Search PubMed⌕ Search

Biomedical subjects

S Raghunathan

Publications and source records attributed to S Raghunathan.

10 recordsLinked to original sources
PubMed2000

Structure of the DNA binding domain of E. coli SSB bound to ssDNA.

The structure of the homotetrameric DNA binding domain of the single stranded DNA binding protein from Escherichia coli (Eco SSB) bound to two 35-mer single stranded DNAs was determined to a resolution of 2.8 A. This structure describes the vast network of interactions that results in the extensive wrapping of single stranded DNA around the SSB tetramer and suggests a structural basis for its various binding modes.

Binding Sites↗
PubMed1997

Crystal structure of the homo-tetrameric DNA binding domain of Escherichia coli single-stranded DNA-binding protein determined by multiwavelength x-ray diffraction on the selenomethionyl protein at 2.9-A resolution.

The crystal structure of the tetrameric DNA-binding domain of the single-stranded DNA binding protein from Escherichia coli was determined at a resolution of 2.9 A using multiwavelength anomalous dispersion. Each monomer in the tetramer is topologically similar to an oligomer-binding fold. Two monomers each contribute three beta-strands to a single six-stranded beta-sheet to form a dimer. Two dimer-dimer interfaces are observed within the crystal. One of these stabilizes the tetramer in solution. The other interface promotes a superhelical structure within the crystal that may reflect tetramer-tetramer interactions involved in the positive cooperative binding of the single-stranded DNA-binding protein to single-stranded DNA.

Amino Acid Sequence↗
PubMed1994

Crystal structure of human class mu glutathione transferase GSTM2-2. Effects of lattice packing on conformational heterogeneity.

The structures of three crystal forms of the class mu human glutathione transferase GSTM2-2 have been determined. X-ray phase information was obtained independently from molecular replacement and from anomalous scattering by a single isomorphous derivative. One crystal form contains a single monomer in the asymmetric unit and has been refined to 1.85 A with an overall R factor of 22.6%. The second form contains a single dimer in the asymmetric unit and has been refined to 3.5 A with an R factor of 20.7%. The third form contains two dimers in the asymmetric unit and has been refined to 3.0 A with an R factor of 25.0%. Although all three crystal forms were grown from solutions that contained glutathione-dinitrobenzene, electron density can only be seen for the glutathione portion of the ligand. The first 202 residues in the seven crystallographically independent monomers of GSTM2-2 are essentially identical in structure. However, heterogeneity in the conformation of the side-chain of Tyr115 is observed in the different monomers. The tertiary structure of residues 1-202 is similar to that of the corresponding region in the class mu isoform of glutathione transferase from rat, GST3-3 (Ji et al. (1992), Biochemistry, 31, 10169-10184). However, significant differences in the conformation of the two enzymes have been observed in the region of the active site that binds hydrophobic substrates. These differences include a 2 A shift in the carboxy terminus of a helix, and significant heterogeneity in the conformation of the last 15 residues of the carboxy terminus. The conformation and degree of disorder of the last 15 residues correlates with the extent of protein-protein contacts within the unit cell.

Amino Acid Sequence↗
PubMed1993

The linker of des-Glu84-calmodulin is bent.

The crystal structure of a mutant calmodulin (CaM) lacking Glu-84 has been refined to R = 0.23 using data measured to 2.9-A resolution. In native CaM the central helix is fully extended, and the molecule is dumbbell shaped. In contrast, the deletion of Glu-84 causes a bend of 95 degrees in the linker region of the central helix at Ile-85. However, EF-hand domains 1 and 2 (lobe 1,2) do not touch lobe 3,4. The length, by alpha-carbon separation, of des-Glu84-CaM is 56 A; that of native CaM is 64 A. The shape of des-Glu84-CaM is similar to that of native CaM, as it is bound to the target peptide of myosin light-chain kinase. This result supports the proposal that the linker region of the central helix of CaM functions as a flexible tether.

Amino Acid Sequence↗
PubMed1991

Inhibition of proteinase K by methoxysuccinyl-Ala-Ala-Pro-Ala-chloromethyl ketone. An x-ray study at 2.2-A resolution.

The crystal structure of the transition state analog complex formed covalently between proteinase K and methoxysuccinyl-Ala-Ala-Pro-Ala-chloromethyl ketone was determined by x-ray diffraction methods at a resolution of 2.2 A and refined by constrained least squares to an R factor of 19.8% for the 11864 structure amplitudes greater than 1 sigma F. The chloromethyl ketone group is covalently linked with the active site functional groups His69(N epsilon) and Ser224(O gamma). The former has substituted for chlorine and the latter has attacked the carbon of the ketone group, thereby forming the tetrahedral carbon atom of the transition state analog. The peptide part of the inhibitor is in an extended conformation and fills subsites S1 to S5 of the substrate recognition site. Its backbone hydrogens bond with strands 100-104 and 132-136 of the substrate recognition site as the central strand of a three-stranded antiparallel beta-pleated sheet. This sheet formation is associated with a movement by approximately 1 A of strand 100-104 which is probably associated with the insertion of the bulky proline side chain. The methoxysuccinyl group is stacked on the phenolic side chain of Tyr104 that is a part of the bottom of the recognition site. Biochemical studies show that shorter inhibitors of this type are less effective than the longer one, because there are fewer hydrogen bonding and van der Waals/stacking interactions.

Amino Acid Chloromethyl Ketones↗
PubMed1991

Three-dimensional structure of the E. coli DNA-binding protein FIS.

The factor for inversion stimulation, FIS, is involved in several cellular processes, including site-specific recombination and transcriptional activation. In the reactions catalysed by the DNA invertases Gin, Hin and Cin, FIS stimulates recombination by binding to an enhancer sequence. Within the enhancer, two FIS dimers (each 2 x 98 amino acids) bind to two 15-base-pair consensus sequences and induce bending of the DNA. Current models propose that the enhancer-FIS complex organizes a specific synapse, either through direct interactions with Gin, or by modelling the substrate into a configuration suitable for recombination. Using X-ray analysis at 2.0 A resolution, we now show that FIS is composed of four alpha helices tightly intertwined to form a globular dimer with two protruding helix-turn-helix motifs. The 24 N-terminal amino acids are so poorly defined in the electron density map as to make interpretation doubtful, indicating that they might act as 'feelers' suitable for DNA or protein (invertase) recognition. We infer from model building that DNA has to bend for tight binding to FIS.

Amino Acid Sequence↗
PubMed1989

Long-range structural changes in proteinase K triggered by calcium ion removal.

The X-ray crystal structure of the subtilisin-type enzyme proteinase K at 1.5 A resolution shows that is has two binding sites for Ca2+. Scatchard analysis indicates that one Ca2+ binds tightly, with pK 7.6 x 10(-8) M-1, and the other only weakly. Although Ca2+ is not directly involved in the catalytic mechanism and is 16.6 A away from the alpha-carbon atoms of the catalytic triad Asp 39-His 69-Ser 224, the activity of proteinase K towards the synthetic substrate succinyl-Ala-Ala-Ala-p-nitroanilide drops slowly to approximately 20% of its original value when it is depleted of Ca2+. This is not due to autolysis of the enzyme. The X-ray crystal structure of Ca2+-free proteinase K shows that removal of Ca2+ from the tight binding site triggers a concerted domino-like movement of five peripheral loops and of two alpha-helices. At a distance of 25 A from this calcium-binding site, the geometry of both the secondary substrate binding site and of the catalytic triad is affected by this movement thereby reducing the activity of the enzyme.

Binding Sites↗
PubMed1983

Hyaluronic acid: molecular conformations and interactions in the orthorhombic and tetragonal forms containing sinuous chains.

The conformation, packing and probable mode of cation binding have been determined for compact potassium hyaluronate chains (average rise per disaccharide h = 0.89 nm) organized in an orthorhombic unit cell (a = 1.173 nm, b = 0.925 nm, c = 3.542 nm). The space group symmetry is P2(1)2(1)2(1) and the unit cell contains two antiparallel polysaccharide chains that are disturbed 4(3) helices. Each chain is stabilized intramolecularly by four hydrogen bonds and between adjacent antiparallel chains there are two intermolecular hydrogen bonds per crystallographic tetrasaccharide repeat. Fourier difference synthesis revealed two potassium ions and two water molecules in each asymmetric unit. Both potassium ions show octahedral co-ordination geometries and link adjacent antiparallel polysaccharide chains. The water molecules provide further intermolecular association through water bridges. A comparison of this potassium hyaluronate structure with the orthorhombic and tetragonal sodium hyaluronate structures containing similar compact hyaluronate chains revealed that the apparently isomorphous orthorhombic sodium and potassium salts did not have the same packing arrangements. The relative orientations of the sinuous hyaluronate chains in the sodium and potassium salts are about 90 degrees apart. The locations of the cations and hence the coordination schemes of K+ and Na+ are therefore very different. These analyses have delineated, for the first time, how hyaluronate chains can respond differentially to two different monovalent cations.

Carbohydrate Conformation↗
PubMed1983

Hyaluronic acid double helix.

The hyaluronic acid double helix, contrary to an earlier visualization, probably incorporates extensively hydrogen-bonded chains and is pinned together by carboxyl-carboxylate hydrogen bonds and water bridges. Transient interactions between stiffened chain segments provided by the formation of double-helical loops could give rise to the characteristic viscoelastic properties of hyaluronic acid solutions.

Carbohydrate Conformation↗