387 research outputs found
Mass and charge fluctuations and black hole entropy
The effects of thermal fluctuations of the mass (horizon area) and electric
charge, on the entropy of non-rotating charged {\it macroscopic} black holes,
are analyzed using a grand canonical ensemble. Restricting to Gaussian
fluctuations around equilibrium, and assuming a power law type of relation
between the black hole mass, charge and horizon area, characterized by two real
positive indices, the grand canonical entropy is shown to acquire a logarithmic
correction with a positive coefficient proportional to the sum of the indices.
However, the root mean squared fluctuations of mass and charge relative to the
mean values of these quantities turn out to be independent of the details of
the assumed mass-area relation. We also comment on possible cancellation
between log (area) corrections arising due to {\it fixed area} quantum
spacetime fluctuations and that due to thermal fluctuations of the area and
other quantities.Comment: 8 pages revtex, no figure
Preliminary X-ray analysis of a new crystal form of the Escherichia coli KDO8P synthase
3-Deoxy-d-manno-octulosonate 8-phosphate (KDO8P) synthase catalyzes the biosynthesis of an essential component of the lipopolysaccharide of all Gram-negative bacteria. The structure and mechanism of KDO8P synthase are being actively studied as this enzyme represents an important target for antibiotic therapy. The structure of the Escherichia coli KDO8P synthase in cubic crystals (space group I23) has recently been determined and the enzyme shown to be a tetramer of identical subunits. However, this information is challenged by biochemical studies, which suggest that the enzyme behaves in solution as a homotrimer. Here, the preparation and preliminary X-ray analysis of monoclinic crystals of KDO8P synthase are reported. The crystals belong to space group P21, with unit-cell parameters a≃ 50, b≃ 140, c≃ 74 Å, β≃ 105°. The structure of KDO8P synthase in the monoclinic crystal form was determined by molecular replacement, using as a search model one of the subunits of the enzyme in the cubic crystals. A tetramer of KDO8P synthase with 222 local symmetry is also present in the asymmetric unit of the P21 crystals, with a solvent content of 43%. The observation that the same quaternary structure of KDO8P synthase is observed in two different crystal forms belonging to distinct crystal systems (monoclinic and cubic) suggests that a tetramer is the native form of the enzyme
Eikonal Particle Scattering and Dilaton Gravity
Approximating light charged point-like particles in terms of (nonextremal)
dilatonic black holes is shown to lead to certain pathologies in Planckian
scattering in the eikonal approximation, which are traced to the presence of a
(naked) curvature singularity in the metric of these black holes. The existence
of such pathologies is confirmed by analyzing the problem in an `external
metric' formulation where an ultrarelativistic point particle scatters off a
dilatonic black hole geometry at large impact parameters. The maladies
disappear almost trivially upon imposing the extremal limit. Attempts to derive
an effective three dimensional `boundary' field theory in the eikonal limit are
stymied by four dimensional (bulk) terms proportional to the light-cone
derivatives of the dilaton field, leading to nontrivial mixing of
electromagnetic and gravitational effects, in contrast to the case of general
relativity. An eikonal scattering amplitude, showing decoupling of these
effects, is shown to be derivable by resummation of graviton, dilaton and
photon exchange ladder diagrams in a linearized version of the theory, for an
asymptotic value of the dilaton field which makes the string coupling constant
non-perturbative.Comment: 22 pages, Revte
Electromagnetic and Gravitational Scattering at Planckian Energies
The scattering of pointlike particles at very large center of mass energies
and fixed low momentum transfers, occurring due to both their electromagnetic
and gravitational interactions is re-examined in the particular case when one
of the particles carries magnetic charge. At Planckian center-of-mass energies,
when gravitational dominance is normally expected, the presence of magnetic
charge is shown to produce dramatic modifications to the scattering cross
section as well as to the holomorphic structure of the scattering amplitude.Comment: 33 pages, Revtex file, no figs; a footnote and two references adde
Quantum Aspects of Black Hole Entropy
This survey intends to cover recent approaches to black hole entropy which
attempt to go beyond the standard semiclassical perspective. Quantum
corrections to the semiclassical Bekenstein-Hawking area law for black hole
entropy, obtained within the quantum geometry framework, are treated in some
detail. Their ramification for the holographic entropy bound for bounded
stationary spacetimes is discussed. Four dimensional supersymmetric extremal
black holes in string-based N=2 supergravity are also discussed, albeit more
briefly.Comment: 13 Pages Revtex with 3 eps figures; based on plenary talk given at
the International Conference on Gravitation and Cosmology, Kharagpur, India,
January, 2000 One reference adde
Protein tyrosine phosphatase receptor delta acts as a neuroblastoma tumor suppressor by destabilizing the aurora kinase a oncogene.
BACKGROUND: Protein tyrosine phosphatase receptor delta (PTPRD) is a member of a large family of protein tyrosine phosphatases which negatively regulate tyrosine phosphorylation. Neuroblastoma is a major childhood cancer arising from precursor cells of the sympathetic nervous system which is known to acquire deletions and alterations in the expression patterns of PTPRD, indicating a potential tumor suppressor function for this gene. The molecular mechanism, however, by which PTPRD renders a tumor suppressor effect in neuroblastoma is unknown.
RESULTS: As a molecular mechanism, we demonstrate that PTPRD interacts with aurora kinase A (AURKA), an oncogenic protein that is over-expressed in multiple forms of cancer, including neuroblastoma. Ectopic up-regulation of PTPRD in neuroblastoma dephosphorylates tyrosine residues in AURKA resulting in a destabilization of this protein culminating in interfering with one of AURKA\u27s primary functions in neuroblastoma, the stabilization of MYCN protein, the gene of which is amplified in approximately 25 to 30% of high risk neuroblastoma.
CONCLUSIONS: PTPRD has a tumor suppressor function in neuroblastoma through AURKA dephosphorylation and destabilization and a downstream destabilization of MYCN protein, representing a novel mechanism for the function of PTPRD in neuroblastoma
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