27,715 research outputs found
Interplay between excitation kinetics and reaction-center dynamics in purple bacteria
Photosynthesis is arguably the fundamental process of Life, since it enables
energy from the Sun to enter the food-chain on Earth. It is a remarkable
non-equilibrium process in which photons are converted to many-body excitations
which traverse a complex biomolecular membrane, getting captured and fueling
chemical reactions within a reaction-center in order to produce nutrients. The
precise nature of these dynamical processes -- which lie at the interface
between quantum and classical behaviour, and involve both noise and
coordination -- are still being explored. Here we focus on a striking recent
empirical finding concerning an illumination-driven transition in the
biomolecular membrane architecture of {\it Rsp. Photometricum} purple bacteria.
Using stochastic realisations to describe a hopping rate model for excitation
transfer, we show numerically and analytically that this surprising shift in
preferred architectures can be traced to the interplay between the excitation
kinetics and the reaction center dynamics. The net effect is that the bacteria
profit from efficient metabolism at low illumination intensities while using
dissipation to avoid an oversupply of energy at high illumination intensities.Comment: 21 pages, 13 figures, accepted for publication in New Journal of
Physic
How cells feel: stochastic model for a molecular mechanosensor
Understanding mechanosensitivity, i.e. how cells sense the stiffness of their
environment is very important, yet there is a fundamental difficulty in
understanding its mechanism: to measure an elastic modulus one requires two
points of application of force - a measuring and a reference point. The cell in
contact with substrate has only one (adhesion) point to work with, and thus a
new method of measurement needs to be invented. The aim of this theoretical
work is to develop a self-consistent physical model for mechanosensitivity, a
process by which a cell detects the mechanical stiffness of its environment
(e.g. a substrate it is attached to via adhesion points) and generates an
appropriate chemical signaling to remodel itself in response to this
environment. The model uses the molecular mechanosensing complex of latent
TGF- attached to the adhesion point as the biomarker. We show that the
underlying Brownian motion in the substrate is the reference element in the
measuring process. The model produces the closed expression for the rate of
release of active TGF-, which depends on the substrate stiffness and the
pulling force coming from the cell in a subtle and non-trivial way. It is
consistent with basic experimental data showing an increase in signal for
stiffer substrates and higher pulling forces. In addition, we find that for
each cell there is a range of stiffness where a homeostatic configuration of
the cell can be achieved, outside of which the cell either relaxes its
cytoskeletal forces and detaches from the very weak substrate, or generates an
increasingly strong pulling force through stress fibers with a positive
feedback loop on very stiff substrates. In this way, the theory offers the
underlying mechanism for the myofibroblast conversion in wound healing and
smooth muscle cell dysfunction in cardiac disease
A biomimetic algorithm for the improved detection of microarray features,
One the major difficulties of microarray technology relate to the processing of large and - importantly - error-loaded images of the dots on the chip surface. Whatever the source of these errors, those obtained in the first stage of data acquisition - segmentation - are passed down to the subsequent processes, with deleterious results. As it has been demonstrated recently that biological systems have evolved algorithms that are mathematically efficient, this contribution attempts to test an algorithm that mimics a bacterial-"patented" algorithm for the search of available space and nutrients to find, "zero-in" and eventually delimitate the features existent on the microarray surface
Physics of thick polymers
We present the results of analytic calculations and numerical simulations of
the behaviour of a new class of chain molecules which we call thick polymers.
The concept of the thickness of such a polymer, viewed as a tube, is
encapsulated by a special three body interaction and impacts on the behaviour
both locally and non-locally. When thick polymers undergo compaction due to an
attractive self-interaction, we find a new type of phase transition between a
compact phase and a swollen phase at zero temperature on increasing the
thickness. In the vicinity of this transition, short tubes form space filling
helices and sheets as observed in protein native state structures. Upon
increasing the chain length, or the number of chains, we numerically find a
crossover from secondary structure motifs to a quite distinct class of
structures akin to the semi-crystalline phase of polymers or amyloid fibers in
polypeptides.Comment: 41 pages, 20 figures. Accepted for publication in J. Pol. Sci.
Method for finding metabolic properties based on the general growth law. Liver examples. A General framework for biological modeling
We propose a method for finding metabolic parameters of cells, organs and
whole organisms, which is based on the earlier discovered general growth law.
Based on the obtained results and analysis of available biological models, we
propose a general framework for modeling biological phenomena and discuss how
it can be used in Virtual Liver Network project. The foundational idea of the
study is that growth of cells, organs, systems and whole organisms, besides
biomolecular machinery, is influenced by biophysical mechanisms acting at
different scale levels. In particular, the general growth law uniquely defines
distribution of nutritional resources between maintenance needs and biomass
synthesis at each phase of growth and at each scale level. We exemplify the
approach considering metabolic properties of growing human and dog livers and
liver transplants. A procedure for verification of obtained results has been
introduced too. We found that two examined dogs have high metabolic rates
consuming about 0.62 and 1 gram of nutrients per cubic centimeter of liver per
day, and verified this using the proposed verification procedure. We also
evaluated consumption rate of nutrients in human livers, determining it to be
about 0.088 gram of nutrients per cubic centimeter of liver per day for males,
and about 0.098 for females. This noticeable difference can be explained by
evolutionary development, which required females to have greater liver
processing capacity to support pregnancy. We also found how much nutrients go
to biomass synthesis and maintenance at each phase of liver and liver
transplant growth. Obtained results demonstrate that the proposed approach can
be used for finding metabolic characteristics of cells, organs, and whole
organisms, which can further serve as important inputs for many applications in
biology (protein expression), biotechnology (synthesis of substances), and
medicine.Comment: 20 pages, 6 figures, 4 table
Erratum: Signal propagation in proteins and relation to equilibrium fluctuations (PLoS Computational Biology (2007) 3, 9, (e172) DOI: 10.1371/journal.pcbi.0030172))
Elastic network (EN) models have been widely used in recent years for describing protein dynamics, based on the premise that the motions naturally accessible to native structures are relevant to biological function. We posit that equilibrium motions also determine communication mechanisms inherent to the network architecture. To this end, we explore the stochastics of a discrete-time, discrete-state Markov process of information transfer across the network of residues. We measure the communication abilities of residue pairs in terms of hit and commute times, i.e., the number of steps it takes on an average to send and receive signals. Functionally active residues are found to possess enhanced communication propensities, evidenced by their short hit times. Furthermore, secondary structural elements emerge as efficient mediators of communication. The present findings provide us with insights on the topological basis of communication in proteins and design principles for efficient signal transduction. While hit/commute times are information-theoretic concepts, a central contribution of this work is to rigorously show that they have physical origins directly relevant to the equilibrium fluctuations of residues predicted by EN models
Molecular theory of solvation: Methodology summary and illustrations
Integral equation theory of molecular liquids based on statistical mechanics
is quite promising as an essential part of multiscale methodology for chemical
and biomolecular nanosystems in solution. Beginning with a molecular
interaction potential force field, it uses diagrammatic analysis of the
solvation free energy to derive integral equations for correlation functions
between molecules in solution in the statistical-mechanical ensemble. The
infinite chain of coupled integral equations for many-body correlation
functions is reduced to a tractable form for 2- or 3-body correlations by
applying the so-called closure relations. Solving these equations produces the
solvation structure with accuracy comparable to molecular simulations that have
converged but has a critical advantage of readily treating the effects and
processes spanning over a large space and slow time scales, by far not feasible
for explicit solvent molecular simulations. One of the versions of this
formalism, the three-dimensional reference interaction site model (3D-RISM)
integral equation complemented with the Kovalenko-Hirata (KH) closure
approximation, yields the solvation structure in terms of 3D maps of
correlation functions, including density distributions, of solvent interaction
sites around a solute (supra)molecule with full consistent account for the
effects of chemical functionalities of all species in the solution. The
solvation free energy and the subsequent thermodynamics are then obtained at
once as a simple integral of the 3D correlation functions by performing
thermodynamic integration analytically.Comment: 24 pages, 10 figures, Revie
- …