768 research outputs found
Michaelis-Menten Relations for Complex Enzymatic Networks
All biological processes are controlled by complex systems of enzymatic
chemical reactions. Although the majority of enzymatic networks have very
elaborate structures, there are many experimental observations indicating that
some turnover rates still follow a simple Michaelis-Menten relation with a
hyperbolic dependence on a substrate concentration. The original
Michaelis-Menten mechanism has been derived as a steady-state approximation for
a single-pathway enzymatic chain. The validity of this mechanism for many
complex enzymatic systems is surprising. To determine general conditions when
this relation might be observed in experiments, enzymatic networks consisting
of coupled parallel pathways are investigated theoretically. It is found that
the Michaelis-Menten equation is satisfied for specific relations between
chemical rates, and it also corresponds to the situation with no fluxes between
parallel pathways. Our results are illustrated for simple models. The
importance of the Michaelis-Menten relationship and derived criteria for
single-molecule experimental studies of enzymatic processes are discussed.Comment: 10 pages, 4 figure
Passive Scalar: Scaling Exponents and Realizability
An isotropic passive scalar field advected by a rapidly-varying velocity
field is studied. The tail of the probability distribution for
the difference in across an inertial-range distance is found
to be Gaussian. Scaling exponents of moments of increase as
or faster at large order , if a mean dissipation conditioned on is
a nondecreasing function of . The computed numerically
under the so-called linear ansatz is found to be realizable. Some classes of
gentle modifications of the linear ansatz are not realizable.Comment: Substantially revised to conform with published version. Revtex (4
pages) with 2 postscript figures. Send email to [email protected]
Fragmentation of High-spin Particle-hole States in 26-Mg
This research was sponsored by the National Science Foundation Grant NSF PHy 87-1440
Fragmentation of High-Spin Particle-Hole States in 26-Mg
This work was supported by the National Science Foundation Grants NSF PHY 78-22774 A03, NSF PHY 81-14339, and by Indiana Universit
Unstable decay and state selection II
The decay of unstable states when several metastable states are available for
occupation is investigated using path-integral techniques. Specifically, a
method is described which allows the probabilities with which the metastable
states are occupied to be calculated by finding optimal paths, and fluctuations
about them, in the weak noise limit. The method is illustrated on a system
described by two coupled Langevin equations, which are found in the study of
instabilities in fluid dynamics and superconductivity. The problem involves a
subtle interplay between non-linearities and noise, and a naive approximation
scheme which does not take this into account is shown to be unsatisfactory. The
use of optimal paths is briefly reviewed and then applied to finding the
conditional probability of ending up in one of the metastable states, having
begun in the unstable state. There are several aspects of the calculation which
distinguish it from most others involving optimal paths: (i) the paths do not
begin and end on an attractor, and moreover, the final point is to a large
extent arbitrary, (ii) the interplay between the fluctuations and the leading
order contribution are at the heart of the method, and (iii) the final result
involves quantities which are not exponentially small in the noise strength.
This final result, which gives the probability of a particular state being
selected in terms of the parameters of the dynamics, is remarkably simple and
agrees well with the results of numerical simulations. The method should be
applicable to similar problems in a number of other areas such as state
selection in lasers, activationless chemical reactions and population dynamics
in fluctuating environments.Comment: 28 pages, 6 figures. Accepted for publication in Phys. Rev.
Fragmentation of High-Spin Particle-Hole States in 26-Mg
This work was supported by the National Science Foundation Grant NSF PHY 81-14339 and by Indiana Universit
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Overview of mathematical approaches used to model bacterial chemotaxis I: the single cell
Mathematical modeling of bacterial chemotaxis systems has been influential and insightful in helping to understand experimental observations. We provide here a comprehensive overview of the range of mathematical approaches used for modeling, within a single bacterium, chemotactic processes caused by changes to external gradients in its environment. Specific areas of the bacterial system which have been studied and modeled are discussed in detail, including the modeling of adaptation in response to attractant gradients, the intracellular phosphorylation cascade, membrane receptor clustering, and spatial modeling of intracellular protein signal transduction. The importance of producing robust models that address adaptation, gain, and sensitivity are also discussed. This review highlights that while mathematical modeling has aided in understanding bacterial chemotaxis on the individual cell scale and guiding experimental design, no single model succeeds in robustly describing all of the basic elements of the cell. We conclude by discussing the importance of this and the future of modeling in this area
Alpha-decay branching ratios of near-threshold states in <sup>19</sup>Ne and the astrophysical rate of <sup>15</sup> O(α, γ )<sup>19</sup>Ne
The 15O(α,γ)19Ne reaction is one of two routes for breakout from the hot CNO cycles into the rp process in accreting neutron stars. Its astrophysical rate depends critically on the decay properties of excited states in 19Ne lying just above the 15O + α threshold. We have measured the α-decay branching ratios for these states using the p(21lNe,t)19Ne reaction at 43 MeV/u.</p
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