4,148 research outputs found
Modeling Networks of Coupled Enzymatic Reactions Using the Total Quasi-Steady State Approximation
In metabolic networks, metabolites are usually present in great excess over the enzymes that catalyze their interconversion, and describing the rates of these reactions by using the MichaelisâMenten rate law is perfectly valid. This rate law assumes that the concentration of enzymeâsubstrate complex (C) is much less than the free substrate concentration (S (0)). However, in protein interaction networks, the enzymes and substrates are all proteins in comparable concentrations, and neglecting C with respect to S (0) is not valid. Borghans, DeBoer, and Segel developed an alternative description of enzyme kinetics that is valid when C is comparable to S (0). We extend this description, which Borghans et al. call the total quasi-steady state approximation, to networks of coupled enzymatic reactions. First, we analyze an isolated GoldbeterâKoshland switch when enzymes and substrates are present in comparable concentrations. Then, on the basis of a real example of the molecular network governing cell cycle progression, we couple two and three GoldbeterâKoshland switches together to study the effects of feedback in networks of protein kinases and phosphatases. Our analysis shows that the total quasi-steady state approximation provides an excellent kinetic formalism for protein interaction networks, because (1) it unveils the modular structure of the enzymatic reactions, (2) it suggests a simple algorithm to formulate correct kinetic equations, and (3) contrary to classical MichaelisâMenten kinetics, it succeeds in faithfully reproducing the dynamics of the network both qualitatively and quantitatively
Probing The Multiphase Interstellar Medium Of The Dwarf Starburst Galaxy NGC 625 With FUSE Spectroscopy
We present new FUSE spectroscopy of the dwarf starburst galaxy NGC 625. These
observations probe multiple phases of the interstellar medium, including the
coronal, ionized, neutral and molecular gas. This nearby (D = 3.9 +/- 0.2 Mpc)
system shows a clear detection of outflowing coronal gas as traced by OVI 1032
Angstrom absorption. The centroid of the OVI profile is blueshifted with
respect to the galaxy systemic velocity by ~ 30 km/sec, suggesting a
low-velocity outflow. The implied OVI velocity extent is found to be 100 +/- 20
km/sec, which is fully consistent with the detected HI outflow velocity found
in radio synthesis observations. We detect multiple lines of diffuse H2
absorption from the ISM of NGC 625; this is one of only a few extragalactic
systems with FUSE detections of H2 lines in the Lyman and Werner bands. We find
a potential abundance offset between the neutral and nebular gas that exceeds
the errors on the derived column densities. Since such an offset has been found
in multiple dwarf galaxies, we discuss the implications of a lower-metallicity
halo surrounding the central star forming regions of dwarf galaxies. The
apparent offset may be due to saturation of the observed OI line, and higher
S/N observations are required to resolve this issue.Comment: ApJ, in press; full-resolution version may be obtained at
http://www.astro.umn.edu/~cannon/n625.fuse.p
Sodium content as a predictor of the advanced evolution of globular cluster stars
The asymptotic giant branch (AGB) phase is the final stage of nuclear burning
for low-mass stars. Although Milky Way globular clusters are now known to
harbour (at least) two generations of stars they still provide relatively
homogeneous samples of stars that are used to constrain stellar evolution
theory. It is predicted by stellar models that the majority of cluster stars
with masses around the current turn-off mass (that is, the mass of the stars
that are currently leaving the main sequence phase) will evolve through the AGB
phase. Here we report that all of the second-generation stars in the globular
cluster NGC 6752 -- 70 per cent of the cluster population -- fail to reach the
AGB phase. Through spectroscopic abundance measurements, we found that every
AGB star in our sample has a low sodium abundance, indicating that they are
exclusively first-generation stars. This implies that many clusters cannot
reliably be used for star counts to test stellar evolution timescales if the
AGB population is included. We have no clear explanation for this observation.Comment: Published in Nature (online 29 May 2013, hard copy 13 June), 12
pages, 3 figures + supplementary information sectio
Emergence of collective intonation in the musical performance of crowds
To be published in EPLTo be published in EP
Energy demand on dairy farms in Ireland
peer-reviewedReducing electricity consumption in Irish milk production is a topical issue for 2 reasons. First, the introduction of a dynamic electricity pricing system, with peak and off-peak prices, will be a reality for 80% of electricity consumers by 2020. The proposed pricing schedule intends to discourage energy consumption during peak periods (i.e., when electricity demand on the national grid is high) and to incentivize energy consumption during off-peak periods. If farmers, for example, carry out their evening milking during the peak period, energy costs may increase, which would affect farm profitability. Second, electricity consumption is identified in contributing to about 25% of energy use along the life cycle of pasture-based milk. The objectives of this study, therefore, were to document electricity use per kilogram of milk sold and to identify strategies that reduce its overall use while maximizing its use in off-peak periods (currently from 0000 to 0900h). We assessed, therefore, average daily and seasonal trends in electricity consumption on 22 Irish dairy farms, through detailed auditing of electricity-consuming processes. To determine the potential of identified strategies to save energy, we also assessed total energy use of Irish milk, which is the sum of the direct (i.e., energy use on farm) and indirect energy use (i.e., energy needed to produce farm inputs). On average, a total of 31.73 MJ was required to produce 1kg of milk solids, of which 20% was direct and 80% was indirect energy use. Electricity accounted for 60% of the direct energy use, and mainly resulted from milk cooling (31%), water heating (23%), and milking (20%). Analysis of trends in electricity consumption revealed that 62% of daily electricity was used at peak periods. Electricity use on Irish dairy farms, therefore, is substantial and centered around milk harvesting. To improve the competitiveness of milk production in a dynamic electricity pricing environment, therefore, management changes and technologies are required that decouple energy use during milking processes from peak periods
Efficacy of pulmonary artery pressure monitoring in patients with chronic heart failure:a meta-analysis of three randomized controlled trials
Aims Adjustment of treatment based on remote monitoring of pulmonary artery (PA) pressure may reduce the risk of hospital admission for heart failure (HF). We have conducted a meta-analysis of large randomized trials investigating this question. Methods A systematic literature search was performed for randomized clinical trials with PA pressure monitoring devices in patients and results with HF. The primary outcome of interest was the total number of HF hospitalizations. Other outcomes assessed were urgent visits leading to treatment with intravenous diuretics, all-cause mortality, and composites. Treatment effects are expressed as hazard ratios, and pooled effect estimates were obtained applying random effects meta-analyses. Three eligible randomized clinical trials were identified that included 1898 outpatients in New York Heart Association functional classes IIâIV, either hospitalized for HF in the prior 12 months or with elevated plasma NT-proBNP concentrations. The mean followup was 14.7 months, 67.8% of the patients were men, and 65.8% had an ejection fraction â€40%. Compared to patients in the control group, the hazard ratio (95% confidence interval) for total HF hospitalizations in those randomized to PA pressure monitoring was 0.70 (0.58â0.86) (P = .0005). The corresponding hazard ratio for the composite of total HF hospitalizations, urgent visits and all-cause mortality was 0.75 (0.61â0.91; P = .0037) and for all-cause mortality 0.92 (0.73â1.16). Subgroup analyses, including ejection fraction phenotype, revealed no evidence of heterogeneity in the treatment effect. Conclusion The use of remote PA pressure monitoring to guide treatment of patients with HF reduces episodes of worsening HF and subsequent hospitalizations.</p
Strings in AdS_3 and the SL(2,R) WZW Model. Part 2: Euclidean Black Hole
We consider the one-loop partition function for Euclidean BTZ black hole
backgrounds or equivalently thermal AdS_3 backgrounds which are quotients of
H_3 (Euclidean AdS_3). The one-loop partition function is modular invariant and
we can read off the spectrum which is consistent to that found in
hep-th/0001053. We see long strings and discrete states in agreement with the
expectations.Comment: 23 pages, 3 figure
Predicting Crystal Structures with Data Mining of Quantum Calculations
Predicting and characterizing the crystal structure of materials is a key
problem in materials research and development. It is typically addressed with
highly accurate quantum mechanical computations on a small set of candidate
structures, or with empirical rules that have been extracted from a large
amount of experimental information, but have limited predictive power. In this
letter, we transfer the concept of heuristic rule extraction to a large library
of ab-initio calculated information, and demonstrate that this can be developed
into a tool for crystal structure prediction.Comment: 4 pages, 3 pic
The Progenitors of Dwarf Spheroidal Galaxies
Dwarf spheroidal (dSph) galaxies present an evolutionary puzzle that we
explore in 40 early- and late-type dwarfs in the Local Group and nearby field.
Although dSphs formed stars over extended periods, today all but one are free
of detectable interstellar matter (ISM), even in the Fornax dSph, where stars
still formed 100 Myr ago. Combining metallicities for red giants with HI data
from the literature, we show that the well-known offset in
luminosity-metallicity (L-Z) relations for dSphs and dwarf irregular (dIrr)
galaxies exists also when comparing only their old stellar populations: dSphs
have higher mean stellar metallicities for a fixed luminosity. Evidently
younger dSphs experienced more efficient enrichment than young dIrrs. Dwarf
galaxies, whose locus in the L-Z diagram is consistent with that of dSphs even
for baryonic luminosities, are the ``transition-type dwarfs'' Phoenix, DDO210,
LGS3, Antlia, and KKR25. They have mixed dIrr/dSph morphologies, low stellar
masses, low angular momentum, and HI contents of less than a few 10^6 solar
masses. Unlike dIrrs, many transition-type dwarfs would closely resemble dSphs
if their gas were removed; they are likely dSph progenitors. As gas removal is
key, we consider the empirical evidence for various gas removal processes. We
suggest that internal gas removal mechanisms are inadequate and favor ram
pressure stripping to make dSphs. A combination of initial conditions and
environment seems to support the formation of dSphs, which appear to form from
small galaxies with active early star formation, whose evolution halts due to
externally induced gas loss. Transition-type dwarfs then are dSphs that kept
their ISM, and therefore should replace dSphs in isolated locations where
stripping is ineffective. (Abridged)Comment: 25 pages in AASTeX two-column preprint style, 1 table, 3 figures.
Accepted for publication in the Astronomical Journal (April 2003 issue
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