Crystal structure of sulerythrin, a rubrerythrin-like protein from a strictly aerobic archaeon, Sulfolobus tokodaii strain 7, shows unexpected domain swapping

Sulerythrin is the first rubrerythrin-like protein to be isolated from an aerobic organism, Sulfolobus tokodaii strain 7, and it lacks a C-terminal rubredoxin-like FeS(4) domain. The protein purified from Sulfolobus cells was crystallized, and the crystal structure was determined at 1.7 A resolution. The dimer of sulerythrin exhibited "domain-swapping" at the loop connecting alphaB and alphaC, hybrid four-helix bundles consisting of alphaA/B and alphaC/D being formed. The structure and atomic identity of the binuclear metal center were determined by means of anomalous scattering analysis. The site contained 1.0 mol of hexacoordinate Fe, 0.80-0.87 mol of tetracoordinate Zn, and 0.73-0.88 mol of putative O(2) per monomer. The metal ions were found at exchanged positions compared to those in the Fe/Zn-containing rubrerythrin from Desulfovibrio vulgaris. The results demonstrate that the binuclear metal center of rubrerythrin-like proteins is plastic in its ability to bind metal ions.     

Displacement of iron by zinc at the diiron site of Desulfovibrio vulgaris rubrerythrin: X-ray crystal structure and anomalous scattering analysis

X-ray crystal structures of recombinant Desulfovibrio (D.) vulgaris rubrerythrin (Rbr) have shown a diiron site, whereas the crystal structure of Rbr "as-isolated" from D. vulgaris was reported to contain a mixed Zn,Fe binuclear site. To investigate the possibility that zinc had displaced iron during isolation or crystallization of the "as-isolated" D. vulgaris Rbr, the X-ray crystal structure of recombinant D. vulgaris all-iron Rbr that had been incubated with excess zinc sulfate prior to crystallization, yielding a protein labeled Zn,FeRbr, was solved. Analysis of the anomalous scattering data obtained at two different wavelengths showed that zinc had displaced a significant proportion of iron from both iron centers of the diiron site, and that no iron had been displaced from the [Fe(SCys)(4)] site. UV-visible absorption spectra of the redissolved Zn,FeRbr crystals showed 30-40% retention of oxo-bridged diferric sites, and the redissolved crystals had 37% of the peroxidase specific activity of the starting all-iron Rbr, which, together with the crystallographic results, indicate a predominant mixture of Fe1,Fe2 and Zn1,Zn2 sites. The structure of the Zn(Fe)1,Fe(Zn)2 binuclear site in the Zn,FeRbr crystals was very similar to that of the Zn,Fe binuclear site reported for the "as-isolated" D. vulgaris Rbr, including tetrahedral four-coordination at the Zn(Fe)1 site. The diiron sites in the recombinant Zn,FeRbr crystals were likely at least partially reduced during synchrotron irradiation. Our results suggest that the mixed-metal binuclear site reported for the "as-isolated" D. vulgaris Rbr could be due to displacement of iron from a native diiron site by adventitious zinc during isolation and/or crystallization, and that reduced diiron and dizinc sites can adopt very similar structures in Rbr.     

Binuclear metal centers in plant purple acid phosphatases: Fe-Mn in sweet potato and Fe-Zn in soybean.

Purple acid phosphatases comprise a family of binuclear metal-containing acid hydrolases, representatives of which have been found in animals, plants, and fungi. The goal of this study was to characterize purple acid phosphatases from sweet potato tubers and soybean seeds and to establish their relationship with the only well-characterized plant purple acid phosphatase, the FeIII-ZnII-containing red kidney bean enzyme. Metal analysis indicated the presence in the purified sweet potato enzyme of 1.0 g-atom of iron, 0.6-0.7 g-atom of manganese, and small amounts of zinc and copper. The soybean enzyme contained 0.8-0.9 g-atom of iron, 0.7-0.8 g-atom of zinc per subunit, and small amounts of manganese, copper, and magnesium. Both enzymes exhibited visible absorption maxima at 550-560 nm, with molar absorption coefficients of 3200 and 3300 M(-1) cm(-1), respectively, very similar to the red kidney bean enzyme. Substrate specificities were markedly different from those of the red kidney bean enzyme. A cloning strategy was developed based on N-terminal sequences of the sweet potato and soybean enzymes and short sequences around the conserved metal ligands of the mammalian and red kidney bean enzymes. Three sequences were obtained, one from soybean and two from sweet potato. All three showed extensive sequence identity (>66%) with red kidney bean purple acid phosphatase, and all of the metal ligands were conserved. The combined results establish that these enzymes are binuclear metalloenzymes: Fe-Mn in the sweet potato enzyme and Fe-Zn in soybean. The sweet potato enzyme is the first well-defined example of an Fe-Mn binuclear center in a protein.     

Crystal structure of Helicobacter pylori neutrophil-activating protein with a di-nuclear ferroxidase center in a zinc or cadmium-bound form

Helicobacter pylori neutrophil-activating protein (HP-NAP) is a Dps-like iron storage protein forming a dodecameric shell, and promotes adhesion of neutrophils to endothelial cells. The crystal structure of HP-NAP in a Zn(2+)- or Cd(2+)-bound form reveals the binding of two zinc or two cadmium ions and their bridged water molecule at the ferroxidase center (FOC). The two zinc ions are coordinated in a tetrahedral manner to the conserved residues among HP-NAP and Dps proteins. The two cadmium ions are coordinated in a trigonal-bipyramidal and distorted octahedral manner. In both structures, the second ion is more weakly coordinated than the first. Another zinc ion is found inside of the negatively-charged threefold-related pore, which is suitable for metal ions to pass through.     

Structural consequences of the active site substitution Cys181 ==> Ser in metallo-beta-lactamase from Bacteroides fragilis

The metallo-beta-lactamases require divalent cations such as zinc or cadmium for hydrolyzing the amide bond of beta-lactam antibiotics. The crystal structure of the Zn2+ -bound enzyme from Bacteroides fragilis contains a binuclear zinc center in the active site. A hydroxide, coordinated to both zinc atoms, is proposed as the moiety that mounts the nucleophilic attack on the carbonyl carbon atom of the beta-lactam bond of the substrate. It was previously reported that the replacement of the active site Cys181 by a serine residue severely impaired catalysis while atomic absorption measurements indicated that binding of the two zinc ions remained intact. Contradicting data emerge from recent mass spectrometry results, which show that only a single zinc ion binds to the C181S metallo-beta-lactamase. In the current study, the C181S mutant enzyme was examined at the atomic level by determining the crystal structure at 2.6 A resolution. The overall structure of the mutant enzyme is the same as that of the wild-type enzyme. At the mutation site, the side chain of Ser181 occupies the same position as that of the side chain of Cys181 in the wild-type protein. One zinc ion, Zn1, is present in the crystal structure; however, the site of the second zinc ion, Zn2 is unoccupied. A water molecule is associated with Zn1, reminiscent of the hydroxide seen in the structure of the wild-type enzyme but farther from the metal. The position of the water molecule is off the plane of the carboxylate group of Asp103; therefore, the water molecule may be less nucleophilic than a water molecule which is coplanar with the carboxylate group.     

Crystal structures of the cadmium- and mercury-substituted metallo-beta-lactamase from Bacteroides fragilis.

The metallo-beta-lactamases require zinc or cadmium for hydrolyzing beta-lactam antibiotics and are inhibited by mercurial compounds. To data, there are no clinically useful inhibitors of this class of enzymes. The crystal structure of the Zn(2+)-bound enzyme from Bacteroides fragilis contains a binuclear zinc center in the active site. A hydroxide, coordinated to both zinc atoms, is proposed as the moiety that mounts the nucleophilic attack on the carbonyl carbon atom of the beta-lactam ring. To study the metal coordination further, the crystal structures of a Cd(2+)-bound enzyme and of an Hg(2+)-soaked zinc-containing enzyme have been determined at 2.1 A and 2.7 A, respectively. Given the diffraction resolution, the Cd(2+)-bound enzyme exhibits the same active-site architecture as that of the Zn(2+)-bound enzyme, consistent with the fact that both forms are enzymatically active. The 10-fold reduction in activity of the Cd(2+)-bound molecule compared with the Zn(2+)-bound enzyme is attributed to fine differences in the charge distribution due to the difference in the ionic radii of the two metals. In contrast, in the Hg(2+)-bound structure, one of the zinc ions, Zn2, was ejected, and the other zinc ion, Zn1, remained in the same site as in the 2-Zn(2+)-bound structure. Instead of the ejected zinc, a mercury ion binds between Cys 104 and Cys 181, 4.8 A away from Zn1 and 3.9 A away from the site where Zn2 is located in the 2-Zn(2+)-bound molecule. The perturbed binuclear metal cluster explains the inactivation of the enzyme by mercury compounds.     

The 2.1-A resolution structure of iron superoxide dismutase from Pseudomonas ovalis

The 2.1-A resolution crystal structure of native uncomplexed iron superoxide dismutase (EC 1.15.1.1) from Pseudomonas ovalis was solved and refined to a final R factor of 24%. The dimeric structure contains one catalytic iron center per monomer with an asymmetric trigonal-bipyramidal coordination of protein ligands to the metal. Each monomer contains two domains, with the trigonal ligands (histidines 74 and 160; aspartate 156) contributed by the large domain and stabilized by an extended hydrogen-bonded network, including residues from opposing monomers. The axial ligand (histidine 26) is found on the small domain and does not participate extensively in the stabilizing H-bond network. The open axial coordination position of the iron is devoid of bound water molecules or anions. The metal is located 0.5 A out of the plane of the trigonal ligands toward histidine 26, providing a slightly skewed coordination away from the iron binding site. The molecule contains a glutamine residue in the active site which is conserved between all iron enzymes sequenced to data but which is conserved among all manganese SODs at a separate position in the sequence. This residue shows the same structural interactions in both cases, implying that iron and manganese SODs are second-site revertants of one another.     

The 1.9 Å crystal structure of the “as isolated” rubrerythrin from <Emphasis Type="Italic">Desulfovibrio vulgaris</Emphasis>: some surprising results

Rubrerythrin is a non-heme iron dimeric protein isolated from the sulfate-reducing bacterium Desulfovibrio vulgaris. Each monomer has one mononuclear iron center similar to rubredoxin and one dinuclear metal center similar to hemerythrin or ribonucleotide reductase. The 1.88 A X-ray structure of the "as isolated" molecule and a uranyl heavy atom derivative have been solved by molecular replacement techniques. The resulting model of the native "as isolated" molecule, including 164 water molecules, has been refined giving a final R factor of 0.197 (R(free) = 0.255). The structure has the same general protein fold, domain structure, and dimeric interactions as previously found for rubrerythrin [1, 2], but it also has some interesting undetected differences at the metal centers. The refined model of the protein structure has a cis peptide between residues 78 and 79. The Fe-Cys4 center has a previously undetected strong seventh N-H...S hydrogen bond in addition to the six N-H...S bonds usually found in rubredoxin. The dinuclear metal center has a hexacoordinate Fe atom and a tetracoordinate Zn atom. Each metal is coordinated by a GluXXHis polypeptide chain segment. The Zn atom binds at a site distinctly different from that found in the structure of a diiron rubrerythrin. Difference electron density for the uranyl derivative shows an extremely large peak adjacent to and replacing the Zn atom, indicating that this particular site is capable of binding other atoms. This feature/ability may give rise to some of the confusing activities ascribed to this molecule.     

Functional control of the binuclear metal site in the metallo-beta-lactamase-like fold by subtle amino acid replacements

At present there are three protein families that share a common structural domain, the alphabeta/betaalpha fold of class B beta-lactamases: zinc beta-lactamases, glyoxalases II, and A-type flavoproteins. A detailed inspection of their superimposed structures was undertaken and showed that although these proteins contain binuclear metal sites in spatially equivalent positions, there are some subtle differences within the first ligand sphere that determine a distinct composition of metals. Although zinc beta-lactamases contain either a mono or a di-zinc center, the catalytically active form of glyoxalase II contains a mixed iron-zinc binuclear center, whereas A-type flavoproteins contain a di-iron site. These variations on the type of metal site found within a common fold are correlated with the subtle variations in the nature of the ligating amino acid residues and are discussed in terms of the different reactions catalyzed by each of the protein families. Correlation of these observations with sequence data results in the definition of a sequence motif that comprises the possible binuclear metal site ligands in this broad family. The evolution of the proteins sharing this common fold and factors modulating reactivity are also discussed.     

Acetate C-C bond formation and decomposition in the anaerobic world: the structure of a central enzyme and its key active-site metal cluster

The structure of carbon monoxide dehydrogenase/acetyl-coenzyme A synthase (CODH/ACS), a central enzyme in the anaerobic metabolism of acetyl-coenzyme A (acetyl-CoA), has been solved to a resolution of 2.2A. The active-site metal cluster responsible for catalyzing acetyl C-C bond synthesis and cleavage, designated the A center, was identified as an Fe(4)S(4) iron sulfur cluster with one of its cysteine thiolates acting as a bridge to an adjacent binuclear metal site. Nickel was found at one position in the binuclear site and the other metal was indicated to be copper - a surprising result, implying a previously unrecognized role for copper. Details of the A center provided new insight into the unusual organometallic mechanism of acetyl C-C bond formation and cleavage, with substantial conformational changes indicated for binding of the large methylcorrinoid protein substrate, and a unique intramolecular channel acting to contain carbon monoxide within the protein and transfer it to the site needed for acetyl-CoA synthesis.     

Crystal structure of phosphatidylglycerophosphatase (PGPase), a putative membrane-bound lipid phosphatase, reveals a novel binuclear metal binding site and two "proton wires"

Phosphatidylglycerophosphatase (PGPase), an enzyme involved in lipid metabolism, catalyzes formation of phosphatidylglycerol from phosphatidylglycerophosphate. Phosphatidylglycerol is a multifunctional phospholipid, found in the biological membranes of many organisms. Here, we report the crystal structure of Listeria monocytogenes PGPase at 1.8 A resolution. PGPase, an all-helical molecule, forms a homotetramer. Each protomer contains an independent active site with two metal ions, Ca(2+) and Mg(2+), forming a hetero-binuclear center located in a hydrophilic cavity near the surface of the molecule. The binuclear center, conserved ligands, metal-bound water molecules, and an Asp-His dyad form the active site. The catalytic mechanism of this enzyme is likely to proceed via binuclear metal activated nucleophilic water. The binuclear metal-binding active-site environment of this structure should provide insights into substrate binding and metal-dependent catalysis. A long channel with inter-linked linear water chains, termed "proton wires," is observed at the tetramer interface. Comparison of similar water chain structures in photosynthetic reaction centers (RCs), Cytochrome f, gramicidin, and bacteriorhodopsin, suggests that PGPase may conduct protons via proton wires.     

The crystal structure of enamidase: a bifunctional enzyme of the nicotinate catabolism

The hydrolysis of 1,4,5,6-tetrahydro-6-oxonicotinate to 2-formylglutarate is a central step in the catabolism of nicotinate in several Clostridia and Proteobacteria. This reaction is catalyzed by the novel enzyme enamidase, a new member of the amidohydrolase superfamily as indicated by its unique reaction, sequence relationship, and the stoichiometric binding of iron and zinc. A hallmark of enamidase is its capability to catalyze a two-step reaction: the initial decyclization of 1,4,5,6-tetrahydro-6-oxonicotinate leading to 2-(enamine)glutarate followed by an additional hydrolysis step yielding (S)-2-formylglutarate. Here, we present the crystal structure of enamidase from Eubacterium barkeri at 1.9 Å resolution, providing a structural basis for catalysis and suggesting a mechanism for its exceptional activity and enantioselectivity. The enzyme forms a 222-symmetric tetramer built up by a dimer of dimers. Each enamidase monomer consists of a composite β-sandwich domain and an (α/β)₈-TIM-barrel domain harboring the active site. With its catalytic binuclear metal center comprising both zinc and iron ions, enamidase represents a special case of subtype II amidohydrolases.     

Structural characterization and biological implications of di-zinc binding in the ferroxidase center of Streptococcus pyogenes Dpr

Dps proteins contain a ferroxidase site that binds and oxidizes iron, thereby preventing hydroxyl radical formation by Fenton reaction. Although the involvement of a di-iron ferroxidase site has been suggested, X-ray crystal structures of various Dps members have shown either one or two iron cations with various occupancies despite the high structural conservation of the site. Similarly, structural studies with zinc, a redox-stable replacement for iron, have shown the binding of either one or two zinc ions. Here, the crystal structure of Streptococcus pyogenes Dpr in complex with zinc reveals the binding of two zinc cations in the ferroxidase center and an additional zinc-binding site at the surface of the protein. The results suggest a structural basis for the protection of Streptococcus pyogenes in zinc stress conditions and provide a clear evidence for a di-zinc and di-iron ferroxidase site in Streptococcus pyogenes Dpr protein.     

Molecular structure of dihydroorotase: a paradigm for catalysis through the use of a binuclear metal center

Dihydroorotase plays a key role in pyrimidine biosynthesis by catalyzing the reversible interconversion of carbamoyl aspartate to dihydroorotate. Here we describe the three-dimensional structure of dihydroorotase from Escherichia coli determined and refined to 1.7 A resolution. Each subunit of the homodimeric enzyme folds into a "TIM" barrel motif with eight strands of parallel beta-sheet flanked on the outer surface by alpha-helices. Unexpectedly, each subunit contains a binuclear zinc center with the metal ions separated by approximately 3.6 A. Lys 102, which is carboxylated, serves as a bridging ligand between the two cations. The more buried or alpha-metal ion in subunit I is surrounded by His 16, His 18, Lys 102, Asp 250, and a solvent molecule (most likely a hydroxide ion) in a trigonal bipyramidal arrangement. The beta-metal ion, which is closer to the solvent, is tetrahedrally ligated by Lys 102, His 139, His 177, and the bridging hydroxide. L-Dihydroorotate is observed bound to subunit I, with its carbonyl oxygen, O4, lying 2.9 A from the beta-metal ion. Important interactions for positioning dihydroorotate into the active site include a salt bridge with the guanidinium group of Arg 20 and various additional electrostatic interactions with both protein backbone and side chain atoms. Strikingly, in subunit II, carbamoyl L-aspartate is observed binding near the binuclear metal center with its carboxylate side chain ligating the two metals and thus displacing the bridging hydroxide ion. From the three-dimensional structures of the enzyme-bound substrate and product, it has been possible to propose a unique catalytic mechanism for dihydroorotase. In the direction of dihydroorotate hydrolysis, the bridging hydroxide attacks the re-face of dihydroorotate with general base assistance by Asp 250. The carbonyl group is polarized for nucleophilic attack by the bridging hydroxide through a direct interaction with the beta-metal ion. During the cyclization of carbamoyl aspartate, Asp 250 initiates the reaction by abstracting a proton from N3 of the substrate. The side chain carboxylate of carbamoyl aspartate is polarized through a direct electrostatic interaction with the binuclear metal center. The ensuing tetrahedral intermediate collapses with C-O bond cleavage and expulsion of the hydroxide which then bridges the binuclear metal center.     

X-ray crystal structure of Desulfovibrio vulgaris rubrerythrin with zinc substituted into the [Fe(SCys)4] site and alternative diiron site structures

The X-ray crystal structure of recombinant Desulfovibrio vulgaris rubrerythrin (Rbr) that was subjected to metal constitution first with zinc and then iron, yielding ZnS(4)Rbr, is reported. A [Zn(SCys)(4)] site with no iron and a diiron site with no appreciable zinc in ZnS(4)Rbr were confirmed by analysis of the anomalous scattering data. Partial reduction of the diiron site occurred during the synchrotron X-ray irradiation at 95 K, resulting in two different diiron site structures in the ZnS(4)Rbr crystal. These two structures can be classified as containing mixed-valent Fe1(III)(mu-OH(-))(mu-GluCO(2)(-))(2)Fe2(II) and Fe1(II)(mu-GluCO(2)(-))(2)Fe2(III)-OH(-) cores. The data do not show any evidence for alternative positions of the protein or solvent ligands. The iron and ligand positions of the solvent-bridged site are close to those of the diferric site in all-iron Rbr. The diiron site with only the two carboxylato bridges differs by an approximately 2 A shift in the position of Fe1, which changes from six- to four-coordination. The Fe1- - -Fe2 distance (3.6 A) in this latter site is significantly longer than that of the site with the additional solvent bridge (3.4 A) but significantly shorter than that previously reported for the diferrous site (4.0 A) in all-iron Rbr. The apparent redox-induced movement of Fe1 at 95 K in the ZnS(4)Rbr crystal implies an extremely low activation barrier, which is consistent with the rapid (approximately 30 s(-1)) room temperature turnover of the all-iron Rbr during its catalysis of two-electron reduction of hydrogen peroxide. ZnS(4)Rbr does not show peroxidase activity, presumably because the [Zn(SCys)(4)] site, unlike the [Fe(SCys)(4)] site, cannot mediate electron transfer to the diiron site. One or both of the diiron site structures in the cryoreduced ZnS(4)Rbr crystal are likely to represent that (those) of transient mixed-valent diiron site(s) that must occur upon return of the diferric to the diferrous oxidation level during peroxidase turnover.     

Hybrid cluster proteins (HCPs) from Desulfovibrio desulfuricans ATCC 27774 and Desulfovibrio vulgaris (Hildenborough): X-ray structures at 1.25 Å resolution using synchrotron radiation

The structures of the hybrid cluster proteins (HCPs) from the sulfate-reducing bacteria Desulfovibrio desulfuricans (ATCC 27774) and Desulfovibrio vulgaris (Hildenborough) have been elucidated at a resolution of 1.25 A using X-ray synchrotron radiation techniques. In the case of the D. desulfuricans protein, protein isolation, purification, crystallization and X-ray data collection were carried out under strict anaerobic conditions, whereas for the D. vulgaris protein the conditions were aerobic. However, both structures are essentially the same, comprising three domains and two iron-sulfur centres. One of these centres situated near the exterior of the molecules in domain 1 is a cubane [4Fe-4S] cluster, whereas the other, located at the interface of the three domains, contains the unusual four-iron cluster initially found in the D. vulgaris protein. Details of the structures and the associated EPR spectroscopy of the D. desulfuricans protein are reported herein. These structures show that the nature of the hybrid cluster, containing both oxygen and sulfur bridges, is independent of the presence of oxygen in the isolation and crystallization procedure and also does not vary significantly with changes in the oxidation state. The structures and amino acid sequences of the HCP are compared with the recently elucidated structure of the catalytic subunit of a carbon monoxide dehydrogenase from Carboxydothermus hydrogenoformans and related dehydrogenases. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00775-001-0326-y.     

Crystal structure of the autocatalytic initiator of glycogen biosynthesis,glycogenin

The crystal structure of a medium-chain NAD(H)-dependent alcohol dehydrogenase (ADH) from an archaeon has been solved by multiwavelength anomalous diffraction, using a selenomethionine-substituted enzyme. The protein (SsADH), extracted from the hyperthermophilic organism Sulfolobus solfataricus, is a homo-tetramer with a crystallographic 222 symmetry. Despite the low level of sequence identity, the overall fold of the monomer is similar to that of the other homologous ADHs of known structure. However, a significant difference is the orientation of the catalytic domain relative to the coenzyme-binding domain that results in a larger interdomain cleft. At the bottom of this cleft, the catalytic zinc ion is coordinated tetrahedrally and lacks the zinc-bound water molecule that is usually found in ADH apoform structures. The fourth coordination position is indeed occupied by a Glu residue, as found in bacterial tetrameric ADHs. Other differences are found in the architecture of the substrate pocket whose entrance is more restricted than in other ADHs. SsADH is the first tetrameric ADH X-ray structure containing a second zinc ion playing a structural role. This latter metal ion shows a peculiar coordination, with a glutamic acid residue replacing one of the four cysteine ligands that are highly conserved throughout the structural zinc-containing dimeric ADHs.     

Evidence for a dual role of an active site histidine in α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase

The previously reported crystal structures of α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) show a five-coordinate Zn(II)(His)(3)(Asp)(OH(2)) active site. The water ligand is H-bonded to a conserved His228 residue adjacent to the metal center in ACMSD from Pseudomonas fluorescens (PfACMSD). Site-directed mutagenesis of His228 to tyrosine and glycine in this study results in a complete or significant loss of activity. Metal analysis shows that H228Y and H228G contain iron rather than zinc, indicating that this residue plays a role in the metal selectivity of the protein. As-isolated H228Y displays a blue color, which is not seen in wild-type ACMSD. Quinone staining and resonance Raman analyses indicate that the blue color originates from Fe(III)-tyrosinate ligand-to-metal charge transfer. Co(II)-substituted H228Y ACMSD is brown in color and exhibits an electron paramagnetic resonance spectrum showing a high-spin Co(II) center with a well-resolved (59)Co (I = 7/2) eight-line hyperfine splitting pattern. The X-ray crystal structures of as-isolated Fe-H228Y (2.8 ?) and Co-substituted (2.4 ?) and Zn-substituted H228Y (2.0 ? resolution) support the spectroscopic assignment of metal ligation of the Tyr228 residue. The crystal structure of Zn-H228G (2.6 ?) was also determined. These four structures show that the water ligand present in WT Zn-ACMSD is either missing (Fe-H228Y, Co-H228Y, and Zn-H228G) or disrupted (Zn-H228Y) in response to the His228 mutation. Together, these results highlight the importance of His228 for PfACMSD's metal specificity as well as maintaining a water molecule as a ligand of the metal center. His228 is thus proposed to play a role in activating the metal-bound water ligand for subsequent nucleophilic attack on the substrate.     

Redox-dependent structural changes in the Azotobacter vinelandii bacterioferritin: new insights into the ferroxidase and iron transport mechanism

In this work, we report the X-ray crystal structure of the aerobically isolated (oxidized) and the anaerobic dithionite-reduced (at pH 8.0) forms of the native Azotobacter vinelandii bacterioferritin to 2.7 and 2.0 A resolution, respectively. Iron K-edge multiple anomalous dispersion (MAD) experiments unequivocally identified the presence of three independent iron-containing sites within the protein structure. Specifically, a dinuclear (ferroxidase) site, a b-type heme site, and the binding of a single iron atom at the four-fold molecular axis of the protein shell were observed. In addition to the novel observation of iron at the four-fold pore, these data also reveal that the oxidized form of the protein has a symmetrical ferroxidase site containing two five-coordinate iron atoms. Each iron atom is ligated by four carboxylate oxygen atoms and a single histidyl nitrogen atom. A single water molecule is found within hydrogen bonding distance of the ferroxidase site that bridges the two iron atoms on the side opposite the histidine ligands. Chemical reduction of the protein under anaerobic conditions results in an increase in the average Fe-Fe distance in the ferroxidase site from approximately 3.5 to approximately 4.0 A and the loss of one of the ligands, H130. In addition, there is significant movement of the bridging water molecule and several other amino acid side chains in the vicinity of the ferroxidase site and along the D helix to the three-fold symmetry axis. In contrast to previous work, the higher-resolution data for the dithionite-reduced structure suggest that the heme may be bound in multiple conformations. Taken together, these data allow a molecular movie of the ferroxidase gating mechanism to be developed and provide further insight into the iron uptake and/or release and mineralization mechanism of bacterioferritins in general.     

Crystal structure of the zinc-, cobalt-, and iron-containing adenylate kinase from <Emphasis Type="Italic">Desulfovibrio gigas</Emphasis>: a novel metal-containing adenylate kinase from Gram-negative bacteria

Adenylate kinases (AK) from Gram-negative bacteria are generally devoid of metal ions in their LID domain. However, three metal ions, zinc, cobalt, and iron, have been found in AK from Gram-negative bacteria. Crystal structures of substrate-free AK from Desulfovibrio gigas with three different metal ions (Zn²⁺, Zn-AK; Co²⁺, Co-AK; and Fe²⁺, Fe-AK) bound in its LID domain have been determined by X-ray crystallography to resolutions 1.8, 2.0, and 3.0 Å, respectively. The zinc and iron forms of the enzyme were crystallized in space group I222, whereas the cobalt-form crystals were C2. The presence of the metals was confirmed by calculation of anomalous difference maps and by X-ray fluorescence scans. The work presented here is the first report of a structure of a metal-containing AK from a Gram-negative bacterium. The native enzyme was crystallized, and only zinc was detected in the LID domain. Co-AK and Fe-AK were obtained by overexpressing the protein in Escherichia coli. Zn-AK and Fe-AK crystallized as monomers in the asymmetric unit, whereas Co-AK crystallized as a dimer. Nevertheless, all three crystal structures are very similar to each other, with the same LID domain topology, the only change being the presence of the different metal atoms. In the absence of any substrate, the LID domain of all holoforms of AK was present in a fully open conformational state. Normal mode analysis was performed to predict fluctuations of the LID domain along the catalytic pathway.