76 research outputs found
Covalently bound substrate at the regulatory site triggers allosteric enzyme activation
The mechanism by which the enzyme pyruvate decarboxylase from yeast is activated allosterically has been elucidated. A total of seven three-dimensional structures of the enzyme, of enzyme variants or of enzyme complexes form two yeast species (three of them reported here for the first time) provide detailed atomic resolution snapshots along the activation coordinate. The prime event is the covalent binding of the substrate pyruvate to the side chain of cysteine 221, thus forming a thiohemiketal. This reaction causes the shift of a neighbouring amino acid, which eventually leads to the rigidification of two otherwise flexible loops, where one of the loops provides two histidine residues necessary to complete the enzymatically competent active site architecture. The structural data are complemented and supported by kinetic investigations and binding studies and provide a consistent picture of the structural changes, which occur upon enzyme activation
Triple GEM Detectors for the Forward Tracker in STAR
Future measurements of the flavor-separated spin structure of the proton via
parity-violating W boson production at RHIC require an upgrade of the forward
tracking system of the STAR detector. This upgrade will allow the
reconstruction of the charge sign of electrons and positrons produced from
decaying W bosons. A design based on six large area triple GEM disks using GEM
foils produced by Tech-Etch Inc. has emerged as a cost-effective solution to
provide the necessary tracking precision. We report first results from a beam
test of three test detectors using Tech-Etch produced GEM foils and a laser
etched two dimensional strip readout. The detectors show good operational
stability, high efficiency and a spacial resolution of around 70 um or better,
exceeding the requirements for the forward tracking upgrade. The influence of
the angle of incidence of the particles on the spatial resolution of the
detectors has also been studied in detail.Comment: 5 pages, 8 figures, presented at the IEEE Nuclear Science Symposium
in Honolulu, HI, USA, October 27 - November 3, 200
Pion, kaon, proton and anti-proton transverse momentum distributions from p+p and d+Au collisions at GeV
Identified mid-rapidity particle spectra of , , and
from 200 GeV p+p and d+Au collisions are reported. A
time-of-flight detector based on multi-gap resistive plate chamber technology
is used for particle identification. The particle-species dependence of the
Cronin effect is observed to be significantly smaller than that at lower
energies. The ratio of the nuclear modification factor () between
protons and charged hadrons () in the transverse momentum
range GeV/c is measured to be
(stat)(syst) in minimum-bias collisions and shows little
centrality dependence. The yield ratio of in minimum-bias d+Au
collisions is found to be a factor of 2 lower than that in Au+Au collisions,
indicating that the Cronin effect alone is not enough to account for the
relative baryon enhancement observed in heavy ion collisions at RHIC.Comment: 6 pages, 4 figures, 1 table. We extended the pion spectra from
transverse momentum 1.8 GeV/c to 3. GeV/
Demonstration of the temporal matter-wave Talbot effect for trapped matter waves
We demonstrate the temporal Talbot effect for trapped matter waves using
ultracold atoms in an optical lattice. We investigate the phase evolution of an
array of essentially non-interacting matter waves and observe matter-wave
collapse and revival in the form of a Talbot interference pattern. By using
long expansion times, we image momentum space with sub-recoil resolution,
allowing us to observe fractional Talbot fringes up to 10th order.Comment: 17 pages, 7 figure
Azimuthal anisotropy at RHIC: the first and fourth harmonics
We report the first observations of the first harmonic (directed flow, v_1),
and the fourth harmonic (v_4), in the azimuthal distribution of particles with
respect to the reaction plane in Au+Au collisions at the Relativistic Heavy Ion
Collider (RHIC). Both measurements were done taking advantage of the large
elliptic flow (v_2) generated at RHIC. From the correlation of v_2 with v_1 it
is determined that v_2 is positive, or {\it in-plane}. The integrated v_4 is
about a factor of 10 smaller than v_2. For the sixth (v_6) and eighth (v_8)
harmonics upper limits on the magnitudes are reported.Comment: 6 pages with 3 figures, as accepted for Phys. Rev. Letters The data
tables are at
http://www.star.bnl.gov/central/publications/pubDetail.php?id=3
Host galaxy identification for supernova surveys
Host galaxy identification is a crucial step for modern supernova (SN) surveys such as the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST), which will discover SNe by the thousands. Spectroscopic resources are limited, so in the absence of real-time SN spectra these surveys must rely on host galaxy spectra to obtain accurate redshifts for the Hubble diagram and to improve photometric classification of SNe. In addition, SN luminosities are known to correlate with host-galaxy properties. Therefore, reliable identification of host galaxies is essential for cosmology and SN science. We simulate SN events and their locations within their host galaxies to develop and test methods for matching SNe to their hosts. We use both real and simulated galaxy catalog data from the Advanced Camera for Surveys General Catalog and MICECATv2.0, respectively. We also incorporate "hostless" SNe residing in undetected faint hosts into our analysis, with an assumed hostless rate of 5%. Our fully automated algorithm is run on catalog data and matches SNe to their hosts with 91% accuracy. We find that including a machine learning component, run after the initial matching algorithm, improves the accuracy (purity) of the matching to 97% with a 2% cost in efficiency (true positive rate). Although the exact results are dependent on the details of the survey and the galaxy catalogs used, the method of identifying host galaxies we outline here can be applied to any transient survey
Mid-rapidity anti-proton to proton ratio from Au+Au collisions at GeV
We report results on the ratio of mid-rapidity anti-proton to proton yields
in Au+Au collisions at \rts = 130 GeV per nucleon pair as measured by the
STAR experiment at RHIC. Within the rapidity and transverse momentum range of
and 0.4 1.0 GeV/, the ratio is essentially independent of
either transverse momentum or rapidity, with an average of for minimum bias collisions. Within errors, no
strong centrality dependence is observed. The results indicate that at this
RHIC energy, although the -\pb pair production becomes important at
mid-rapidity, a significant excess of baryons over anti-baryons is still
present.Comment: 5 pages, 3 figures, accepted by Phys. Rev. Let
Transverse-momentum correlations on from mean- fluctuations in Au-Au collisions at 200 GeV
We present first measurements of the pseudorapidity and azimuth
bin-size dependence of event-wise mean transverse momentum
fluctuations for Au-Au collisions at GeV. We invert that
dependence to obtain autocorrelations on differences
interpreted to represent velocity/temperature
distributions on (). The general form of the autocorrelations
suggests that the basic correlation mechanism is parton fragmentation. The
autocorrelations vary strongly with collision centrality, which suggests that
fragmentation is strongly modified by a dissipative medium in the more central
Au-Au collisions relative to peripheral or p-p collisions. \\Comment: 7 pages, 3 figure
The influence of several amino acid residues at the regulatory site of yeast pyruvate decarboxylase on the allosteric substrate activation of this enzyme
Pyruvate decarboxylase (EC 4.1.1.1) is an allosteric enzyme whose catalytic activity is strictly controlled by its substrate pyruvate binding at a separate regulatory site. Chemical enzyme modification and cross-linking, kinetic characterization of several PDC variants, and the determination of their crystal structures in complex with substrates or artificial activators led to a comprehensive understanding of the mechanism of allosteric activation at an atomic level [1-5, 8, 9]. Particularly, the signal transfer pathway from the regulatory to the active site as triggered by activator binding at the regulatory site and resulting in rearrangements of several amino acid residues at the catalytic site could be traced [6, 7]. To understand the role of individual amino acid residues for substrate activation we generated a number of variants with single amino acid substitutions at the regulatory site, e.g. C221A/C222A, H92F, H225F, H310F, A287G, and S311A. The first of the two adjacent cysteine residues, C221, was identified as the core of the activator binding site. Three surrounding histidine residues create a positively charged trap that guides the activator molecule to the binding pocket at C221. Together with the histidine residues, A287 and S311 are able to interact with the activator molecule bound covalently at C221. Structure and function of the variants have been analysed applying detailed steady state and transient kinetic approaches as well as small-angle X-ray solution scattering measurements. The structural integrity of the variants was not influenced by the site specific mutations [10]. While A287G shows a wild type-like behaviour all other variants clearly deviate in their kinetic properties from the wild type. The results obtained allow a deeper look at the details of substrate activation.References1 Hübner, G., König, S. and Schellenberger, A. (1988) Biomed. Biochim. Acta 47, 9-18.2 Lu, G., Dobritzsch, D., Baumann, S., Schneider, G. and König, S. (2000) Eur. J. Biochem. 267, 861-868.3 Killenberg-Jabs, M., König, S., Eberhardt, I., Hohmann, S. and Hübner, G. (1997) Biochemistry 36, 1900-1905.4 Dobritzsch, D., König, S., Schneider, G. and Lu, G. (1998) J. Biol. Chem. 273, 20196-20204.5 Krieger, F., Spinka, M., Golbik, R., Hübner, G. and König, S. (2002) Eur. J. Biochem. 269, 3256-3263.6 Kutter, S., Weiss, M. S., Wille, G., Golbik, R., Spinka, M. and König, S. (2009) J. Biol. Chem. 284, 12136-12144.7 König, S., Spinka, M. and Kutter, S. (2009) J. Mol. Catal. B Enz. 61, 100-110.8 Baburina, I., Gao, Y., Hu, Z., Jordan, F., Hohmann, S. and Furey, W. (1994) Biochemistry 33, 5630-5635.9 Sergienko, E. A. and Jordan, F. (2001) Biochemistry 40, 7355-7368, 7369-7381, 7382-7403.10 König, S, Svergun, D. I., Volkov, V. V., Feigin, L. A. and Koch, M. H. J. (1998) Biochemistry 37, 5329-5334
Activation of thiamin diphosphate and FAD in the phosphatedependent pyruvate oxidase from Lactobacillus plantarum
The phosphate- and oxygen-dependent pyruvate oxidase from Lactobacillus plantarum is a homotetrameric enzyme that binds 1 FAD and 1 thiamine diphosphate per subunit. A kinetic analysis of the partial reactions in the overall oxidative conversion of pyruvate to acetyl phosphate and CO2 shows an indirect activation of the thiamine diphosphate by FAD that is mediated by the protein moiety. The rate constant of the initial step, the deprotonation of C2-H of thiamine diphosphate, increases 10-fold in the binary apoenzyme-thiamine diphosphate complex to 10-2 s-1. Acceleration of this step beyond the observed overall catalytic rate constant to 20 s-1 requires enzyme-bound FAD. FAD appears to bind in a two-step mechanism. The primarily bound form allows formation of hydroxyethylthiamine diphosphate but not the transfer of electrons from this intermediate to O2. This intermediate form can be mimicked using 5-deaza-FAD, which is inactive toward O2 but active in an assay using 2,6-dichlorophenolindophenol as electron acceptor. This analogue also promotes the rate constant of C2-H dissociation of thiamine diphosphate in pyruvate oxidase beyond the overall enzyme turnover. Formation of the catalytically competent FAD-thiamine-pyruvate oxidase ternary complex requires a second step, which was detected at low temperature
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