62 research outputs found
High rates of fuel consumption are not required by insulating motifs to suppress retroactivity in biochemical circuits
Retroactivity arises when the coupling of a molecular network
to a downstream network results in signal propagation back from
to . The phenomenon represents a breakdown in
modularity of biochemical circuits and hampers the rational design of complex
functional networks. Considering simple models of signal-transduction
architectures, we demonstrate the strong dependence of retroactivity on the
properties of the upstream system, and explore the cost and efficacy of
fuel-consuming insulating motifs that can mitigate retroactive effects. We find
that simple insulating motifs can suppress retroactivity at a low fuel cost by
coupling only weakly to the upstream system . However, this design
approach reduces the signalling network's robustness to perturbations from leak
reactions, and potentially compromises its ability to respond to
rapidly-varying signals.Comment: 26 pages, 19 figures, To appear in Engineering Biolog
The robustness of proofreading to crowding-induced pseudo-processivity in the MAPK pathway
Double phosphorylation of protein kinases is a common feature of signalling
cascades. This motif may reduce cross-talk between signalling pathways, as the
second phosphorylation site allows for proofreading, especially when
phosphorylation is distributive rather than processive. Recent studies suggest
that phosphorylation can be `pseudo-processive' in the crowded cellular
environment, as rebinding after the first phosphorylation is enhanced by slow
diffusion. Here, we use a simple model with unsaturated reactants to show that
specificity for one substrate over another drops as rebinding increases and
pseudo-processive behavior becomes possible. However, this loss of specificity
with increased rebinding is typically also observed if two distinct enzyme
species are required for phosphorylation, i.e. when the system is necessarily
distributive. Thus the loss of specificity is due to an intrinsic reduction in
selectivity with increased rebinding, which benefits inefficient reactions,
rather than pseudo-processivity itself. We also show that proofreading can
remain effective when the intended signalling pathway exhibits high levels of
rebinding-induced pseudo-processivity, unlike other proposed advantages of the
dual phosphorylation motif.Comment: To appear in Biohys.
Thermodynamics of deterministic finite automata operating locally and periodically
Real-world digital computers have operational constraints that cause nonzero
entropy production (EP). In particular, almost all real-world computers are
"periodic" in that they iteratively undergo the same physical process, and are
"local" in that not all physical variables that are statistically coupled are
also directly coupled physically. These constraints are so universal because
the ability to decompose a complex computation into small, iterative logical
updates is what makes digital computers so powerful. Here we first derive
expressions for the nonzero EP caused by these two particular constraints in
physical implementations of deterministic finite automata (DFA), a foundational
system of computer science theory. We then relate this minimal EP to the
computational characteristics of the DFA. Specifically, we show that DFA divide
into two classes: those with an invertible local update map, which have zero
local and periodic EP, and those with a non-invertible local update map, which
have high minimal EP. We also demonstrate the thermodynamic advantages of
implementing a DFA with a physical process that is agnostic about the inputs
that it processes. \end{abstract
The thermodynamics of computational copying in biochemical systems
Living cells use readout molecules to record the state of receptor proteins,
similar to measurements or copies in typical computational devices. But is this
analogy rigorous? Can cells be optimally efficient, and if not, why? We show
that, as in computation, a canonical biochemical readout network generates
correlations; extracting no work from these correlations sets a lower bound on
dissipation. For general input, the biochemical network cannot reach this
bound, even with arbitrarily slow reactions or weak thermodynamic driving. It
faces an accuracy-dissipation trade-off that is qualitatively distinct from and
worse than implied by the bound, and more complex steady-state copy processes
cannot perform better. Nonetheless, the cost remains close to the thermodynamic
bound unless accuracy is extremely high. Additionally, we show that
biomolecular reactions could be used in thermodynamically optimal devices under
exogenous manipulation of chemical fuels, suggesting an experimental system for
testing computational thermodynamics.Comment: Accepted versio
Force-induced rupture of a DNA duplex
The rupture of double-stranded DNA under stress is a key process in
biophysics and nanotechnology. In this article we consider the shear-induced
rupture of short DNA duplexes, a system that has been given new importance by
recently designed force sensors and nanotechnological devices. We argue that
rupture must be understood as an activated process, where the duplex state is
metastable and the strands will separate in a finite time that depends on the
duplex length and the force applied. Thus, the critical shearing force required
to rupture a duplex within a given experiment depends strongly on the time
scale of observation. We use simple models of DNA to demonstrate that this
approach naturally captures the experimentally observed dependence of the
critical force on duplex length for a given observation time. In particular,
the critical force is zero for the shortest duplexes, before rising sharply and
then plateauing in the long length limit. The prevailing approach, based on
identifying when the presence of each additional base pair within the duplex is
thermodynamically unfavorable rather than allowing for metastability, does not
predict a time-scale-dependent critical force and does not naturally
incorporate a critical force of zero for the shortest duplexes. Additionally,
motivated by a recently proposed force sensor, we investigate application of
stress to a duplex in a mixed mode that interpolates between shearing and
unzipping. As with pure shearing, the critical force depends on the time scale
of observation; at a fixed time scale and duplex length, the critical force
exhibits a sigmoidal dependence on the fraction of the duplex that is subject
to shearing.Comment: 10 pages, 6 figure
Modelling DNA Origami Self-Assembly at the Domain Level
We present a modelling framework, and basic model parameterization, for the
study of DNA origami folding at the level of DNA domains. Our approach is
explicitly kinetic and does not assume a specific folding pathway. The binding
of each staple is associated with a free-energy change that depends on staple
sequence, the possibility of coaxial stacking with neighbouring domains, and
the entropic cost of constraining the scaffold by inserting staple crossovers.
A rigorous thermodynamic model is difficult to implement as a result of the
complex, multiply connected geometry of the scaffold: we present a solution to
this problem for planar origami. Coaxial stacking and entropic terms,
particularly when loop closure exponents are taken to be larger than those for
ideal chains, introduce interactions between staples. These cooperative
interactions lead to the prediction of sharp assembly transitions with notable
hysteresis that are consistent with experimental observations. We show that the
model reproduces the experimentally observed consequences of reducing staple
concentration, accelerated cooling and absent staples. We also present a
simpler methodology that gives consistent results and can be used to study a
wider range of systems including non-planar origami
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