Crystal structure of rice Rubisco and implications for activation induced by positive effectors NADPH and 6-phosphogluconate.

The key enzyme of plant photosynthesis, D-ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), must be activated to become catalytically competent via the carbamylation of Lys201 of the large subunit and subsequent stabilization by Mg(2+) coordination. Many biochemical studies have reported that reduced nicotinamide adenine dinucleotide phosphate (NADPH) and 6-phosphogluconate (6PG) function as positive effectors to promote activation. However, the structural mechanism remains unknown. Here, we have determined the crystal structures of activated rice Rubisco in complex with NADPH, 6PG, or 2-carboxy-D-arabinitol 1,5-bisphosphate (2CABP). The structures of the NADPH and 6PG complexes adopt open-state conformations, in which loop 6 at the catalytic site and some other loops are disordered. The structure of the 2CABP complex is in a closed state, similar to the previous 2CABP-bound activated structures from other sources. The catalytic sites of the NADPH and 6PG complexes are fully activated, despite the fact that bicarbonate (NaHCO(3)) was not added into the crystallization solution. In the catalytic site, NADPH does not interact with Mg(2+) directly but interacts with Mg(2+)-coordinated water molecules, while 6PG interacts with Mg(2+) directly. These observations suggest that the two effectors promote Rubisco activation by stabilizing the complex of Mg(2+) and the carbamylated Lys201 with unique interactions and preventing its dissociation. The structure also reveals that the relaxed complex of the effectors (NADPH or 6PG), distinct from the tight-binding mode of 2CABP, would allow rapid exchange of the effectors in the catalytic sites by substrate D-ribulose 1,5-bisphosphate for catalysis in physiological conditions.

Structure of the apo decarbamylated form of 2,3-diketo-5-methylthiopentyl-1-phosphate enolase from Bacillus subtilis.

2,3-Diketo-5-methylthiopentyl-1-phosphate enolase (DK-MTP-1P enolase), a RuBisCO-like protein (RLP), catalyzes the enolization of 2,3-diketo-5-methylthiopentyl-1-phosphate. The crystal structure of the apo decarbamylated form (E form) of Bacillus subtilis DK-MTP-1P enolase (Bs-DK-MTP-1P enolase) has been determined at 2.3 A resolution. The overall structure of the E form of Bs-DK-MTP-1P enolase highly resembles that of Geobacillus kaustophilus DK-MTP-1P enolase (Gk-DK-MTP-1P enolase), with the exception of a few insertions or deletions and of a few residues at the active site. In the E form of Bs-DK-MTP-1P enolase, Lys150 (equivalent to Lys175 in RuBisCO) at the active site adopts a conformation that is distinct from those observed in the other forms of Gk-DK-MTP-1P enolase. This unusual conformational change appears to be induced by changes in the varphi and psi angles of Gly151, which is conserved in the sequences of the Bs-DK-MTP-1P and Gk-DK-MTP-1P enolases but not in those of RuBisCOs. The loop at 303-312, equivalent to the catalytic loop termed ;loop-6' in RuBisCO, is in a closed conformation in the E form of Bs-DK-MTP-1P enolase. The closed conformation appears to be stabilized by Pro312, which is conserved in the sequences of several RLPs (equivalent to Glu338 in RuBisCO). Based on these results, the characteristic structural features of DK-MTP-1P enolase are discussed.

Crystallization and preliminary X-ray analysis of 2,3-diketo-5-methylthiopentyl-1-phosphate enolase from Bacillus subtilis.

2,3-Diketo-5-methylthiopentyl-1-phosphate enolase (DK-MTP-1P enolase) from Bacillus subtilis was crystallized using the hanging-drop vapour-diffusion method. Crystals grew using PEG 3350 as the precipitant at 293 K. The crystals diffracted to 2.3 A resolution at 100 K using synchrotron radiation and were found to belong to the monoclinic space group P2(1), with unit-cell parameters a = 79.3, b = 91.5, c = 107.0 A, beta = 90.8 degrees. The asymmetric unit contained four molecules of DK-MTP-1P enolase, with a V(M) value of 2.2 A(3) Da(-1) and a solvent content of 43%.

Mechanistic insights into the hydrolysis and synthesis of ceramide by neutral ceramidase.

Ceramidase (CDase; EC 3.5.1.23) hydrolyzes ceramide to generate sphingosine and fatty acid. The enzyme plays a regulatory role in a variety of physiological events in eukaryotes and also functions as an exotoxin in particular bacteria. The crystal structures of neutral CDase from Pseudomonas aeruginosa (PaCD) in the C2-ceramide-bound and -unbound forms were determined at 2.2 and 1.4 A resolutions, respectively. PaCD consists of two domains, and the Zn(2+)- and Mg(2+)/Ca(2+)-binding sites are found within the center of the N-terminal domain and the interface between the domains, respectively. The structural comparison between the C2-ceramide-bound and unbound forms revealed an open-closed conformational change occurring to loop I upon binding of C2-ceramide. In the closed state, this loop sits above the Zn(2+) coordination site and over the opening to the substrate binding site. Mutational analyses of residues surrounding the Zn(2+) of PaCD and rat neutral CDase revealed that the cleavage or creation of the N-acyl linkage of ceramide follows a similar mechanism as observed for the Zn(2+)-dependent carboxypeptidases. The results provide an understanding of the molecular mechanism of hydrolysis and synthesis of ceramide by the enzyme. Furthermore, insights into the actions of PaCD and eukaryotic neutral CDases as an exotoxin and mediators of sphingolipid signaling are also revealed, respectively.

Maize C4-form phosphoenolpyruvate carboxylase engineered to be functional in C3 plants: mutations for diminished sensitivity to feedback inhibitors and for increased substrate affinity.

Introducing a C(4)-like pathway into C(3) plants is one of the proposed strategies for the enhancement of photosynthetic productivity. For this purpose it is necessary to provide each component enzyme that exerts strong activity in the targeted C(3) plants. Here, a maize C(4)-form phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) was engineered for its regulatory and catalytic properties so as to be functional in the cells of C(3) plants. Firstly, amino acid residues Lys-835 and Arg-894 of maize PEPC, which correspond to Lys-773 and Arg-832 of Escherichia coli PEPC, respectively, were replaced by Gly, since they had been shown to be involved in the binding of allosteric inhibitors, malate or aspartate, by our X-ray crystallographic analysis of E. coli PEPC. The resulting mutant enzymes were active but their sensitivities to the inhibitors were greatly diminished. Secondly, a Ser residue (S780) characteristically conserved in all C(4)-form PEPC was replaced by Ala conserved in C(3)- and root-form PEPCs to decrease the half-maximal concentration (S(0.5)) of PEP. The double mutant enzyme (S780A/K835G) showed diminished sensitivity to malate and decreased S(0.5)(PEP) with equal maximal catalytic activity (V(m)) to the wild-type PEPC, which will be quite useful as a component of the C(4)-like pathway to be introduced into C(3) plants.

Crystal structure of 5-methylthioribose 1-phosphate isomerase product complex from Bacillus subtilis: implications for catalytic mechanism.

The methionine salvage pathway (MSP) plays a crucial role in recycling a sulphahydryl derivative of the nucleoside. Recently, the genes and reactions in MSP from Bacillus subtilis have been identified, where 5-methylthioribose 1-phosphate isomerase (M1Pi) catalyzes a conversion of 5-methylthioribose 1-phosphate (MTR-1-P) to 5-methylthioribulose 1-phosphate (MTRu-1-P). Herein, we report the crystal structures of B. subtilis M1Pi (Bs-M1Pi) in complex with its product MTRu-1-P, and a sulfate at 2.4 and 2.7 A resolution, respectively. The electron density clearly shows the presence of each compound in the active site. The structural comparison with other homologous proteins explains how the substrate uptake of Bs-M1Pi may be induced by an open/closed transition of the active site. The highly conserved residues at the active site, namely, Cys160 and Asp240 are most likely to be involved in catalysis. The structural analysis sheds light on its catalytic mechanism of M1Pi.

Structural basis for the decarboxylation of orotidine 5'-monophosphate (OMP) by Plasmodium falciparum OMP decarboxylase.

Orotidine 5'-monophoshate decarboxylase (OMPDC) catalyses the decarboxylation of orotidine 5'-monophosphate (OMP) to uridine 5'-monophosphate (UMP). Here, we report the X-ray analysis of apo, substrate or product-complex forms of OMPDC from Plasmodium falciparum (PfOMPDC) at 2.7, 2.65 and 2.65 A, respectively. The structural analysis provides the substrate recognition mechanism with dynamic structural changes, as well as the rearrangement of the hydrogen bond array at the active site. The structural basis of substrate or product binding to PfOMPDC will help to uncover the decarboxylation mechanism and facilitate structure-based optimization of antimalarial drugs.

Enzymatic characterization of 5-methylthioribose 1-phosphate isomerase from Bacillus subtilis.

The product of the mtnA gene of Bacillus subtilis catalyzes the isomerization of 5-methylthioribose 1-phosphate (MTR-1-P) to 5-methylthioribulose 1-phosphate (MTRu-1-P). The catalysis of MtnA is a novel isomerization of an aldose phosphate harboring a phosphate group on the hemiacetal group. This enzyme is distributed widely among bacteria through higher eukaryotes. The isomerase reaction analyzed using the recombinant B. subtilis enzyme showed a Michaelis constant for MTR-1-P of 138 microM, and showed that the maximum velocity of the reaction was 20.4 micromol min(-1) (mg protein)(-1). The optimum reaction temperature and reaction pH were 35 degrees C and 8.1. The activation energy of the reaction was calculated to be 68.7 kJ mol(-1). The enzyme, with a molecular mass of 76 kDa, was composed of two subunits. The equilibrium constant in the reversible isomerase reaction [MTRu-1-P]/[MTR-1-P] was 6. We discuss the possible reaction mechanism.

Enhanced lasing properties of dissymmetric Eu(III) complex with bidentate phosphine ligands.

The luminescent and lasing properties of Eu(III) complexes were enhanced by using an dissymmetric Eu(III) complex. The photophysical properties (the emission spectral shapes, the emission lifetimes, the emission quantum yields, and the stimulated emission cross section (SEC)) were found to be dependent on the geometrical structures of Eu(III) complexes. The geometrical structures of Eu(III) complexes were determined by X-ray single crystal analyses. The symmetrical group of Eu(hfa)3(BIPHEPO) (tris(hexafluoroacetylacetonato)europium(III) 1,1'-biphenyl-2,2'-diylbis(diphenylphosphine oxide)) was found to be C1, which was more dissymmetric than Eu(hfa)3(TPPO)2 (tris(hexafluoroacetylacetonato)europium(III) 1,2-phenylenebis(diphenylphosphine oxide): C2 symmetry) and Eu(hfa)3(OPPO)2 (tris(hexafluoroacetylacetonato)europium(III) 1,2-phenylenebis(diphenylphosphine oxide): C2 symmetry). The analytical data were supported by Judd-Ofelt analysis. The most dissymmetrical Eu(III) complex, Eu(hfa)3(BIPHEPO), showed large electron transition probability and large SEC (4.64 x 10(-20) cm2). The SEC of Eu(hfa)3(BIPHEPO) was superior to even the values of Nd-glass laser for practical use (1.6-4.5 x 10(-20) cm2). The lasing properties of Eu(III) complexes in polymer thin film were measured by photopumping of a Nd:YAG laser (355 nm). The threshold energy of lasing oscillation was found to be 0.05 mJ. The increasing rate of the lasing intensity of Eu(hfa)3(BIPHEPO) as a function of the excitation energy was much larger than that of Eu(hfa)3(TPPO)2 and Eu(hfa)3(OPPO)2. The dissymmetrical structure of Eu(hfa)3(BIPHEPO) promoted the enhancement of the lasing property.

Structure and function of a hexameric copper-containing nitrite reductase.

Dissimilatory nitrite reductase (NIR) is a key enzyme in denitrification, catalyzing the first step that leads to gaseous products (NO, N(2)O, and N(2)). We have determined the crystal structure of a Cu-containing NIR from a methylotrophic denitrifying bacterium, Hyphomicrobium denitrificans, at 2.2-A resolution. The overall structure of this H. denitrificans NIR reveals a trigonal prism-shaped molecule in which a monomer consisting of 447 residues and three Cu atoms is organized into a unique hexamer (i.e., a tightly associated dimer of trimers). Each monomer is composed of an N-terminal region containing a Greek key beta-barrel folding domain, cupredoxin domain I, and a C-terminal region containing cupredoxin domains II and III. Both cupredoxin domains I and II bind one type 1 Cu and are combined with a long loop comprising 31 amino acid residues. The type 2 Cu is ligated at the interface between domain II of one monomer and domain III of an adjacent monomer. Between the two trimeric C-terminal regions are three interfaces formed by an interaction between the domains I, and the type 1 Cu in the domain is required for dimerization of the trimer. The type 1 Cu in domain II functions as an electron acceptor from an electron donor protein and then transfers an electron to the type 2 Cu, binding the substrate to reduce nitrite to NO. The discussion of the intermolecular electron transfer process from cytochrome c(550) to the H. denitrificans NIR is based on x-ray crystallographic and kinetic results.

Crystallization and preliminary crystallographic analysis of orotidine 5'-monophosphate decarboxylase from the human malaria parasite Plasmodium falciparum.

Orotidine 5'-monophosphate (OMP) decarboxylase (OMPDC; EC 4.1.1.23) catalyzes the final step in the de novo synthesis of uridine 5'-monophosphate (UMP) and defects in the enzyme are lethal in the malaria parasite Plasmodium falciparum. Active recombinant P. falciparum OMPDC (PfOMPDC) was crystallized by the seeding method in a hanging drop using PEG 3000 as a precipitant. A complete set of diffraction data from a native crystal was collected to 2.7 A resolution at 100 K using synchrotron radiation at the Swiss Light Source. The crystal exhibits trigonal symmetry (space group R3), with hexagonal unit-cell parameters a = b = 201.81, c = 44.03 A. With a dimer in the asymmetric unit, the solvent content is 46% (V(M) = 2.3 A3 Da(-1)).

Crystal structure of TBP-interacting protein (Tk-TIP26) and implications for its inhibition mechanism of the interaction between TBP and TATA-DNA.

TATA-binding protein (TBP)-interacting protein from the hyperthermophilic archaeon Thermococcus kodakaraensis strain KOD1 (Tk-TIP26) is a possible transcription regulatory protein in Thermococcales. Here, we report the crystal structure of Tk-TIP26 determined at 2.3 A resolution with multiple-wavelength anomalous dispersion (MAD) method. The overall structure of Tk-TIP26 consists of two domains. The N-terminal domain forms an alpha/beta structure, in which three alpha-helices enclose the central beta-sheet. The topology of this domain is similar to that of holliday junction resolvase Hjc from Pyrococcus furiosus. The C-terminal domain comprises three alpha-helices, six beta-strands, and two 3(10)-helices. In the dimer structure of Tk-TIP26, two molecules are related with the crystallographic twofold axis, and these molecules rigidly interact with each other via hydrogen bonds. The complex of Tk-TIP26/Tk-TBP is isolated and analyzed by SDS-PAGE and gel filtration column chromatography, resulting in a stoichiometric ratio of the interaction between Tk-TIP26 and Tk-TBP of 4:2, i.e., two dimer molecules of Tk-TIP26 formed a complex with one dimeric TBP. The electrostatic surfaces of Tk-TIP26 and TBP from Pyrocuccus woesei (PwTBP) allowed us to build a model of the Tk-TIP26/TBP complex, and to propose the inhibition mechanism where two dimer molecules of Tk-TIP26 bind to a dimeric TBP, preventing its binding to TATA-DNA.

Structural mechanism for coordination of proofreading and polymerase activities in archaeal DNA polymerases.

A novel mechanism for controlling the proofreading and polymerase activities of archaeal DNA polymerases was studied. The 3'-5'exonuclease (proofreading) activity and PCR performance of the family B DNA polymerase from Thermococcus kodakaraensis KOD1 (previously Pyrococcus kodakaraensis KOD1) were altered efficiently by mutation of a "unique loop" in the exonuclease domain. Interestingly, eight different H147 mutants showed considerable variations in respect to their 3'-5'exonuclease activity, from 9% to 276%, as against that of the wild-type (WT) enzyme. We determined the 2.75A crystal structure of the H147E mutant of KOD DNA polymerase that shows 30% of the 3'-5'exonuclease activity, excellent PCR performance and WT-like fidelity. The structural data indicate that the properties of the H147E mutant were altered by a conformational change of the Editing-cleft caused by an interaction between the unique loop and the Thumb domain. Our data suggest that electrostatic and hydrophobic interactions between the unique loop of the exonuclease domain and the tip of the Thumb domain are essential for determining the properties of these DNA polymerases.

Structural and mutational analysis of Trypanosoma brucei prostaglandin H2 reductase provides insight into the catalytic mechanism of aldo-ketoreductases.

Trypanosoma brucei prostaglandin F2alpha synthase is an aldo-ketoreductase that catalyzes the reduction of prostaglandin H2 to PGF2alpha in addition to that of 9,10-phenanthrenequinone. We report the crystal structure of TbPGFS.NADP+.citrate at 2.1 angstroms resolution. TbPGFS adopts a parallel (alpha/beta)8-barrel fold lacking the protrudent loops and possesses a hydrophobic core active site that contains a catalytic tetrad of tyrosine, lysine, histidine, and aspartate, which is highly conserved among AKRs. Site-directed mutagenesis of the catalytic tetrad residues revealed that a dyad of Lys77 and His110, and a triad of Tyr52, Lys77, and His110 are essential for the reduction of PGH2 and 9,10-PQ, respectively. Structural and kinetic analysis revealed that His110, acts as the general acid catalyst for PGH2 reduction and that Lys77 facilitates His110 protonation through a water molecule, while exerting an electrostatic repulsion against His110 that maintains the spatial arrangement which allows the formation of a hydrogen bond between His110 and C11 that carbonyl of PGH2. We also show Tyr52 acts as the general acid catalyst for 9,10-PQ reduction, and thus we not only elucidate the catalytic mechanism of a PGH2 reductase but also provide an insight into the catalytic specificity of AKRs.

Structural reorganization of the copper binding site involving Thr15 of mavicyanin from Cucurbita pepo medullosa (zucchini) upon reduction.

Mavicyanin, a glycosylated protein isolated from Cucurbita pepo medullosa (zucchini), is a member of the phytocyanin subfamily containing one polypeptide chain of 109 amino residues and an unusual type-I Cu site in which the copper ligands are His45, Cys86, His91, and Gln96. The crystal structures of oxidized and reduced mavicyanin were determined at 1.6 and 1.9 A resolution, respectively. Mavicyanin has a core structure of seven polypeptide beta-strands arranged as a beta-sandwich organized into two beta-sheets, and the structure considerably resembles that of stellacyanin from cucumber (CST) or cucumber basic protein (CBP). A flexible region was not observed on superimpositioning of the oxidized and reduced mavicyanin structures. However, the Cu(II)-epsilon-O-Gln96 bond length was extended by 0.47 A, and the Thr15 residue was rotated by 60.0 degrees and O-gamma1-Thr15 moved from a distance of 4.78 to 2.58 A from the ligand Gln96 forming a new hydrogen bond between O-gamma1-Thr15 and epsilon-O-Gln96 upon reduction. The reorganization of copper coordination geometry of mavicyanin upon reduction arouses reduction potential decreased above pH 8 [Battistuzzi et al. (2001) J. Inorg. Biochem. 83, 223-227]. The rotation of Thr15 and the hydrogen bonding with the ligand Gln96 may constitute structural evidence of the decrease in the reduction potential at high pH.

Maize phosphoenolpyruvate carboxylase. Mutations at the putative binding site for glucose 6-phosphate caused desensitization and abolished responsiveness to regulatory phosphorylation.

Phosphoenolpyruvate carboxylases (PEPC, EC 4.1.1.31) from higher plants are regulated by both allosteric effects and reversible phosphorylation. Previous x-ray crystallographic analysis of Zea mays PEPC has revealed a binding site for sulfate ion, speculated to be the site for an allosteric activator, glucose 6-phosphate (Glc-6-P) (Matsumura, H., Xie, Y., Shirakata, S., Inoue, T., Yoshinaga, T., Ueno, Y., Izui, K., and Kai, Y. (2002) Structure (Lond.) 10, 1721-1730). Because kinetic experiments have also supported this notion, each of the four basic residues (Arg-183, -184, -231, and -372' on the adjacent subunit) located at or near the binding site was replaced by Gln, and the kinetic properties of recombinant mutant enzymes were investigated. Complete desensitization to Glc-6-P was observed for R183Q, R184Q, R183Q/R184Q (double mutant), and R372Q, as was a marked decrease in the sensitivity for R231Q. The heterotropic effect of Glc-6-P on an allosteric inhibitor, l-malate, was also abolished, but sensitivity to Gly, another allosteric activator of monocot PEPC, was essentially not affected, suggesting the distinctness of their binding sites. Considering the kinetic and structural data, Arg-183 and Arg-231 were suggested to be involved directly in the binding with phosphate group of Glc-6-P, and the residues Arg-184 and Arg-372 were thought to be involved in making up the site for Glc-6-P and/or in the transmission of an allosteric regulatory signal. Most unexpectedly, the mutant enzymes had almost lost responsiveness to regulatory phosphorylation at Ser-15. An apparent lack of kinetic competition between the phosphate groups of Glc-6-P and of phospho-Ser at 15 suggested the distinctness of their binding sites. The possible roles of these Arg residues are discussed.

Crystallization and preliminary X-ray diffraction analysis of thioredoxin peroxidase from the aerobic hyperthermophilic archaeon Aeropyrum pernix K1.

Thioredoxin peroxidase is a member of the peroxiredoxin family and plays a dominant role in a hydrogen peroxide metabolism. A recombinant form of the hyperthermostable thioredoxin peroxidase from the aerobic hyperthermophilic archaeon Aeropyrum pernix K1, a polypeptide consisting of 250 amino acids, was purified. The C207S mutant protein was crystallized by the hanging-drop vapour-diffusion method using potassium sodium tartrate as the precipitant at 298 K. Diffraction data were collected and processed to 2.7 A resolution. The crystal belongs to space group P1, with unit-cell parameters a = 126.2, b = 126.3, c = 213.7 A, alpha = 80.4, beta = 80.3, gamma = 70.7 degrees. Calculation of the self-rotation function showed that the protein quaternary structure includes a fivefold axis and five twofold axes.