Entropic Comparison of Atomic-Resolution Electron Tomography of Crystals and Amorphous Materials.

Electron tomography bears promise for widespread determination of the three-dimensional arrangement of atoms in solids. However, it remains unclear whether methods successful for crystals are optimal for amorphous solids. Here, we explore the relative difficulty encountered in atomic-resolution tomography of crystalline and amorphous nanoparticles. We define an informational entropy to reveal the inherent importance of low-entropy zone-axis projections in the reconstruction of crystals. In turn, we propose considerations for optimal sampling for tomography of ordered and disordered materials.

Three-dimensional structure of myosin subfragment-1: a molecular motor.

Directed movement is a characteristic of many living organisms and occurs as a result of the transformation of chemical energy into mechanical energy. Myosin is one of three families of molecular motors that are responsible for cellular motility. The three-dimensional structure of the head portion of myosin, or subfragment-1, which contains both the actin and nucleotide binding sites, is described. This structure of a molecular motor was determined by single crystal x-ray diffraction. The data provide a structural framework for understanding the molecular basis of motility.

Fast imaging of laboratory core floods using 3D compressed sensing RARE MRI.

Three-dimensional (3D) imaging of the fluid distributions within the rock is essential to enable the unambiguous interpretation of core flooding data. Magnetic resonance imaging (MRI) has been widely used to image fluid saturation in rock cores; however, conventional acquisition strategies are typically too slow to capture the dynamic nature of the displacement processes that are of interest. Using Compressed Sensing (CS), it is possible to reconstruct a near-perfect image from significantly fewer measurements than was previously thought necessary, and this can result in a significant reduction in the image acquisition times. In the present study, a method using the Rapid Acquisition with Relaxation Enhancement (RARE) pulse sequence with CS to provide 3D images of the fluid saturation in rock core samples during laboratory core floods is demonstrated. An objective method using image quality metrics for the determination of the most suitable regularisation functional to be used in the CS reconstructions is reported. It is shown that for the present application, Total Variation outperforms the Haar and Daubechies3 wavelet families in terms of the agreement of their respective CS reconstructions with a fully-sampled reference image. Using the CS-RARE approach, 3D images of the fluid saturation in the rock core have been acquired in 16min. The CS-RARE technique has been applied to image the residual water saturation in the rock during a water-water displacement core flood. With a flow rate corresponding to an interstitial velocity of vi=1.89±0.03ftday(-1), 0.1 pore volumes were injected over the course of each image acquisition, a four-fold reduction when compared to a fully-sampled RARE acquisition. Finally, the 3D CS-RARE technique has been used to image the drainage of dodecane into the water-saturated rock in which the dynamics of the coalescence of discrete clusters of the non-wetting phase are clearly observed. The enhancement in the temporal resolution that has been achieved using the CS-RARE approach enables dynamic transport processes pertinent to laboratory core floods to be investigated in 3D on a time-scale and with a spatial resolution that, until now, has not been possible.

Crystallization, structure determination and least-squares refinement to 1.75 A resolution of the fatty-acid-binding protein isolated from Manduca sexta L.

The molecular structure of an insect fatty-acid-binding protein isolated from Manduca sexta L. has been determined and refined to a nominal resolution of 1.75 A. Crystals used in the investigation were grown from 1.6 M-ammonium sulfate solutions buffered at pH 4.5 with 50 mM-sodium succinate, and belonged to space group P2(1) with unit cell dimensions of a = 27.5 A, b = 71.0 A, c = 28.7 A and beta = 90.8 degrees. An electron density map, phased with four heavy-atom derivatives and calculated to 2.5 A resolution, allowed for complete tracing of the 131 amino acid residue polypeptide chain. Subsequent least-squares refinement of the model reduced the R-factor from 46.0% to 17.3% using all measured X-ray data from 30.0 A to 1.75 A. Approximately 92% of the amino acid residues fall into classical secondary structural elements including ten strands of anti-parallel beta-pleated sheet, two alpha-helices, one type I turn, three type II turns, four type II' turns and one type III turn. As in other fatty-acid-binding proteins, the overall molecular architecture of the insect molecule consists of ten strands of anti-parallel beta-pleated sheet forming two layers that are nearly orthogonal to one another. A helix-turn-helix motif at the N-terminal portion of the protein flanks one side of the up-and-down beta-barrel. The functional group of the fatty acid is within hydrogen-bonding distance of Gln39, Tyr129, Arg127 and a sulfate molecule, while the aliphatic portion of the ligand is surrounded by hydrophobic amino acid residues lining the beta-barrel. The binding of the carboxylic acid portion of the ligand is very similar to that observed in P2 myelin protein and the murine adipocyte lipid-binding protein, but the positioning of the hydrocarbon tail after approximately C6 is completely different.

Molecular structure of cytochrome c2 isolated from Rhodobacter capsulatus determined at 2.5 A resolution.

The molecular structure of the cytochrome c2, isolated from the purple photosynthetic bacterium Rhodobacter capsulatus, has been solved to a nominal resolution of 2.5 A and refined to a crystallographic R-factor of 16.8% for all observed X-ray data. Crystals used for this investigation belong to the space group R32 with two molecules in the asymmetric unit and unit cell dimensions of a = b = 100.03 A, c = 162.10 A as expressed in the hexagonal setting. An interpretable electron density map calculated at 2.5 A resolution was obtained by the combination of multiple isomorphous replacement with four heavy atom derivatives, molecular averaging and solvent flattening. At this stage of the structural analysis the electron densities corresponding to the side-chains are well ordered except for several surface lysine, glutamate and aspartate residues. Like other c-type cytochromes, the secondary structure of the protein consists of five alpha-helices forming a basket around the heme prosthetic group with one heme edge exposed to the solvent. The overall alpha-carbon trace of the molecule is very similar to that observed for the bacterial cytochrome c2, isolated from Rhodospirillum rubrum, with the exception of a loop, delineated by amino acid residues 21 to 32, that forms a two stranded beta-sheet-like motif in the Rb. capsulatus protein. As observed in the eukaryotic cytochrome c proteins, but not in the cytochrome c2 from Rsp. rubrum, there are two evolutionarily conserved solvent molecules buried within the heme binding pocket.

Long-term changes in open field behaviour following a single social defeat in rats can be reversed by sleep deprivation.

The long-term consequences of a single social defeat on open field behaviour in rats were studied, with special emphasis on the time course of stress-induced changes. Animals were subjected to social defeat by placing them into the territory of an aggressive male conspecific for 1 h. After the defeat session experimental animals were returned to their home cage and their own room, receiving no further cues from the resident. Other animals serving as controls were placed in a clean and empty cage for 1 h. Five-minute open field tests were performed on days 1, 2, 7, 14, and 28 after defeat, with independent groups of rats. Locomotion of the animals was recorded and analyzed with an automated video system. Social defeat resulted in a strong subsequent reduction in open field activity, which lasted till at least 7 days after the conflict. Differences in total travelled distance were no longer significant 2 weeks after the conflict. The latency for moving to the outer ring of the open field arena after the start of the test was still significantly longer 4 weeks after defeat. The stress-induced reduction in open field locomotion could be reversed by 12-h sleep deprivation during the resting phase, an intervention known to have antidepressant effects in humans. Possible relevance of the present findings with respect to human affective disorders is discussed.

An outpatient maneuver to treat bloody effluent during continuous ambulatory peritoneal dialysis (CAPD).

Asymptomatic episodes of grossly bloody effluent during continuous ambulatory peritoneal dialysis (CAPD) can be treated by following a simple therapeutic maneuver. The patient performs one to three rapid exchanges using unwarmed (room temperature), 1.5% dextrose-containing dialysate. No dwell time is employed. This treatment proves successful in a variety of clinical settings, and no adverse effects have been noted. Infusion of unwarmed dialysate likely induces peritoneal vasoconstriction and thus favors hemostasis. Bleeding of nonperitoneal etiology, such as renal cyst hemorrhage or retrograde menstruation, proves resistant.

Blood feeding on large grazers affects the transmission of Borrelia burgdorferi sensu lato by Ixodes ricinus.

The presence of Ixodes ricinus and their associated Borrelia infections on large grazers was investigated. Carcases of freshly shot red deer, mouflon and wild boar were examined for the presence of any stage of I. ricinus. Questing ticks were collected from locations where red deer and wild boar are known to occur. Presence of Borrelia burgdorferi s.l. DNA was examined in a fraction of the collected ticks. Larvae, nymphs and adult ticks were found on the three large grazers. Red deer had the highest tick burden, with many of the nymphs and adult females attached for engorgement. Most larvae had not attached. The mean number of ticks on the animals varied from 13 to 67. Ticks were highly aggregated amongst the animals: some animals had no ticks, while others had high numbers. Larvae and nymphs were mostly found on the ears, while adult ticks were attached to the axillae. The Borrelia infection rate of questing nymphs was 8.5%. Unengorged wandering nymphs on deer had a Borrelia infection rate of 12.5%, while only 0.9% of feeding nymphs carried a Borrelia infection. The infection rate of unengorged adult male ticks was 4.5%, and that of feeding female ticks was 0.7%. The data suggest that ticks feeding on red deer and wild boar lose their Borrelia infections. The implications of the results are discussed with respect to Borrelia epidemiology and maintenance of a Borrelia reservoir as well as the role of reproductive hosts for Ixodes ricinus.

X-Ray structure of the cytochrome c2 isolated from Paracoccus denitrificans refined to 1.7-A resolution.

The cytochrome c2 (formerly c550) isolated from Paracoccus denitrificans is one of the larger bacterial c-type proteins examined thus far. The molecular structure of this cytochrome has been redetermined and refined to 1.7-A resolution with a crystallographic R-factor of 17.5% for all measured X-ray data. Like other, smaller c-type cytochromes, the molecule consists of five alpha-helices that wrap around the heme group. In addition, this bacterial cytochrome contains two strands of anti-parallel beta-sheet, five Type I turns, and three Type II turns. The present model differs from the originally determined structure in several regions including the N-terminus, the loop delineated by Asp 25 to Lys 31, the region defined by Trp 86 to Val 88, and the C-terminus. A total of 103 water molecules has been positioned into the electron density map. Six of these waters are directly involved in heme binding.

Structural investigation of the antibiotic and ATP-binding sites in kanamycin nucleotidyltransferase.

Kanamycin nucleotidyltransferase (KNTase) is a plasmid-coded enzyme responsible for some types of bacterial resistance to aminoglycosides. The enzyme deactivates various antibiotics by transferring a nucleoside monophosphate group from ATP to the 4'-hydroxyl group of the drug. Detailed knowledge of the interactions between the protein and the substrates may lead to the design of aminoglycosides less susceptible to bacterial deactivation. Here we describe the structure of KNTase complexed with both the nonhydrolyzable nucleotide analog AMPCPP and kanamycin. Crystals employed in the investigation were grown from poly(ethylene glycol) solutions and belonged to the space group P2(1)2(1)2(1) with unit cell dimensions of a = 57.3 A, b = 102.2 A, c = 101.8 A, and one dimer in the asymmetric unit. Least-squares refinement of the model at 2.5 A resolution reduced the crystallographic R factor to 16.8%. The binding pockets for both the nucleotide and the antibiotic are extensively exposed to the solvent and are composed of amino acid residues contributed by both subunits in the dimer. There are few specific interactions between the protein and the adenine ring of the nucleotide; rather the AMPCPP molecule is locked into position by extensive hydrogen bonding between the alpha-, beta-, and gamma-phosphates and protein side chains. This, in part, may explain the observation that the enzyme can utilize other nucleotides such as GTP and UTP. The 4'-hydroxyl group of the antibiotic is approximately 5 A from the alpha-phosphorus of the nucleotide and is in the proper orientation for a single in-line displacement attack at the phosphorus.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular structure of a high potential cytochrome c2 isolated from Rhodopila globiformis.

Unlike their mitochondrial counterparts, the c-type cytochromes typically isolated from photosynthetic nonsulfur purple bacteria display a wide range of oxidation-reduction potentials. Here we describe the X-ray crystallographic analysis of the cytochrome c2 isolated from Rhodopila globiformis. This particular c-type cytochrome was selected for study because of its anomalously high redox potential of +450 mV. Crystals employed in the investigation belonged to the space group I4(1) with unit cell dimensions of a = b = 79.2 A, c = 75.2 A, and two molecules in the asymmetric unit. The structure was solved by the techniques of multiple isomorphous replacement with two heavy-atom derivatives and electron density modification procedures. Least-squares refinement of the model reduced the R-factor to 18.7% for all measured X-ray data from 30.0 to 2.2 A. The overall structural motif of the protein is composed of five alpha-helices, one type I turn, and six type II turns. As in other cytochromes c, there are two conserved water molecules located in the heme-binding pocket. Overall, the three-dimensional structure of the R. globiformis molecule is more similar to the eukaryotic c-type cytochromes than to other bacterial proteins.

Three-dimensional structure of phosphotriesterase: an enzyme capable of detoxifying organophosphate nerve agents.

Organophosphates, such as parathion and paraoxon, constitute the largest class of insecticides currently used in industrialized nations. In addition, many of these compounds are known to inhibit mammalian acetylcholinesterases thereby acting as nerve agents. Consequently, organophosphate-degrading enzymes are of considerable interest in light of their ability to detoxify such compounds. Here we report the three-dimensional structure of such an enzyme, namely, phosphotriesterase, as determined by single crystal X-ray diffraction analysis to 2.1-A resolution. Crystals employed in this investigation belonged to the space group P2(1)2(1)2 with unit cell dimensions of a = 80.3 A, b = 93.4 A, and c = 44.8 A and one molecule per asymmetric unit. The structure was solved by multiple isomorphous replacement with two heavy-atom derivatives and refined to a crystallographic R factor of 18.0%. As observed in various other enzymes, the overall fold of the molecule consists of an alpha/beta barrel with eight strands of parallel beta-pleated sheet. In addition, there are two antiparallel beta-strands at the N-terminus. The molecular model of phosphotriesterase presented here provides the initial structural framework necessary toward understanding the enzyme's broad substrate specificities and its catalytic mechanism.

The binding of substrate analogs to phosphotriesterase.

Phosphotriesterase (PTE) from Pseudomonas diminuta catalyzes the detoxification of organophosphates such as the widely utilized insecticide paraoxon and the chemical warfare agent sarin. The three-dimensional structure of the enzyme is known from high resolution x-ray crystallographic analyses. Each subunit of the homodimer folds into a so-called TIM barrel, with eight strands of parallel beta-sheet. The two zinc ions required for activity are positioned at the C-terminal portion of the beta-barrel. Here, we describe the three-dimensional structure of PTE complexed with the inhibitor diisopropyl methyl phosphonate, which serves as a mimic for sarin. Additionally, the structure of the enzyme complexed with triethyl phosphate is also presented. In the case of the PTE-diisopropyl methyl phosphonate complex, the phosphoryl oxygen of the inhibitor coordinates to the more solvent-exposed zinc ion (2.5 A), thereby lending support to the presumed catalytic mechanism involving metal coordination of the substrate. In the PTE-triethyl phosphate complex, the phosphoryl oxygen of the inhibitor is positioned at 3.4 A from the more solvent-exposed zinc ion. The two structures described in this report provide additional molecular understanding for the ability of this remarkable enzyme to hydrolyze such a wide range of organophosphorus substrates.

New reactions in the crotonase superfamily: structure of methylmalonyl CoA decarboxylase from Escherichia coli.

The molecular structure of methylmalonyl CoA decarboxylase (MMCD), a newly defined member of the crotonase superfamily encoded by the Escherichia coli genome, has been solved by X-ray crystallographic analyses to a resolution of 1.85 A for the unliganded form and to a resolution of 2.7 A for a complex with an inert thioether analogue of methylmalonyl CoA. Like two other structurally characterized members of the crotonase superfamily (crotonase and dienoyl CoA isomerase), MMCD is a hexamer (dimer of trimers) with each polypeptide chain composed of two structural motifs. The larger N-terminal domain contains the active site while the smaller C-terminal motif is alpha-helical and involved primarily in trimerization. Unlike the other members of the crotonase superfamily, however, the C-terminal motif is folded back onto the N-terminal domain such that each active site is wholly contained within a single subunit. The carboxylate group of the thioether analogue of methylmalonyl CoA is hydrogen bonded to the peptidic NH group of Gly 110 and the imidazole ring of His 66. From modeling studies, it appears that Tyr 140 is positioned within the active site to participate in the decarboxylation reaction by orienting the carboxylate group of methylmalonyl CoA so that it is orthogonal to the plane of the thioester carbonyl group. Surprisingly, while the active site of MMCD contains Glu 113, which is homologous to the general acid/base Glu 144 in the active site of crotonase, its carboxylate side chain is hydrogen bonded to Arg 86, suggesting that it is not directly involved in catalysis. The new constellation of putative functional groups observed in the active site of MMCD underscores the diversity of function in this superfamily.

Methodological implications of testing anaerobe susceptibility to cephalosporins (cefazolin, cefamandole, cefoxitin).

By simultaneously performing broth dilution, agar dilution, and agar diffusion tests with Bacteroidaceae and Peptococcaceae, the influence of methodology upon the outcome of susceptibility testing to cefazolin, cefamandole and cefoxitin was studied. With beta-lactamase positive and negative Bacteroidaceae, the results of the broth dilution and agar dilution tests were in good agreement for cefoxitin. However, when tested with cefaxolin and cefamandole, beta-lactamase positive Bacteroides strains had mostly high broth dilution MICs and relatively low agar dilution MICs. The statistical analysis of the relationship between zone size and broth dilution or agar dilution MICs frequently showed lack of stochastic linearity or relatively low correlation coefficients.

The structure of carbamoyl phosphate synthetase determined to 2.1 A resolution.

Carbamoyl phosphate synthetase catalyzes the formation of carbamoyl phosphate from one molecule of bicarbonate, two molecules of Mg2+ATP and one molecule of glutamine or ammonia depending upon the particular form of the enzyme under investigation. As isolated from Escherichia coli, the enzyme is an alpha,beta-heterodimer consisting of a small subunit that hydrolyzes glutamine and a large subunit that catalyzes the two required phosphorylation events. Here the three-dimensional structure of carbamoyl phosphate synthetase from E. coli refined to 2.1 A resolution with an R factor of 17.9% is described. The small subunit is distinctly bilobal with a catalytic triad (Cys269, His353 and Glu355) situated between the two structural domains. As observed in those enzymes belonging to the alpha/beta-hydrolase family, the active-site nucleophile, Cys269, is perched at the top of a tight turn. The large subunit consists of four structural units: the carboxyphosphate synthetic component, the oligomerization domain, the carbamoyl phosphate synthetic component and the allosteric domain. Both the carboxyphosphate and carbamoyl phosphate synthetic components bind Mn2+ADP. In the carboxyphosphate synthetic component, the two observed Mn2+ ions are both octahedrally coordinated by oxygen-containing ligands and are bridged by the carboxylate side chain of Glu299. Glu215 plays a key allosteric role by coordinating to the physiologically important potassium ion and hydrogen bonding to the ribose hydroxyl groups of ADP. In the carbamoyl phosphate synthetic component, the single observed Mn2+ ion is also octahedrally coordinated by oxygen-containing ligands and Glu761 plays a similar role to that of Glu215. The carboxyphosphate and carbamoyl phosphate synthetic components, while topologically equivalent, are structurally different, as would be expected in light of their separate biochemical functions.

Molecular structure of the oxidized high-potential iron-sulfur protein isolated from Ectothiorhodospira vacuolata.

The high-potential iron-sulfur protein (iso-form II) isolated from Ectothiorhodospira vacuolata has been crystallized and its three-dimensional structure determined by molecular replacement procedures and refined to 1.8-A resolution with a crystallographic R factor of 16.3%. Crystals employed in the investigation belonged to the space group C222(1) with unit cell dimensions of a = 58.4 A, b = 64.7 A, and c = 39.3 A and one molecule per asymmetric unit. Like those HiPIPs structurally characterized thus far, the E. vacuolata molecule contains mostly reverse turns that wrap around the iron-sulfur cluster with cysteine residues 34, 37, 51, and 65 ligating the metal center to the polypeptide chain. There are 57 ordered solvent molecules, most of which lie at the surface of the protein. Two of these water molecules play important structural roles by stabilizing the loops located between Asp 42 and Lys 57. The metal center binding pocket is decidedly hydrophobic with the closest solvent molecule being 6.9 A from S2 of the [4Fe-4S] cluster. The E. vacuolata HiPIP molecules pack in the crystalline lattice as dimers with their iron-sulfur centers approximately 17.5 A apart. On the basis of biochemical properties, it was anticipated that the E. vacuolata HiPIP would be structurally more similar to the HiPIP isolated from Ectothiorhodospira halophila than to the protein obtained from Chromatium vinosum. In fact, the E. vacuolata molecule is as structurally close to the C. vinosum HiPIP as it is to the E. halophila protein due to the presence of various insertions and deletions that disrupt local folding.(ABSTRACT TRUNCATED AT 250 WORDS)

Three-dimensional structure of the zinc-containing phosphotriesterase with the bound substrate analog diethyl 4-methylbenzylphosphonate.

Phosphotriesterase from Pseudomonas diminuta catalyzes the hydrolysis of paraoxon and related acetylcholinesterase inhibitors with rate enhancements that approach 10(12). The enzyme requires a binuclear metal center for activity and as isolated contains 2 equiv of zinc per subunit. Here we describe the three-dimensional structure of the Zn2+/Zn2+-substituted enzyme complexed with the substrate analog diethyl 4-methylbenzylphosphonate. Crystals employed in the investigation belonged to the space group C2 with unit cell dimensions of a = 129.6 A, b = 91.4 A, c = 69.4 A, beta = 91.9 degrees, and two subunits in the asymmetric unit. The model was refined by least-squares analysis to a nominal resolution of 2.1 A and a crystallographic R-factor of 15.4% for all measured X-ray data. As in the previously reported structure of the cadmium-containing enzyme, the bridging ligands are a carbamylated lysine residue (Lys 169) and a hydroxide. The zinc ions are separated by 3.3 A. The more buried zinc ion is surrounded by His 55, His 57, Lys 169, Asp 301, and the bridging hydroxide in a trigonal bipyramidal arrangement as described for the cadmium-substituted enzyme. Unlike the octahedral coordination observed for the more solvent-exposed cadmium ion, however, the second zinc is tetrahedrally ligated to Lys 169, His 201, His 230, and the bridging hydroxide. The diethyl 4-methylbenzylphosphonate occupies a site near the binuclear metal center with the phosphoryl oxygen of the substrate analog situated at 3.5 A from the more solvent-exposed zinc ion. The aromatic portion of the inhibitor binds in a fairly hydrophobic pocket. A striking feature of the active site pocket is the lack of direct electrostatic interactions between the inhibitor and the protein. This most likely explains the broad substrate specificity exhibited by phosphotriesterase. The position of the inhibitor within the active site suggests that the nucleophile for the hydrolysis reaction is the metal-bound hydroxide.

Ligand-induced domain movement in pyruvate kinase: structure of the enzyme from rabbit muscle with Mg2+, K+, and L-phospholactate at 2.7 A resolution.

The structure of rabbit muscle pyruvate kinase crystallized as a complex with Mg2+, K+, and L-phospholactate (L-P-lactate) has been solved and refined to 2.7 A resolution. The crystals, grown from solutions of polyethylene glycol 8000 at pH 7.5, belong to the space group P2(1) and have unit cell parameters a = 144.4 A, b = 112.6 A, c = 171.2 A, and beta = 93.7 degrees. The asymmetric unit contains two tetramers. The crystal structure reveals that the eight subunits within the asymmetric unit adopt several different conformations. These conformations are characterized by differences in the relative positions of protein domains A and B, resulting in different degrees of closure of the active site cleft that occupies the interface between these two domains. The global conformational differences may be described as rotations of the B domain with respect to the (beta/alpha)8-barrel of the A domain. Carbon atoms of the backbone in domain B rotate >20 degrees from the most open to the most closed subunit. The different conformations among subunits within the asymmetric unit are accompanied by 3-3.8 A shifts in the position of Mg2+ and a significant change in the orientation of the phenyl ring of Phe 243. In all of the subunits, Mg2+ coordinates to the protein through the carboxylate side chains of Glu 271 and Asp 295. In the subunit having the most closed conformation, Mg2+ also coordinates to the carboxylate oxygen, the bridging ester oxygen, and a nonbridging phosphoryl oxygen of L-P-lactate. Mg2+ to L-P-lactate coordination is missing in subunits exhibiting a more open conformation. K+ coordinates to four protein ligands and to a phosphoryl oxygen of the L-P-lactate. The position and liganding of K+ are unaffected by the different conformations of the subunits. The side chain of Arg 72, Mg2+, and K+ provides a locus of positive charge for the phosphate moiety of the analog in the closed subunit.

Structure of the bis(Mg2+)-ATP-oxalate complex of the rabbit muscle pyruvate kinase at 2.1 A resolution: ATP binding over a barrel.

Pyruvate kinase from rabbit muscle has been cocrystallized as a complex with MgIIATP, oxalate, Mg2+, and either K+ or Na+. Crystals with either Na+ or K+ belong to the space group P2(1)2(1)2(1), and the asymmetric units contain two tetramers. The structures were solved by molecular replacement and refined to 2.1 (K+) and 2.35 A (Na+) resolution. The structures of the Na+ and K+ complexes are virtually isomorphous. Each of the eight subunits within the asymmetric unit contains MgIIoxalate as a bidentate complex linked to the protein through coordination of Mg2+ to the carboxylates of Glu 271 and Asp 295. Six of the subunits also contain an alpha,beta,gamma-tridentate complex of MgIIATP, and the active-site cleft, located between domains A and B, is closed in these subunits. In the remaining two subunits MgIIATP is missing, and the active-site cleft is open. Closure of the active-site cleft in the fully liganded subunits includes a rotation of 41 degrees of the B domain relative to the A domain. alpha-Carbons of residues in the B domain undergo movements of up to 17.8 A (Lys 124) in the cleft closure. Lys 206, Arg 119, and Asp 177 from the B domain move several angstroms from their positions in the open conformation to contact the MgIIATP complex in the active site. The gamma-phosphate of ATP coordinates to both magnesium ions and to the monovalent cation, K+ or Na+. A Mg2+-coordinated oxygen from the MgIIoxalate complex lies 3.0 A from Pgamma of ATP, and this oxygen is positioned for an in-line attack on the phosphorus. The side chains of Lys 269 and Arg 119 are positioned to provide leaving-group activation in the forward and reverse directions. There is no obvious candidate for the acid/base catalyst near the 2-si face of the prospective enolate of the normal substrate. A functional group linked through solvent and side-chain hydroxyls may function in a proton relay.