A mixed valence form of the iron cluster in the B2 protein of ribonucleotide reductase from Escherichia coli

A mixed valent form of the iron cluster (Fe(II)Fe(III) in the B2 protein of ribonucleotide reductase has been isolated and characterized. The irons in this state of the protein are ferromagnetically coupled as indicated by the observation of a novel S = 9/2 EPR spectrum. This is the first ferromagnetically coupled Fe(II)Fe(III) cluster reported for a protein and the first observation of the mixed valence form of ribonucleotide reductase.

An EXAFS study of the interaction of substrate with the ferric active site of protocatechuate 3,4-dioxygenase

X-ray crystallographic studies of the intradiol cleaving protocatechuate 3,4-dioxygenase from Pseudomonas aeruginosa have shown that the enzyme has a trigonal bipyramidal ferric active site with two histidines, two tyrosines, and a solvent molecule as ligands [Ohlendorf, D.H., Lipscomb, J.D., & Weber, P.C. (1988) Nature 336, 403-405]. Fe K-edge EXAFS studies of the spectroscopically similar protocatechuate 3,4-dioxygenase from Brevibacterium fuscum are consistent with a pentacoordinate geometry of the iron active site with 3 O/N ligands at 1.90 A and 2 O/N ligands at 2.08 A. The 2.08-A bonds are assigned to the two histidines, while the 1.90-A bonds are associated with the two tyrosines and the coordinated solvent. The short Fe-O distance for the solvent suggests that it coordinates as hydroxide rather than water. When the inhibitor terephthalate is bound to the enzyme, the XANES data indicate that the ferric site becomes 6-coordinate and the EXAFS data show a beat pattern which can only be simulated with an additional Fe-O/N interaction at 2.46 A. Together, the data suggest that the oxygens of the carboxylate group in terephthalate displace the hydroxide and chelate to the ferric site but in an asymmetric fashion. In contrast, protocatechuate 3,4-dioxygenase remains 5-coordinate upon the addition of the slow substrate homoprotocatechuic acid (HPCA). Previous EPR data have indicated that HPCA forms an iron chelate via the two hydroxyl functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Mössbauer and EPR studies of the binuclear iron center in ribonucleotide reductase from Escherichia coli. A new iron-to-protein stoichiometry

57Fe-enriched ribonucleotide reductase subunit B2 from Escherichia coli strain N6405/pSPS2 has been characterized by Mössbauer and EPR spectroscopy in its native diferric state and in a new differous form. The native protein exhibits two Mössbauer doublets in a 1:1 ratio with parameters that are in excellent agreement with those reported for the wild-type protein (Atkin, C. L., Thelander, L., Reichard, P., and Lang, G. (1983) J. Biol. Chem. 248, 7464-7472); in addition, our studies show the absence of adventitiously bound iron. The iron content in the present samples approached 4 per B2 subunit, and the tyrosyl radical content exceeded 1 per B2 subunit. The higher values are attributed to the use of a new epsilon 280 for the protein and more efficient methods for iron extraction. We thus propose that subunit B2 has two binuclear iron clusters, each associated with its own tyrosyl radical, in contradistinction from the prevailing model. Reduction of the native protein with dithionite or reconstitution of the apoprotein with Fe(II) afforded a protein complex with Mössbauer parameters, delta EQ = 3.13 mm/s and delta = 1.26 mm/s at 4.2 K, and a low field EPR signal associated with an integer spin system. These spectral properties resemble those of methane monooxygenase in its diferrous form. Upon exposure to O2, the reduced subunit B2 readily converts to the diferric state and yields active enzyme.

Resonance Raman studies on the blue-green-colored bovine adrenal tyrosine 3-monooxygenase (tyrosine hydroxylase). Evidence that the feedback inhibitors adrenaline and noradrenaline are coordinated to iron

Tyrosine 3-monooxygenase (tyrosine hydroxylase) is a non-heme iron, tetrahydropterin-dependent enzyme which catalyzes the rate-limiting step in the biosynthesis of catecholamines. The highly purified bovine adrenal enzyme contains an unusual blue-green chromophore with lambda max at around 700 nm (epsilon = 1.3 (mM subunit enzyme)-1 cm-1). On excitation at 605.2 nm, resonance-enhanced Raman vibrations are observed at 454, 494, 527, 604, 635, 835, 1130, 1271, 1320, 1426, and 1476 cm-1. The excitation profiles of the modes of 1276 and 1476 cm-1 (from 488 to 620 nm) follow the contour of the 700 nm absorption band. The vibrations observed strongly indicate the presence of a bidentate catecholamine-Fe(III) complex in the enzyme as isolated which gives rise to the characteristic charge-transfer transitions. This is further supported by the release of 0.11 +/- 0.04 mol of noradrenaline and 0.25 +/- 0.06 mol of adrenaline per mol of enzyme subunit on denaturation of the enzyme. The energies of the catecholate to Fe(III) charge-transfer transitions indicate a mixture of histidines and carboxylate(s) coordinated to the iron center in tyrosine hydroxylase. At neutral pH, the enzymatic activity was inhibited more than 50% by 10 microM dopamine, noradrenaline, and adrenaline. The high affinity of the catecholamines to the nonphosphorylated form of tyrosine hydroxylase may have significance in vivo since catecholamines are potent feedback inhibitors of the enzyme.

Magnetization and electron paramagnetic resonance studies of reduced uteroferrin and its "EPR-silent" phosphate complex

The exchange coupling of reduced uteroferrin has been measured (19.8(5) cm-1 S1.S2) using recently developed techniques for studying metalloprotein magnetization. A spin Hamiltonian describing the coupled binuclear Fe(II).Fe(III) center has been used to fit the low and high field magnetization data, the EPR g values, and the highly anisotropic effective hyperfine tensor of the ferric site. The exchange coupling of the phosphate complex of reduced uteroferrin has also been measured (6.0(5) cm-1 S1.S2) using the same techniques. The smaller exchange coupling of the phosphate complex is comparable with the zero field splittings of the iron sites. This results in increased sensitivity of the system g values (found by calculation from the spin Hamiltonian) to variations of the zero field splitting parameters arising from heterogeneities in the protein microenvironment. Consequently, there is a very significant (9-fold) increase in the "effective g strain" of the system compared to the situation in the absence of phosphate. This, together with the larger g anisotropy (g = (1.06, 1.51, 2.27)), gives rise to an EPR signal for the phosphate complex of reduced uteroferrin which is extremely broad and difficult to detect but which has now been identified for the first time.

4-Hydroxyphenylpyruvate dioxygenase is an iron-tyrosinate protein

A resonance Raman investigation into the blue chromophore of 4-hydroxyphenylpyruvate dioxygenase, a non-heme iron enzyme from Pseudomonas P. J. 874, reveals the presence of enhanced vibrations characteristic of tyrosinate coordination to the iron center. The excitation profiles for these features show that they are associated with the 595 nm absorption feature. EPR studies of this enzyme indicate the presence of a high-spin ferric center in a rhombic environment, as evidenced by a signal at g = 4.3 with the correct intensity for the measured iron content. This enzyme thus belongs to the emerging class of iron-tyrosinate proteins.

The interaction of phosphate with uteroferrin. Characterization of a reduced uteroferrin-phosphate complex

The interaction of phosphate with reduced uteroferrin has been re-examined in light of disagreements on the oxidation state of the binuclear iron cluster (Keough, D. T., Beck, J. L., de Jersey, J., and Zerner, B. (1982) Biochem. Biophys. Res. Commun. 108, 1643-1648; Antanaitis, B. C., and Aisen, P. (1985) J. Biol. Chem. 260, 751-756). Our results based on Mossbauer observations and the kinetics of spectral change and activity loss show clearly that phosphate binds to reduced uteroferrin to form a reduced uteroferrin-phosphate complex. This complex exhibits a pair of quadrupole doublets at 119 K with parameters typical of a high spin ferric and a high spin ferrous center, respectively, but distinct from those of the native reduced enzyme. The reduced phosphate complex exhibits a pH-dependent visible absorption maximum ranging from 530 to 561 nm. In air, the reduced phosphate complex converts to the oxidized phosphate complex with a first order rate constant of 4 X 10(-3) min-1, as monitored by spectral changes and loss of enzyme activity.

18O studies of pyrogallol cleavage by catechol 1,2-dioxygenase

18O labeling studies on the catechol 1,2-dioxygenase-catalyzed oxidative cleavage of pyrogallol demonstrate that the enzyme functions both as a dioxygenase and a monooxygenase in this reaction. Two products are observed, 2-pyrone-6-carboxylic acid, 99% singly labeled at the carboxylate, and 2-hydroxy-cis,cis-muconic acid, 74% doubly labeled (one 18O at each carboxylate) and 24% single labeled (one 18O at either carboxylate). The labeling pattern observed shows that 2-pyrone-6-carboxylic acid cannot be derived enzymatically from the lactonization of the 2-hydroxy-cis,cis-muconic acid, thus eliminating the dioxetane as an intermediate in the dioxygenase mechanism. The observations are interpreted to indicate the intermediacy of 2-hydroxymuconic anhydride. This anhydride or the corresponding muconyl enzyme species must be sufficiently long-lived to allow the exchange of labeled hydroxide with solvent. Evidence for mechanism-based enzyme inactivation by a pyrogallol-derived intermediate is also presented.

Evidence for synergistic anion binding to iron in ovotransferrin complexes from resonance Raman and extended X-ray absorption fine structure analysis

The 2,3-dihydroxybenzoate and thioglycolate complexes of iron(III)-ovotransferrin have been studied with resonance Raman and extended x-ray absorption fine structure spectroscopies, respectively, to obtain evidence for the coordination of the synergistic anion to the iron center. The dihydroxybenzoate complex exhibits resonance-enhanced Raman vibrations arising from both the endogenous tyrosinates and the added dihydroxybenzoate. A comparison of the extended x-ray absorption fine structure spectra of the carbonate and thioglycolate complexes shows a large feature at about 1.95 A assigned to Fe-(O,N) interactions. The latter complex exhibits an added feature at 2.32 A assigned to an Fe-S interaction. These experiments demonstrate that the Lewis base functions in the synergistic anions coordinate to the iron in ovotransferrin.

Rapid reaction studies on the oxygenation reactions of catechol dioxygenase

The reaction of oxygen with catechol 1,2-dioxygenase from Pseudomonas arvilla ATCC 23974 in complex with catechol, 4-methylcatechol, and 4-fluorocatechol has been studied using single turnover stopped flow spectrophotometry. Two sequential enzyme intermediates have been resolved and their visible spectra characterized by computer-assisted methods. These intermediates are spectrally similar to those observed in a similar study with protocatechuate dioxygenase (Bull, C., Ballou, D. P., and Otsuka, S. J. Biol. Chem. 256, 12681-12686 (1981), although the first intermediate seen with the latter enzyme was not observed in this study. The rate of formation of intermediate I is oxygen-dependent and also accelerated by electron-donating substituents on the C-4 of the substrate. This is consistent with the proposed substrate reduction of dioxygen to form a hydroperoxide. Intermediate I is thus suggested to be a 6-hydroperoxycyclohexa-3,5-diene-1-one. The decay of intermediate I is also accelerated by electron donors and is consistent with the rearrangement of intermediate hydroperoxide via an acyl migration mechanism. It is inconsistent with mechanisms involving nucleophilic attack at the carbonyl carbon. Intermediate II is proposed to be an enzyme-product complex based on the resemblance of its visible spectra to those of the benzoate complex of catechol 1,2-dioxygenase and enzyme-product complexes of protocatechuate dioxygenase. Careful 18O2-labeling experiments have shown that no label is lost to the solvent, implying that no free hydroxide forms during catalysis.

1H NMR studies of porcine uteroferrin. Magnetic interactions and active site structure

Pink (reduced) uteroferrin exhibits well resolved paramagnetic NMR spectra with resonances ranging from 90 ppm downfield to 70 ppm upfield. The intensities of these signals depend on the degree of reduction and correlate well with the intensity of the EPR signals with gave = 1.74. Analyses of chemical shifts and the temperature dependence of the paramagnetically shifted resonances indicate that the Fe(III)-Fe(II) cluster in the reduced protein exhibits weak antiferromagnetic exchange coupling (-J approximately equal to 10 cm-1), in agreement with the estimate derived from the temperature dependence of the EPR signal intensity. Purple (oxidized) uteroferrin, on the other hand, exhibits no discernible paramagnetically shifted resonances, reflecting either strong antiferromagnetic coupling or an unfavorable electron spin-lattice relaxation time. Evans susceptibility comparisons between pink and purple uteroferrin show that the Fe(III)-Fe(III) cluster in the oxidized protein is more strongly coupled (-J greater than 40 cm-1). This value concurs with low temperature magnetic susceptibility measurements on both the porcine and splenic purple acid phosphatases. The isotropically shifted protons of tyrosine coordinated to the cluster are assigned by comparison with synthetic complexes. Tyrosine, earlier implicated as a ligand by resonance Raman spectroscopy, appears to coordinate only to the ferric site in pink uteroferrin. This is consistent with the relatively invariant extinction coefficients of uteroferrin in its oxidized and reduced forms and the ease of reduction of the nonchromophoric iron compared to its chromophoric partner. Other possible ligands to the cluster include histidine, suggested by the presence of downfield-shifted solvent-exchangeable resonances with appropriate isotropic shifts.

3,4-Dihydroxyphenylacetate 2,3-dioxygenase. A manganese(II) dioxygenase from Bacillus brevis

3,4-Dihydroxyphenylacetate 2,3-dioxygenase, an enzyme which catalyzes the extradiol cleavage of catechols, has been purified from Bacillus brevis. Like other extradiol-cleaving dioxygenases, this enzyme has a molecular weight of 140,000 with four subunits of 36,000 each. Unlike the other enzymes, this dioxygenase is not activated by added ferrous ion, not inhibited by cyanide or diethyldithiocarbamate, and not inactivated by H2O2. X-ray fluorescence and atomic absorption analyses show the enzyme to contain approximately 2 g atoms of manganese per mol of protein. EPR spectra are consistent with a manganese(II) center in an environment of low symmetry. This is the first report of an oxygen-activating manganese enzyme.

Resonance Raman studies on protocatechuate 3,4-dioxygenase-inhibitor complexes

Resonance Raman spectra of a number of protocatechuate 3,4-dioxygenase-inhibitor complexes were studied by use of the available lines of an argon and a krypton laser. Three types of inhibitors were investigated-hydroxybenzoates, dicarboxylates, and 4-nitrocatechol. The hydroxybenzoate study shows that the hydroxy group in 3-hydroxybenzoate does not coordinate to the active site iron, in agreement with earlier suggestions, and confirms the coordination of the hydroxy group in the isomeric 4-hydroxybenzoate. The dicarboxylate study demonstrates that both glutarate and terephthalate perturb the active-site environment, shifting the charge-transfer interaction to lower energy. The pH dependence of terephthalate binding as well as the spectral similarities of the dicarboxylate complexes to the ESO2 intermediate provides further evidence for the suggestion that this intermediate is a tightly bound enzyme-product complex. The 4-nitrocatechol study indicates that, unlike the substrate catechols, 4-nitrocatechol does not bind to the iron; a binding configuration wherein the acidic phenolate group interacts with the carboxylate binding site has been suggested by others. Finally the spectra of the 4-hydroxybenzoate and terephthalate complexes demonstrate the presence of two tyrosines coordinated to the active-site iron as suggested by others; these tyrosines have different vCO's and excitation profiles.

Resonance Raman studies of pyrocatechase-inhibitor complexes

The resonance Raman spectra of native pyrocatechase and its benzoate and phenolate complexes were investigated by using the available lines of an argon and a krypton laser. The data provide evidence for the presence of two distinct tyrosines coordinated to the active-site iron. The two tyrosines exhibit different upsilon CO values which show maximum resonance enhancements at different excitation wavelengths. Moreover, one tyrosine is more susceptible to changes in the active-site environment. Pyrocatechase is the only example thus far among iron-tyrosinate proteins where the tyrosines coordinating the iron are distinguishable.

Small cell carcinoma of the lung: primary site and hepatic metastases both detected on Tc-99m-pyrophosphate bone scan

A case of small cell anaplastic carcinoma of the lung with hepatic secondaries has been found to accumulate Tc-99m-pyrophosphate in both lung primary and hepatic metastases and to our knowledge is the first such case reported in the literature. There was no radiographic evidence of calcification in the tumor or hepatic metastases.

Mössbauer studies of cytochrome c' from Rhodospirillum rubrum

Cytochrome c' from Rhodospirillum rubrum has been investigated in the ferric form with Mössbauer and EPR spectroscopy. In the pH range from 6 to 9.5, three species are observed which belong to two pH-dependent equilibria with pK values near 6 and 8.5. The pK = 6 transition is resolved only with high-field Mössbauer spectroscopy. For the three species we have determined the zero-field splitting parameters and the hyperfine coupling constants. The data were fitted to a spin Hamiltonian which takes into account a weak mixing of excited S = 3/2 states into the sextet ground manifold. The low temperature spectra clearly show that the quadruple coupling constant deltaEQ is positive for ferricytochrome c' and thus in accord with all other high-spin ferric heme proteins.

Protocatechuate 3,4-dioxygenase. Inhibitor studies and mechanistic implications

Protocatechuate 3,4-dioxygenase (EC 1.13.11.3) from Pseudomonas aeruginosa catalyzes the cleavage of 3,4-dihydroxybenzoate (protocatechuate) into beta-carboxy-cis,cis-muconate. The inhibition constants, Ki, of a series of substrate analogues were measured in order to assess the relative importance of the various functional groups on the substrate. Though important for binding, the carboxylate group is not essential for activity. Compounds with para hydroxy groups are better inhibitors than their meta isomers. Our studies of the enzyme-inhibitor complexes indicate that the 4-OH group of the substrate binds to the active-site iron. Taken together, Mössbauer, EPR, and kinetic data suggest a mechanism where substrate reaction with oxygen is preceded by metal activation of substrate.

Mössbauer and EPR spectroscopy of protocatechuate 3,4-dioxygenase from Pseudomonas aeruginosa

Protocatechuate 3,4-dioxygenase (EC 1.13.11.3) from Pseudomonas aeruginosa has been investigated by EPR and Mössbauer spectroscopy. Low temperature Mössbauer data on the native enzyme (Fe3+, S = 5/2) yields a hyperfine field Hsat=-525 kG at the nucleus. This observation is inconsistent with earlier suggestions, based on EPR data of a rubredoxin-like ligand environment around the iron, i.e. a tetrahedral sulfur coordination. Likewise, the dithionite-reduced enzyme has Mössbauer parameters unlike those of reduced rubredoxin. We conclude that the iron atoms are in a previously unrecognized environment. The ternary complex of the enzyme with 3,4-dihydroxyphenylpropionate and O2 yields EPR signals at g = 6.7 and g = 5.3; these signals result from an excited state Kramers doublet. The kinetics of the disappearance of these signals parallels product formation and the decay of the ternary complex as observed in the optical spectrum. The Mössbauer and EPR data on the ternary complex establish the iron atoms to be a high-spin ferric state characterized by a large and negative zero-field splitting, D = approximately -2 cm-1.

Deoxyfluoroketohexoses: 4-deoxy-4-fluoro-D-sorbose and -tagatose and 5-deoxy-5-fluoro-L-sorbose

4-Deoxy-4-fluoro-alpha-D-sorbose (6) was prepared in crystalline form by the action of potassium hydrogen fluoride on 3,4-anhydro-1,2-O-isopropylidene-beta-D-psicopyranose (3) followed by deacetonation. Under identical conditions, 3,4-anhydro-1,2-O-isopropylidene-beta-D-tagatopyranose (7) underwent epoxide migration to give 4,5-anhydro-1,2-O-isopropylidene-beta-D-fructopyranose (12), which after deacetonation yielded 4-deoxy-4-fluoro-D-tagatose (15) and 5-deoxy-5-fluoro-alpha-L-sorbopyranose (16), the latter as the crystalline, free sugar. The action of glycol-cleavage reagents on the isopropylidene acetals of the deoxyfluoro sugars was consistent with the assigned structures. The structures were established by 13-C n.m.r. studies of the free deoxyfluoro sugars 6 and 16 and of the isopropylidene acetal 13, and by 1-H n.m.r. studies on the acetylated isopropylidene acetals 5 diacetate, 13 diacetate, and 14 diacetate. 5-Deoxy-5-fluoro-L-sorbose (16) was biologically active, producing in mice effects characteristic of deoxyfluorotrioses and of fluoroacetate. 4-Deoxy-4-fluoro-D-tagatose (15) and 4-deoxy-4-fluoro-D-sorbose (6) produced no apparent effects in mice up to a dose of 500mg/kg. The implications of these findings with respect to transport, phosphorylation, and the action of aldolase on ketohexoses are discussed.

Guanosine 5'-diphosphate, 3'-diphosphate: assignment of structure by 13C nuclear magnetic resonance spectroscopy

Guanosine tetraphosphate, recently discovered to mediate the regulatory relationship between protein synthesis and RNA accumulation in various bacteria, has been synthesized in vitro in large quantities and analyzed by natural-abundance (13)C nuclear magnetic resonance spectroscopy in order to confirm its structure and establish the positions of phosphate attachment. These studies have established its structure as guanosine 5'-diphosphate, 3'-diphosphate.