14,221 research outputs found
Lorentz's model with dissipative collisions
Propagation of a particle accelerated by an external field through a
scattering medium is studied within the generalized Lorentz model allowing
inelastic collisions. Energy losses at collisions are proportional to
, where is the restitution coefficient. For
(elastic collisions) there is no stationary state. It is proved in
one dimension that when the stationary state exists . The
corresponding velocity distribution changes from a highly asymmetric
half-gaussian () to an asymptotically symmetric distribution , for . The identical scaling
behavior in the limit of weak inelasticity is derived in three dimensions by a
self-consistent perturbation analysis, in accordance with the behavior of
rigorously evaluated moments. The dependence on the external field scales out
in any dimension, predicting in particular the stationary current to be
proportional to the square root of the external acceleration.Comment: 13 pages, no figures, submitted to Physica
In search of Robert Bruce, part III: medieval royal burial at Dunfermline and the tomb investigations of 1818–19
This article challenges two orthodoxies concerning Dunfermline's medieval church and abbey. It argues that David I and Malcolm IV were buried not with Robert Bruce, his queen, and Alexander III, but with Queen Margaret (died 1093), her husband, three of David's brothers, and Alexander III's first wife, as the earliest royal burial cluster. This argument dovetails with a reappraisal of the evolution of the medieval abbey. It is concluded that when dedicated in 1150, David's abbey was largely coextensive with his mother's church, only achieving cruciform status by 1180 and Queen Margaret's first translation. Reassigning David I and Malcom IV to another burial cluster strengthens the case for identifying the incumbent of the tomb discovered and excavated at Dunfermline in 1818–19 as Robert Bruce, as also argued for in the two other articles in this series
The Trypanosoma cruzi enzyme TcGPXI is a glycosomal peroxidase and can be linked to trypanothione reduction by glutathione or tryparedoxin.
Trypanosoma cruzi glutathione-dependent peroxidase I (TcGPXI) can reduce fatty acid, phospholipid, and short chain organic hydroperoxides utilizing a novel redox cycle in which enzyme activity is linked to the reduction of trypanothione, a parasite-specific thiol, by glutathione. Here we show that TcGPXI activity can also be linked to trypanothione reduction by an alternative pathway involving the thioredoxin-like protein tryparedoxin. The presence of this new pathway was first detected using dialyzed soluble fractions of parasite extract. Tryparedoxin was identified as the intermediate molecule following purification, sequence analysis, antibody studies, and reconstitution of the redox cycle in vitro. The system can be readily saturated by trypanothione, the rate-limiting step being the interaction of trypanothione with the tryparedoxin. Both tryparedoxin and TcGPXI operate by a ping-pong mechanism. Overexpression of TcGPXI in transfected parasites confers increased resistance to exogenous hydroperoxides. TcGPXI contains a carboxyl-terminal tripeptide (ARI) that could act as a targeting signal for the glycosome, a kinetoplastid-specific organelle. Using immunofluorescence, tagged fluorescent proteins, and biochemical fractionation, we have demonstrated that TcGPXI is localized to both the glycosome and the cytosol. The ability of TcGPXI to use alternative electron donors may reflect their availability at the corresponding subcellular sites
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Profiles of SUMO and ubiquitin conjugation in an Alzheimer's disease model
â–º Global levels of ubiquitinated proteins increased in the hippocampus of Tg2576 mice. â–º No global changes in either SUMO-1 or SUMO-2/3 conjugation in any brain regions analysed. â–º SUMO conjugating and deconjugating enzymes, UBC9 and SENP-1, unaltered in Tg2576 mice. â–º Total levels of AMPA and kainate receptors were also unaffected in Tg2576 mice. â–º Posttranslational modification by ubiquitin may play a role in Alzheimer's disease
Genomic presence of recombinant porcine endogenous retrovirus in transmitting miniature swine
The replication of porcine endogenous retrovirus (PERV) in human cell lines suggests a potential infectious risk in xenotransplantation. PERV isolated from human cells following cocultivation with porcine peripheral blood mononuclear cells is a recombinant of PERV-A and PERV-C. We describe two different recombinant PERV-AC sequences in the cellular DNA of some transmitting miniature swine. This is the first evidence of PERV-AC recombinant virus in porcine genomic DNA that may have resulted from autoinfection following exogenous viral recombination. Infectious risk in xenotransplantation will be defined by the activity of PERV loci in vivo
SOC in a population model with global control
We study a plant population model introduced recently by J. Wallinga [OIKOS
{\bf 74}, 377 (1995)]. It is similar to the contact process (`simple epidemic',
`directed percolation'), but instead of using an infection or recovery rate as
control parameter, the population size is controlled directly and globally by
removing excess plants. We show that the model is very closely related to
directed percolation (DP). Anomalous scaling laws appear in the limit of large
populations, small densities, and long times. These laws, associated critical
exponents, and even some non-universal parameters, can be related to those of
DP. As in invasion percolation and in other models where the r\^oles of control
and order parameters are interchanged, the critical value of the wetting
probability is obtained in the scaling limit as singular point in the
distribution of infection rates. We show that a mean field type approximation
leads to a model studied by Y.C. Zhang et al. [J. Stat. Phys. {\bf 58}, 849
(1990)]. Finally, we verify the claim of Wallinga that family extinction in a
marginally surviving population is governed by DP scaling laws, and speculate
on applications to human mitochondrial DNA.Comment: 19 pages, with 10 ps-figured include
Structural dynamics and ligand binding in kynurenine-3- monooxygenase
Kynurenine 3-monooxygenase is a FAD-dependent aromatic hydroxylase (FAH) which is a
widely suggested therapeutic target for controlling the balance of bioactive metabolite
levels produced by the mammalian kynurenine pathway (KP). Prior to starting this work
no structural information was known for the enzyme, with studies of the human form
complicated by the presence of a C-terminal transmembrane helix. The bacterial
Pseudomonas fluorescens enzyme (PfKMO) lacks the transmembrane region and has
been previously characterised by Crozier-Reabe and Moran [1, 2]. Therefore PfKMO,
which shares 32 % sequence identity with the human enzyme, was selected as a target
for structure solution. Initial substrate bound PfKMO crystals showed poor X-ray
diffraction. Subsequent growth optimisation and the generation of a C252S/C461S
PfKMO mutant (dm2) yielded crystals suitable for structure solution. Selenomethioninelabelled
substrate bound dm2 crystals were used to solve the first structure to a
resolution of 3.40 Ã…. With just one protein molecule per asymmetric unit, a high solvent
content was responsible for the poor diffraction properties of this crystal form. The
overall fold resembled that of other FAH enzymes with a Rossmann-fold based FADbinding
domain above a buried substrate binding pocket. Interestingly PfKMO possesses
an additional, novel C-terminal domain that caps the back of the substrate-binding
pocket on the opposite side to the flavin. Residues proposed to be involved in substrate
binding were identified and shown to be highly conserved among mammalian KMO
sequences.
Subsequently single crystals of substrate-free dm2 PfKMO were obtained and showed
significantly stronger diffraction due to new lattice packing in an orthorhombic space
group bearing four molecules per asymmetric unit. The structure was solved to a
resolution of 2.26 Ã… and revealed a clear conformational change of the novel C-terminal
domain. This movement opens a potential route of substrate/product exchange between
bulk solvent and the active site. The investigation of a set of C-terminal mutants further
explored the relevance and mechanics of the conformational change. In addition the
presence of chloride ions in the substrate-free crystal growth solution caused a small
number of localised subtle alterations to the structure, with a potential chloride binding
site identified adjacent to the flavin cofactor. This may have relevance for the observed
inhibition of PfKMO activity by monovalent anions – a feature widely common to FAH
enzymes [3].
The first discovered KMO inhibitors were analogues of the substrate L-Kyn, however one
such compound (m-NBA) was recently shown to instigate uncoupled NADPH oxidation
leading to the release of cytotoxic hydrogen peroxide [1]. A set of substrate analogues
were tested and characterised for inhibition of PfKMO. The picture was shown to be
complex as some substrate analogues completely inhibited the enzyme whilst the
binding of some still stimulated low-levels of NADPH oxidation. Crystallographic studies
with m-NBA and 3,4-dichlorobenzoylalanine (3,4-CBA) bound revealed indistinguishable
structures from that of substrate-bound PfKMO. These studies suggest that the analogue
3,4CBA is a potent PfKMO inhibitor whose therapeutic potential may be re-visited. The
previous most potent KMO inhibitor whose structure was not analogous to the substrate
was Ro 61-8048 [4], which unfortunately did not pass pre-clinical safety tests. A novel
series of 1,2,4-oxadiazole amides based on the structure of Ro 61-8048 was created by
Gavin Milne (PhD, University of St Andrews) and tested on PfKMO. Rounds of refinement
led to the discovery and refinement of low nanomolar competitive inhibitors of the
bacterial enzyme. PfKMO was co-crystallised with each of the four most potent
compounds forming a third different lattice arrangement, which yielded structures to
resolutions of 2.15-2.40 Ã…. The structures displayed conformational changes resembling
the substrate-free fold possibly caused by displacement of a crucial substrate-binding
residue, R84.
Overall the wealth of structural data obtained may be transferable to predictions about
the structural features of human KMO and to the rational design of therapeutic
inhibitors. The potent novel inhibitors tested may additionally present a new exciting
development for the therapeutic inhibition of human KMO
A solenoidal electron spectrometer for a precision measurement of the neutron -asymmetry with ultracold neutrons
We describe an electron spectrometer designed for a precision measurement of
the neutron -asymmetry with spin-polarized ultracold neutrons. The
spectrometer consists of a 1.0-Tesla solenoidal field with two identical
multiwire proportional chamber and plastic scintillator electron detector
packages situated within 0.6-Tesla field-expansion regions. Select results from
performance studies of the spectrometer with calibration sources are reported.Comment: 30 pages, 19 figures, 1 table, submitted to NIM
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