sulfide is incorporated into an activated amino acid acceptor, such as O-acetylserine, O-acetylhomoserine, or O-succinylhomoserine, to form cysteine or homocysteine (3-5).The assimilatory sulfate reduction pathway is present in plants, fungi, and yeast and in a wide range of eubacteria but is mi...

Adenosine 5-phosphosulfate reductase (APR) catalyzes the two-electron reduction of adenosine 5-phosphosulfate to sulfite and AMP, which represents the key step of sulfate assimilation in higher plants. Recombinant APRs from both Lemna minor and Arabidopsis thaliana were overexpressed in Escherichia coli and isolated as yellow-brown proteins. UV-visible spectra of these recombinant proteins indicated the presence of iron-sulfur centers, whereas flavin was absent. This result was confirmed by quantitative analysis of iron and acid-labile sulfide, suggesting a [4Fe-4S] cluster as the cofactor. EPR spectroscopy of freshly purified enzyme showed, however, only a minor signal at g ‫؍‬ 2.01. Therefore, Mö ssbauer spectra of 57 Fe-enriched APR were obtained at 4.2 K in magnetic fields of up to 7 tesla, which were assigned to a diamagnetic [4Fe-4S] 2؉ cluster. This cluster was unusual because only three of the iron sites exhibited the same Mö ssbauer parameters. The fourth iron site gave, because of the bistability of the fit, a significantly smaller isomer shift or larger quadrupole splitting than the other three sites. Thus, plant assimilatory APR represents a novel type of adenosine 5-phosphosulfate reductase with a [4Fe-4S] center as the sole cofactor, which is clearly different from the dissimilatory adenosine 5-phosphosulfate reductases found in sulfate reducing bacteria.Sulfur is an essential element, which is found in nature mostly in its oxidized inorganic form of sulfate. In living organisms, however, most sulfur is in the reduced form of organic thiols. Plants, yeast, and most prokaryotes are able to reduce sulfate to sulfide and incorporate it into organic compounds. The sulfate assimilation pathway was first resolved in bacteria, such as Escherichia coli and Salmonella typhimurium (1, 2). For reduction sulfate must be activated in two steps; ATP sulfurylase forms adenosine 5Ј-phosphosulfate (APS), 1 which is further phosphorylated by APS kinase to form adenosine 3Ј-phosphate 5Ј-phosphosulfate (PAPS). PAPS is reduced in a thioredoxin-dependent reaction by PAPS reductase (EC 1.8.99) to sulfite. Sulfite is reduced by a NADPH-dependent sulfite reductase (EC 1.8.7.1) to sulfide, which is incorporated into the amino acid skeleton of O-acetyl-L-serine, thus forming cysteine (3). Plants and algae were shown to utilize APS rather than PAPS as sulfonyl donor; the corresponding enzyme was originally called APS sulfotransferase because S-sulfoglutathione was detected among the reaction products (4, 5). APS sulfotransferase was shown to be highly regulated and to play a key role in controlling sulfate reduction in plants (6). Nevertheless, a PAPS-dependent pathway of sulfate reduction could not be excluded, especially when the purification of PAPS reductase from spinach had been reported (7). In attempts to clone plant PAPS reductase by complementation of E. coli CysH mutants, a small family of three cDNA clones was obtained from Arabidopsis thaliana (8, 9). These cDNA clones encoded isoforms of an enzyme with a N-ter...

Nuclear inelastic scattering (NIS) spectra were recorded for a monocrystal of the spin-crossover complex [Fe(tptMetame)] (ClO (4))(2) (tptMetame = 1,1,1-tris([N-(2-pyridylmethyl)-N-methylamino]-methyl)ethane) at T = 30 K (low-spin state) and at room temperature (high-spin state) for different crystal orientations. The high energy resolution (0.65 meV) allowed us to resolve individual molecular vibrations which were unambiguously identified by density functional calculations. From the NIS spectra for the first time the angular-resolved iron-partial density of phonon states (PDOS) was extracted. The PDOS corroborates a vibrational entropy difference as driving force of the spin transition.

The nature and properties of the iron-sulphur (Fe-S) cluster in as-prepared and reduced biotin synthase of Escherichia coli have been investigated by Mo «ssbauer spectroscopy. Our data clearly demonstrate that in the as-prepared sample, the cluster is present as

The EPR and magnetic Mössbauer spectra of a series of axial ligand complexes of tetrakis(2,6-dimethoxyphenyl)porphyrinatoiron(III), [(2,6-(OMe)(2))(4)TPPFeL(2)](+), where L= N-methylimidazole, 2-methylimidazole, or 4-(dimethylamino)pyridine, of one axial ligand complex of tetraphenylporphyrin, the bis(4-cyanopyridine) complex [TPPFe(4-CNPy)(2)](+), and of one axial ligand complex of tetraphenylchlorin, [TPCFe(ImH)(2)](+), where ImH=imidazole, have been investigated and compared to those of low-spin Fe(III) porphyrinates and ferriheme proteins reported in the literature. On the basis of this and previous complementary spectroscopic investigations, three types of complexes have been identified: those having (d(xy))(2)(d(xz),d(yz))(3) electronic ground states with axial ligands aligned in perpendicular planes (Type I), those having (d(xy))(2)(d(xz),d(yz))(3) electronic ground states with axial ligands aligned in parallel planes (Type II), and those having the novel (d(xz),d(yz))(4)(d(xy))(1) electronic ground state (Type III). A subset of the latter type, with planar axial ligands aligned parallel to each other or strong macrocycle asymmetry that yield rhombic EPR spectra, cannot be created using the porphyrinate ligand. Type I centers are characterized by "large g(max)" EPR spectra with g>3.2 and well-resolved, widely spread magnetic Mössbauer spectra having A(zz)/ g(N)mu(N)>680 kG, with A(xx) negative in sign but much smaller in magnitude than A(zz), while Type II centers have well-resolved rhombic EPR spectra with g(zz)=2.4-3.1 and also less-resolved magnetic Mössbauer spectra, and usually have A(zz)/ g(Nmu(N) in the range of 440-660 kG (but in certain cases as small as 180 kG) and A(xx) again negative in sign but only somewhat smaller (but occasionally larger in magnitude) than A(zz), and Type III centers have axial EPR spectra with g( upper left and right quadrants ) approximately 2.6 or smaller and g( vertical line )<1.0-1.95, but often not resolved, and less-resolved magnetic Mössbauer spectra having A(zz)/ g(N)mu(N) in the range of 270-400 kG, and A(xx) again negative in sign but much smaller in magnitude than A(zz). An exception to this rule is [TPPFe(4-CNPy)(2)](+), which has A(xx)/ g(N)mu(N)=-565 kG, A(yy)/ g(N)mu(N)=629 kG, and A(zz)/ g(N)mu(N)=4 kG. A subset of Type II complexes (Type II') have rhombicities ( V/Delta) much greater than 0.67 and A(zz)/ g(N)mu(N) ranging from 320 to 170 kG, with A(xx) also negative but with the magnitude of A(xx) significantly larger than that of A(zz). These classifications are also observed for a variety of ferriheme proteins, and they lead to linear correlations between A(zz) and either A(xx), g(zz), or V/Delta for Types I and II (but not for A(zz) versus V/Delta for Type II'). Not enough data are yet available on Type III complexes to determine what, if any, correlations may be observed.

Biotin synthase, the enzyme that catalyzes the last step of the biosynthesis of biotin, contains only [2Fe-2S](2+) clusters when isolated under aerobic conditions. Previous results showed that reconstitution with an excess of FeCl(3) and Na(2)S under reducing and anaerobic conditions leads to either [4Fe-4S](2+), [4Fe-4S](+), or a mixture of [4Fe-4S](2+) and [2Fe-2S](2+) clusters. To determine whether any of these possibilities or other different cluster configuration could correspond to the physiological in vivo state, we have used (57)Fe Mössbauer spectroscopy to investigate the clusters of biotin synthase in whole cells. The results show that, in aerobically grown cells, biotin synthase contains a mixture of [4Fe-4S](2+) and [2Fe-2S](2+) clusters. A mixed [4Fe-4S](2+):[2Fe-2S](2+) cluster form has already been observed under certain in vitro conditions, and it has been proposed that both clusters might each play a significant role in the mechanism of biotin synthase. Their presence in vivo is now another argument in favor of this mixed cluster form.

A model heme complex, bis(3-aminopyrazole)tetraphenylporphinatoiron(III) chloride, [TPPFe (NH2PzH)2]Cl, for which the EPR g-values lead to a rhombicity V/delta = 1.2 if gzz is the largest g-value, have been investigated by electron spin echo envelope modulation (ESEEM) and Mössbauer spectroscopies. The ESEEM studies focus on the proton sum frequency peaks at near twice the proton Larmor frequency. Analysis of the distant proton peak (mainly due to the pyrrole-H) at exactly twice the proton Larmor frequency shows conclusively that gzz is aligned along the normal to the porphyrin plane, and thus the electron configuration is (dxy)2(dxz,dyz)3, with gzz > gyy > gxx. This system is thus another violation to Taylor's "proper axis system" rule. The near proton (the alpha-H and N-H of the axial ligands) peaks provide distance information for those protons from the metal. Magnetic Mössbauer studies of the same complex confirm the (dxy)2(dxz,dyz)3 ground state and indicate that, as is the case for cytochrome P450cam, Axx is the largest magnitude A-value, and is negative in sign. Other low-spin iron(III) porphyrinates also have Axx of negative sign, but usually the magnitude is only about half that of Azz, which is always positive in sign.

Nuclear inelastic scattering (NIS) measurements were performed on a guanidium nitroprusside ((CN(3)H(6))(2)[Fe(CN)(5)NO], GNP) monocrystal at 77 K after the sample was illuminated with blue light (450 nm) at 50 K to populate the two metastable states, MS(1) and MS(2), of the nitroprusside anion. A second measurement was performed at 77 K after warming up the illuminated crystal to 250 K where the metastable states decay to the groundstate. The measured spectra were compared with simulated NIS spectra that were calculated by using density functional methods. Comparison of measured and simulated spectra provides strong evidence for the isonitrosyl structure of the metastable MS(1) state proposed by Carducci et al. (Carducci, M. D.; Pressprich, M. R.; Coppens, P. J. Am. Chem. Soc. 1997, 119, 2669-2678).