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1.
Serine acetyltransferase is a member of the left-handed parallel beta-helix family of enzymes that catalyzes the committed step in the de novo synthesis of l-cysteine in bacteria and plants. The enzyme has an ordered kinetic mechanism with acetyl CoA bound prior to l-serine and O-acetyl-l-serine released prior to CoA. The rate-limiting step along the reaction pathway is the nucleophilic attack of the serine hydroxyl on the thioester of acetyl CoA. Product release contributes to rate-limitation at saturating concentrations of reactants. The reaction is catalyzed by an active site general base with a pK of 7, which accepts a proton from the serine hydroxyl as a tetrahedral intermediate is formed between the reactants, and donates it to the thiol of CoA as the intermediate collapses to give products. This mechanism is likely the same for all O-acyltransferases that catalyze their reaction by direct attack of the alcohol on the acyl donor, using an active-site histidine as the general base. Serine acetyltransferase is regulated by feedback inhibition by the end product l-cysteine, which acts by binding to the serine site in the active site and inducing a conformational change that prevents reactant binding. The enzyme also associates with O-acetylserine sulfhydrylase, the final enzyme in the biosynthetic pathway, which contributes to stabilizing the acetyltransferase. 相似文献
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CD and NMR studies on heterochiral oligodeoxynucleotides (d/l-ODNs) forming quadruplex structures are reported. Heterochiral ODNs, based on sequence TGGGGT, are able to form stable either right- or left-handed quadruplexes depending on d/l ratio and residues position. Results suggest that the 3′-end and the core of the G-run are more important than the 5′-end in determining the quadruplex handness. Particularly, oligonucleotide TDGDGLGLGDTD (L34) at low temperatures forms a well-defined left-handed quadruplex, notwithstanding it is mostly composed by natural d residues. This structure is characterized by three all-anti G-tetrads and one all-syn G-tetrad. 相似文献
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Summary Non-glycine residues with positive -angles have been identified in four proteins, barley serine proteinase inhibitor CI-2, bacterial ribonuclease (barnase) ofBacillus amyloliquefaciens, hen egg white lysozyme and a basic protein from barley seed (barwin) by use of nuclear magnetic resonance spectroscopy. By accurate measurements of the coupling constant
and integration of the nuclear Overhauser HN-H cross peak, positive -angles could be determined reliably to 60°±30°, in full agreement with the crystal structures for lysozyme, barnase and serine proteinase inhibitor CI-2. The work emphasizes that positive -angles can also occur in non-glycine residues and in the four proteins, positive -angles have been observed for the residue types aspartic acid, asparagine, arginine, serine, glutamine, histidine, tyrosine, tryptophan and phenylalanine. The measured
coupling constants and the intensity of the intraresidue HN-H NOEs agree well with the solution structures of three of the proteins, using the existing parametrization of the Karplus curve (Pardi, A., Billeter, M. and Wüthrich, K. (1984)J. Mol. Biol.,180, 741–751; Ludvigsen, S., Andersen, K.V. and Poulsen, F.M. (1991)J. Mol. Biol.,217, 731–736). 相似文献
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Alessandro D’Urso Jung Kyu Choi Fidelis N. Ngwa Roberto Purrello 《Biochemical and biophysical research communications》2010,397(2):329-332
The left-handed Z-DNA form of the short unmodified alternating guanine-cytosine oligonucleotides, 5′-(dGdC)24 and 5′-(dGdC)18, was selectively detected under physiological ionic strength and pH conditions using the anionic nickel(II) porphyrin, NiTPPS. No spectroscopic signal was observed for NiTPPS with any right-handed oligonucleotides under identical conditions. The 48mer 5′-(dGdC)24 Z-form was detected at concentrations as low as 100 nM. The binding of NiTPPS to the B- and Z-oligonucleotides was studied quantitatively by UV-vis absorption and circular dichroism spectroscopies. NiTPPS was found to be a universal DNA binder, with binding affinity and geometry depending on the ionic composition of the solution, rather than on the DNA helical twist. This is the first example of a successful spectroscopic detection of the Z-DNA of short unmodified oligonucleotides under physiological pH and ionic strength conditions. 相似文献
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Ohishi H Tozuka Y Da-Yang Z Ishida T Nakatani K 《Biochemical and biophysical research communications》2007,358(1):24-28
Several crystal structure analyses of complexes of synthetic polyamine compounds, including N(1)-(2-(2-aminoethylamino))ethyl)ethane-1,2-diamine PA(222) and N(1)-(2-(2-(2-aminoethylamino)ethylamino)ethyl)ethane-1,2-diamine PA(2222), and left-handed Z-DNA d(CGCGCG)(2) have been reported. However, until now, there have been no examples of naturally occurring polyamines bound to the minor groove of the left-handed Z-DNA of d(CGCGCG)(2) molecule. We have found that spermidine, a natural polyamine, is connected to the minor groove of left-handed Z-DNA of d(CGCGCG)(2) molecule in a crystalline complex grown at 10 degrees C. The electron density of the DNA molecule was clear enough to determine that the spermidine was connected in the minor groove of two symmetry related molecules of left-handed Z-DNA d(CGCGCG)(2). This is the first example that a spermidine molecule can form a bridge conformation between two symmetry related molecules of left-handed Z-DNA d(CGCGCG)(2) in the minor groove. 相似文献
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Four novel metal coordination polymers, [Zn(2-PBIM)(OH-BDC)]n (1), [Cd(2-PBIM)(OH-BDC)]n (2), [Mn(2-PBIM)(OH-BDC)]n·1.5nH2O (3), [Cu(2-PBIM)(OH-BDC)]n·nH2O (4) [2-PBIM = 2-(2-pyridyl)benzimidazole; OH-H2BDC = 5-hydroxy-1,3-benzenedicarboxylic acid], have been synthesized under hydrothermal conditions and structurally characterized by single crystal X-ray diffraction analysis, elemental analysis, IR spectroscopy, and thermogravimetric analysis. Compound 1 possesses left-handed screw (M-helix) and right-handed screw (P-helix) chains that are further connected though intermolecular hydrogen bonds and π-π stacking interactions resulting in a three-dimensional (3D) network. Compound 2 has a two-dimensional metal-organic framework which is connected into a 3D network by intermolecular hydrogen bonds and π-π stacking interactions. Compounds 3 and 4 are isostructural and possess one-dimensional (1D) channels. Free 2-PBIM and OH-H2BDC ligands and complexes 1, 2 show fluorescent emissions in the visible and near-infrared region. Magnetic susceptibility measurements indicate that both 3 and 4 exhibit antiferromagnetic coupling between adjacent center atoms. 相似文献
8.
Alexandre Vial 《Plasmonics (Norwell, Mass.)》2006,1(2-4):129-134
We introduce formulae in suitable form for the numerical computation of Mie coefficients and the scattered electrical field in the general case of a sphere with permitivity and permeability . The small particle limit is also investigated. We compute the extinction cross section of a negative index sphere, as well as intensity maps. Excitation of surface plasmons featuring narrow linewidths is observed, which can be attributed to the permitivity or the permeability of the sphere. 相似文献
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Dimerization of the neu/ErbB-2 receptor tyrosine kinase is a necessary but not a sufficient step for signaling. Despite the
efforts expended to identify the molecular interactions responsible for receptor-receptor contacts and particularly those
involving the transmembrane domain, structural details are still unknown. In this work, molecular dynamics simulations of
the helical transmembrane domain (TM) of neu and ErbB-2 receptors are used to predict their dimer structure both in the wild
and oncogenic forms. A global conformational search method, applied to define the best orientations of parallel helices, showed
an energetically favorable configuration with the specific mutation site within the interface, common for both the nontransforming
and the transforming neu/ErbB-2 TM dimers. Starting from this configuration, a total of 10 simulations, about 1.4 ns each,
performed in vacuum, without any constraints, show that the two helices preferentially wrap in left-handed interactions with
a packing angle at about 20°. The resulting structures are nonsymmetric and the hydrogen bond network analysis shows that
helices experience π local distortions that facilitate inter-helix hydrogen bond interactions and may result in a change in
the helix packing, leading to a symmetric interface. For the mutated sequences, we show that the Glu side chain interacts
directly with its cognate or with carbonyl groups of the facing backbone. We show that the connectivity between interfacial
residues conforms to the knobs-into-holes packing mode of transmembrane helices. The dimeric interface described in our models
is discussed with respect to mutagenesis studies.
Received: 12 March 1999 / Revised version: 23 August 1999 / Accepted: 23 August 1999 相似文献