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W N Hunter

Publications and source records attributed to W N Hunter.

At least 55 records · Page 3Linked to original sources
PubMed1994

Crystal and molecular structure of r(CGCGAAUUAGCG): an RNA duplex containing two G(anti).A(anti) base pairs.

BACKGROUND: Non-Watson-Crick base pair associations contribute significantly to the stabilization of RNA tertiary structure. The conformation adopted by such pairs appears to be a function of both the sequence and the secondary structure of the RNA molecule. G.A mispairs adopt G(anti).A(anti) configurations in some circumstances, such as the ends of helical regions of rRNAs, but in other circumstances probably adopt an unusual configuration in which the inter-base hydrogen bonds involve functional groups from other bases. We investigated the structure of G.A pairs in a synthetic RNA dodecamer, r(CGCGAAUUAGCG), which forms a duplex containing two such mismatches. RESULTS: The structure of the RNA duplex was determined by single crystal X-ray diffraction techniques to a resolution in the range 7.0-1.8A, and found to be an A-type helical structure with 10 Watson-Crick pairs and two G.A mispairs. The mispairs adopt the G(anti).A(anti) conformation, held together by two obvious hydrogen bonds. Unlike analogous base pairs seen in a DNA duplex, they do not exhibit a high propeller twist and may therefore be further stabilized by weak, reverse, three-center hydrogen bonds. CONCLUSIONS: G(anti).A(anti) mispairs are held together by two hydrogen of guanine and the N6 and N1 of adenine. If the mispairs do not exhibit high propeller twist they may be further stabilized by inter-base reverse three-centre hydrogen bonds. These interactions, and other hydrogen bonds seen in our study, may be important in modelling the structure of RNA molecules and their interactions with other molecules.

Animals↗
PubMed1994

Guanine-1,N6-ethenoadenine base pairs in the crystal structure of d(CGCGAATT(epsilon dA)GCG).

A single-crystal X-ray analysis of the synthetic oligomer d(CGCGAATT(epsilon dA)GCG) (epsilon dA = 1,N6-ethenoadenosine) has been carried out. The B-form duplex crystallizes in the orthorhombic space group P2(1)2(1)2(1) with unit cell dimensions a = 24.31 A, b = 39.65 A, and c = 63.05 A. Refinement has converged with R = 0.182 for 2837 reflections in the resolution range 7.0-2.25 A for a model consisting of the duplex, one Mg2+ ion, and 127 water molecules. The structure contains two G.epsilon dA base pairings which adopt a G(anti).epsilon dA(syn) conformation. The geometry of the two mispairs suggests that the G.epsilon dA pairing are held together by three interbase hydrogen bonds. These are N2(G)-H...N1(epsilon dA), N1(G)...N9(epsilon dA), and O6(G)...H-C8(epsilon dA). The last interaction serves to alleviate the destabilizing effect that would occur due to the presence of an unfulfilled hydrogen bond acceptor. A superposition of the G(4).epsilon dA-(21) base pair found in this structure and the Watson-Crick G(4).C(21) base pair observed in the native dodecamer d(CGCGAATTCGCG) indicates a significant difference in the sugar/phosphate backbone. However, the overall conformations of the two duplexes remain similar, suggesting that the modified base pairs are accommodated into the double helix mainly by alterations of the backbone conformation. Such structural rearrangement of the backbone, upon incorporation of epsilon dA, may provide a signal to the 3-methyladenine-DNA glycosylase that repairs such lesions.

Adenine↗
PubMed1994

Structure of trypanothione reductase from Crithidia fasciculata at 2.6 A resolution; enzyme-NADP interactions at 2.8 A resolution.

Trypanothione reductase is an FAD-dependent disulfide oxidoreductase which catalyses the reduction of trypanothione using NADPH as co-factor. The enzyme is unique to protozoan parasites from the genera Trypanosoma and Leishmania and is an important target for the design of improved antitrypanocidal drugs. We present details of the structure of trypanothione reductase from Crithidia fasciculata solved by molecular replacement, using human glutathione reductase as a search model, and refined to an R factor of 16.1% with data between 8.0 and 2.6 A resolution. The model comprises two subunits (one containing 487 residues, the other 486), an FAD prosthetic group, plus 392 solvent molecules. The last four C-terminal residues are not seen in either subunit and the density is poor for the N-terminal residue of subunit B. The model has a root-mean-square deviation from ideality of 0.016 A for bond lengths and 3.2 degrees for bond angles. Each subunit was independently refined in the latter stages of the analysis but the subunits remain similar as indicated by the root-mean-square deviation of 0.35 A for C(alpha) atoms. Trypanothione reductase has 36% sequence identity with human glutathione reductase and the root-mean-square deviation between the 462 C(alpha) atoms in the secondary structural units common to the two proteins is 1.1 A. However, there are large differences in the loop regions and significant shifts in the orientation of the four domains within each subunit. Domain II, which binds the dinucleotide co-factor, and domain IV, which forms the interface between the two subunits, are both rotated by approximately 5 degrees with respect to domain I, which binds the FAD moiety, when compared with glutathione reductase. Crystals of trypanothione reductase have been soaked in the dinucleotide co-factor NADPH and N(1)-glutathionylspermidine disulfide substrate and the structure of the resulting complex determined at 2.8 A resolution. Strong density is observed for the adenosine end of the co-factor which forms many charged interactions with the protein though the density for the nicotinamide moiety is more diffuse. The mode of binding indicates that NADP is bound to the enzyme in a similar conformation to that observed with human glutathione reductase.

Journal Article↗
PubMed1993

The crystal structure of N4-methylcytosine.guanosine base-pairs in the synthetic hexanucleotide d(CGCGm4CG).

The structure of d(CGCGm4CG) were m4C = N4-methylcytosine has been determined by crystallographic methods. The crystals are multifaced prisms, with orthorhombic space group P2(1)2(1)2(1) and unit cell dimensions of a = 17.98, b = 30.77 and c = 44.75A. The asymmetric unit consists of one duplex of hexanucleotide and 49 waters. The R-factor is 0.189 for 1495 reflections with F > or = sigma(F) to a resolution limit of 1.8A. The double helix has a Z-DNA type structure which appears to be intermediate in structure to the two previously characterised structure types for Z-DNA hexamers. The two m4C.G base-pairs adopt structures that are very similar to those of the equivalent base-pairs in the structure of the native sequence d(CGCGCG) except for the presence of the methyl groups which are trans to the N3 atoms of their parent nucleotides and protrude into the solvent region. The introduction of the modified base-pairs into the d(CGCGCG) duplex appears to have a minimal effect on the overall base-pair morphology of the Z-DNA duplex.

Base Composition↗
PubMed1993

Crystal and molecular structure of d(CGTAGATCTACG) at 2.25 A resolution.

Single crystal X-ray diffraction methods have been used to determine the structure of the synthetic DNA dodecamer d(CGTAGATCTACG). The dodecamer presents a new crystal form of B-DNA in the monoclinic system, space group C2 with a = 64.83 A, b = 35.36 A, c = 25.35 A and beta = 92.24 degrees. Structure solution was by molecular replacement and the refinement used a combination of rigid body treatment, molecular dynamics simulated annealing and restrained least-squares methods. The refinement has been concluded with an R-factor of 13.8% for 2120 reflections (78% of what is theoretically available) with F > or = 2 sigma (F) in the resolution range 7.0 to 2.25 A. The asymmetric unit comprises a B-form duplex (24 nucleotides, molecular weight 7.2 kDa), 136.5 water molecules and a single magnesium ion at 50% occupancy. The DNA model has root-mean-square derivations from standard bond lengths of 0.011 A. The central part of the structure is d(GATC), a tetrad sequence involved in methylation and mismatch repair. A comparison with other structures containing such a tetrad indicates polymorphism at the tetrad level. In our structure a narrow minor groove with a pronounced hydration pattern is observed. Such features may be of importance in the recognition of the sequence by specific enzymes. A noteworthy feature of the hydration is the identification of several pentagonal arrangements of hydrogen bonding groups. Details of conformational parameters, base stacking patterns, intermolecular interactions and hydration are presented and comparisons with related structures are given.

Base Sequence↗
PubMed1993

Refined structure of cadmium-substituted concanavalin A at 2.0 A resolution.

The three-dimensional structure of cadmium-substituted concanavalin A has been refined using X-PLOR. The R factor on all data between 8 and 2 A is 17.1%. The protein crystallizes in space group I222 with cell dimensions a = 88.7, b = 86.5 and c = 62.5 A and has one protein subunit per asymmetric unit. The final structure contains 237 amino acids, two Cd ions, one Ca ion and 144 water molecules. One Cd ion occupies the transition-metal binding site and the second occupies an additional site, the coordinates of which were first reported by Weinzierl & Kalb [FEBS Lett. (1971), 18, 268-270]. The additional Cd ion is bound with distorted octahedral symmetry and bridges the cleft between the two monomers which form the conventional dimer of concanavalin A. This study provides a detailed analysis of the refined structure of saccharide-free concanavalin A and is the basis for comparison with saccharide complexes reported elsewhere.

Journal Article↗
PubMed1993

Anthracycline-DNA interactions at unfavourable base-pair triplet-binding sites: structures of d(CGGCCG)/daunomycin and d(TGGCCA)/adriamycin complexes.

The structures of two hexanucleotide-anthracycline complexes d(CGGCCG)/daunomycin and d(TGGCCA)/adriamycin have been determined using single-crystal X-ray diffraction techniques. In both cases the anthracycline molecule is bound to non-preferred d(YGG) base-pair triplet sites. For both complexes the crystals are tetragonal and belong to the space group P4(1)2(1)2. Unit-cell dimensions are a = 28.07 (2), c = 53.35 (1) and a = 28.01 (1), c = 52.99 (1) A, respectively, and the asymmetric unit of both structures consists of one strand of DNA, one drug molecule and approximately 50 water molecules. For the d(CGGCCG) complex the refinement converged with an R factor of 0.21 for 1108 reflections with F >/= 2sigma(F) in the resolution range 7.0-1.9 A. For the complex involving d(TGGCCA) the final R value was 0.22 for 1475 reflections in the range 7.0-1.7 A with the same criterion for observed data. Both complexes are essentially isomorphous with related structures but differ in terms of the number of favourable van der Waals interactions of the amino sugars of the drug molecules with the DNA duplexes and the formation in the minor groove of heterodromic pentagonal arrangements of hydrogen bonds involving water molecules which link the amino sugars to the DNA. These differences in structure are used to rationalize the lack of affinity of daunomycin-type anthracyclines for d(YGG) and d(YGC) sites.

Journal Article↗
PubMed1993

The structure of an idarubicin-d(TGATCA) complex at high resolution.

The crystal structure of the DNA hexamer d(TGATCA) complexed with the anthracycline antibiotic idarubicin has been determined at 1.6 A resolution. The asymmetric unit consists of a single hexamer oligonucleotide strand, one drug molecule and 35 water molecules. The complex crystallizes in the tetragonal space group P4(1)2(1)2, Z = 8 with lattice dimensions of a = b = 28.19 (3), c = 52.77 (4) A, V = 41 935 A(3). The structure is isomorphous with a series of hexamer-anthracycline complexes and was solved by molecular replacement. Restrained least-squares methods interspersed with computer-graphics map inspection and model manipulation were used to refine the structure. The R factor is 0.22 for 2032 reflections with F >/= 3sigma(F) in the resolution range 8.0-1.6 A. The self-complementary DNA forms a distorted B-DNA double helix with two idarubicin molecules intercalated in the d(TpG) steps of the duplex. The duplex is formed by utilization of a crystallographic twofold axis of symmetry. The idarubicin chromophore is oriented at right angles to the long axis of the DNA base pairs with the anthracycline amino-sugar moiety positioned in the minor groove. Our structure determination allows for comparison with a d(CGATCG)-idarubicin complex recently reported. Despite the sequence alteration at the intercalation step, the structures are very similar. The geometry of the intercalation and the nature of the interactions are conserved irrespective of the DNA sequence involved in the binding.

Journal Article↗
PubMed1993

Substrate interactions between trypanothione reductase and N1-glutathionylspermidine disulphide at 0.28-nm resolution.

The enzyme trypanothione reductase has been identified as a prime target for the rational design of inhibitors which may have clinical use in the treatment of tropical diseases caused by the genera Trypanosoma and Leishmania. To aid the design or identification of new inhibitors of this enzyme we have elucidated the structural detail of a trypanothione reductase complexed with one of the naturally occurring substrates, N1-glutathionylspermidine disulphide, by single-crystal X-ray diffraction methods at 0.28-nm resolution. The model for the Crithidia fasciculata enzyme-substrate complex has an R-factor of 14.8% and root-mean-square deviations of 0.0015 nm and 3.3 degrees on bond lengths and angles respectively. Hydrogen bonding and van der Waals interactions between the enzyme and substrate are dominated by the amino acid side chains. The substrate binds in a rigid active site such that one glutathione moiety is in a V-shape, the other in an extended conformation. One spermidine moiety binds closely to a hydrophobic patch in the active site formed by a tryptophan and a methionine. Distances between the methionine S delta and the terminal N of this spermidine suggest that a hydrogen bond may supplement the hydrophobic interactions in this part of the active site.

Animals↗
PubMed1992

The conformational variability of an adenosine.inosine base-pair in a synthetic DNA dodecamer.

A crystal structure analysis of the synthetic deoxydodecamer d(CGCAAATTIGCG) which contains two adenosine.inosine (A.I) mispairs has revealed that, in this sequence, the A.I base-pairs adopt a A(anti).I(syn) configuration. The refinement converged at R = 0.158 for 2004 reflections with F greater than or equal to 2 sigma(F) in the range 7.0-2.5A for a model consisting of the DNA duplex and 71 water molecules. A notable feature of the structure is the presence of an almost complete spine of hydration spanning the minor groove of the whole of the (AAATTI)2 core region of the duplex. pH-dependent ultraviolet melting studies have suggested that the base-pair observed in the crystal structure is, in fact, a protonated AH+ (anti).I(syn) species and that the A.I base-pairs in the sequence studied display the same conformational variability as A.G mispairs in the sequence d(CGCAAATTGGCG). The AH+(anti).I(syn) base-pair predominates below pH 6.5 and an A(anti).I(anti) mispair is the major species present between pH 6.5 and 8.0. The protonated base-pairs are held together by two hydrogen bonds one between N6(A) and O6(I) and the other between N1(A) and N7(I). This second hydrogen bond is a direct result of the protonation of the N1 of adenosine. The ultraviolet melting studies indicate that the A(anti).I(anti) base-pair is more stable than the A(anti).G(anti) base-pair but that the AH+(anti).I(syn) base pair is less stable than its AH+(anti).G(syn) analogue. Possible reasons for this observation are discussed.

Adenosine↗
PubMed1992

Conformation of guanine-8-oxoadenine base pairs in the crystal structure of d(CGCGAATT(O8A)GCG).

The structure of the synthetic deoxydodecamer d(CGCGAATT(O8A)GCG)2 (O8A = 8-oxoadenine) has been determined by single-crystal X-ray diffraction techniques. The oligonucleotide crystallizes in the orthorhombic space group P2(1)2(1)2(1) with cell dimensions of a = 25.48 A, b = 41.84 A, and c = 64.91 A. The refinement has converged with an R-factor of 0.151 for 1119 reflections in the resolution range 8.0-2.25 A. Sixty-seven solvent molecules were located during the course of the refinement. The B-DNA helix consists of ten Watson-Crick base pairs and two guanine-8-oxoadenine (G.O8A) base pairs. In order to achieve hydrogen-bonding complementarity between the two bases, an unusual G(anti).O8A-(syn) wobble conformation is adopted. It is proposed that the G.O8A mispairs are held together by a network of four interbase hydrogen bonds which are the result of the formation of two reverse three-center hydrogen-bonding systems. These involve one carbonyl oxygen lone pair interacting with two hydrogen atoms. In a departure from previous observations of the characteristics of purine-purine anti-syn base pairs, lambda 1 and lambda 2, the angles between the glycosidic bonds and the C1'-C1' vector, are symmetric. A reassessment of the other purine-purine mispairs suggests that similar three-center hydrogen bonds may occur and make a contribution to stabilizing other base pairings.

Adenine↗
PubMed1992

Active site of trypanothione reductase. A target for rational drug design.

The X-ray crystal structure of the enzyme trypanothione reductase, isolated from the trypanosomatid organism Crithidia fasciculata, has been solved by molecular replacement. The search model was the crystal structure of human glutathione reductase that shares approximately 40% sequence identity. The trypanosomal enzyme crystallizes in the tetragonal space group P4(1) with unit cell lengths of a = 128.9 A and c = 92.3 A. The asymmetric unit consists of a homodimer of approximate molecular mass 108 kDa. We present the structural detail of the active site as derived from the crystallographic model obtained at an intermediate stage of the analysis using diffraction data to 2.8 A resolution with an R-factor of 23.2%. This model has root-mean-square deviations from ideal geometry of 0.026 A for bond lengths and 4.7 degrees for bond angles. The trypanosomid enzyme assumes a similar biological function to glutathione reductase and, although similar in topology to human glutathione reductase, has an enlarged active site and a number of amino acid differences, steric and electrostatic, which allows it to process only the unique substrate trypanothione and not glutathione. This protein represents a prime target for chemotherapy of several debilitating tropical diseases caused by protozoan parasites belonging to the genera Trypanosoma and Leishmania. The structural differences between the parasite and host enzymes and their substrates thus provides a rational basis for the design of new drugs active against trypanosomes. In addition, our model explains the results of site-directed mutagenesis experiments, carried out on recombinant trypanothione reductase and glutathione reductases, designed by consideration of the crystal structure of human glutathione reductase.

Amino Acid Sequence↗
PubMed1992

The molecular structure of a 4'-epiadriamycin complex with d(TGATCA) at 1.7A resolution: comparison with the structure of 4'-epiadriamycin d(TGTACA) and d(CGATCG) complexes.

The structure of the complex between d(TGATCA) and the anthracycline 4'-epiadriamycin has been determined by crystallographic methods. The crystals are tetragonal, space group P4(1)2(1)2 with unit cell dimensions of a = 28.01, c = 52.95A. The asymmetric unit consists of one strand of hexanucleotide, one molecule of 4'-epiadriamycin and 34 waters. The R-factor is 20.2% for 1694 reflections with F greater than or equal to 2 sigma F to 1.7A. Two asymmetric units associate to generate a duplex complexed with two drug molecules at the d(TpG) steps of the duplex. The chromophore intercalates between these base pairs with the anthracycline amino-sugar positioned in the minor groove. The double helix is a distorted B-DNA type structure. Our structure determination of d(TGATCA) complexed to 4'-epiadriamycin allows for comparison with the previously reported structures of 4'-epiadriamycin bound to d(TGTACA) and to d(CGATCG). The three complexes are similar in gross features and the intercalation geometry is the same irrespective of whether a d(CpG) or d(TpG) sequence is involved. However, the orientation of the amino-sugar displays a dependence on the sequence adjacent to the intercalation site. The flexibility of this amino-sugar may help explain why this class of antibiotics displays a relative insensitivity to base sequence when they bind to DNA.

Epirubicin↗
PubMed1992

Initiating a crystallographic study of a class II fructose-1,6-bisphosphate aldolase.

We have reproducibly crystallized the metal-dependent Class II fructose-1,6-bisphosphate aldolase from Escherichia coli. Crystals in the shape of truncated hexagonal bipyramids have unit cell dimensions of a = b = 78.4 A, c = 290.6 A and are suitable for a detailed structural analysis. The space group has been identified as P6(1)22 or enantiomorph. Data sets to approximately 2.9 A resolution have been recorded using both the Rigaku R-AXIS IIc image plate area detector coupled to a copper target rotating anode X-ray source and using the MAR image plate systems with synchrotron radiation at the EMBL outstation DESY in Hamburg, and at S.R.S. Daresbury. Diffraction beyond 2.5 A has been observed when large freshly grown crystals are used with the synchrotron beam. A data set to this resolution has been collected. Several putative heavy-atom derivative data sets have also been measured using synchrotron radiation facilities and analysis of these data sets is in progress.

Crystallization↗
PubMed1992

Anthracycline binding to DNA. High-resolution structure of d(TGTACA) complexed with 4'-epiadriamycin.

Crystallographic methods have been applied to determine the high-resolution structure of the complex formed between the self-complementary oligonucleotide d(TGTACA) and the anthracycline antibiotic 4'-epiadriamycin. The complex crystallises in the tetragonal system, space group P4(1)2(1)2 with a = 2.802 nm and c = 5.293 nm, and an asymmetric unit consisting of a single DNA strand, one drug molecule and 34 solvent molecules. The refinement converged with an R factor of 0.17 for the 2381 reflections with F greater than or equal to 3 sigma F in the resolution range 0.70-0.14 nm. Two asymmetric units associate such that a distorted B-DNA-type hexanucleotide duplex is formed incorporating two drug molecules that are intercalated at the TpG steps. The amino sugar of 4'-epiadriamycin binds in the minor groove of the duplex and displays different interactions from those observed in previously determined structures. Interactions between the hydrophilic groups of the amino sugar and the oligonucleotide are all mediated by solvent molecules. Ultraviolet melting measurements and comparison with other anthracycline-DNA complexes suggest that these indirect interactions have a powerful stabilising effect on the complex.

Base Sequence↗
PubMed1990

Initiating a crystallographic study of trypanothione reductase.

We have obtained well-ordered single crystals of the flavoenzyme trypanothione reductase from Crithidia fasciculata. The crystals are tetragonal rods with unit cell dimensions a = 128.6 A, c = 92.5 A. The diffraction pattern corresponds to a primitive lattice. Laue class 4/m. Diffraction to better than 2.4 A has been recorded at the Daresbury Synchrotron. The accurate elucidation of the three-dimensional structure of this enzyme is required to support the rational design of compounds active against a variety of tropical diseases caused by trypanosomal parasites.

Animals↗
PubMed1989

DNA-drug interactions. The crystal structure of d(CGATCG) complexed with daunomycin.

The structure of a d(CGATCG)-daunomycin complex has been determined by single crystal X-ray diffraction techniques. Refinement, with the location of 40 solvent molecules, using data up to 1.5 A, converged with a final crystallographic residual, R = 0.25 (RW = 0.22). The tetragonal crystals are in space group P4(1)2(1)2, with cell dimensions of a = 27.98 A and c = 52.87 A. The self-complementary d(CGATCG) forms a distorted right-handed helix with a daunomycin molecule intercalated at each d(CpG) step. The daunomycin aglycon chromophore is oriented at right-angles to the long axis of the DNA base-pairs. This head-on intercalation is stabilized by direct hydrogen bonds and indirectly via solvent-mediated, hydrogen-bonding interactions between the chromophore and its intercalation site base-pairs. The cyclohexene ring and amino sugar substituent lie in the minor groove. The amino sugar N-3' forms a hydrogen bond with O-2 of the next neighbouring thymine. This electrostatic interaction helps position the sugar in a way that results in extensive van der Waals contacts between the drug and the DNA. There is no interaction between daunosamine and the DNA sugar-phosphate backbone. We present full experimental details and all relevant conformational parameters, and use the comparison with a d(CGTACG)-daunomycin complex to rationalize some neighbouring sequence effects involved in daunomycin binding.

Base Sequence↗