scholarly journals Transient times in linear metabolic pathways under constant affinity constraints

1997 ◽  
Vol 327 (2) ◽  
pp. 493-498 ◽  
Author(s):  
Mónica LLORÉNS ◽  
C. Juan NUÑO ◽  
Francisco MONTERO
Keyword(s):  
Steady State ◽  
Metabolic Pathways ◽  
Analytical Study ◽  
Time Dependent ◽  
State Function ◽  
Transient Time ◽  
Input Flux ◽  
Linear Reaction ◽  
Transition Times ◽  
Reaction Schemes

In the early seventies, Easterby began the analytical study of transition times for linear reaction schemes [Easterby (1973) Biochim. Biophys. Acta 293, 552-558]. In this pioneer work and in subsequent papers, a state function (the transient time) was used to measure the period before the stationary state, for systems constrained to work under both constant and variable input flux, was reached. Despite the undoubted usefulness of this quantity to describe the time-dependent features of these kinds of systems, its application to the study of chemical reactions under other constraints is questionable. In the present work, a generalization of these magnitudes to linear metabolic pathways functioning under a constant-affinity constraint is carried out. It is proved that classical definitions of transient times do not reflect the actual properties of the transition to the steady state in systems evolving under this restriction. Alternatively, a more adequate framework for interpretation of the transient times for systems with both constant and variable input flux is suggested. Within this context, new definitions that reflect more accurately the transient characteristics of constant affinity systems are stated. Finally, the meaning of these transient times is discussed.

Equilibrium enzymes in metabolic pathways

1996 ◽  
Vol 74 (3) ◽  
pp. 411-416 ◽  
Author(s):  
S. P. J. Brooks
Keyword(s):  
Steady State ◽  
Kinetic Parameters ◽  
Metabolic Pathways ◽  
Enzyme Locus ◽  
Steady State Flux ◽  
Equilibrium Reactions ◽  
Metabolic Sequence ◽  
Far From Equilibrium ◽  
Catalytic Capacity ◽  
Reaction Schemes

It is commonly believed that certain reactions in a metabolic sequence may be at or close to equilibrium because of the large excess of catalytic capacity compared to the flux through these enzyme loci. Simple algebraic manipulations can show that the equilibrium and steady state conditions are mutually exclusive. However, solution of the complete reaction schemes for model "equilibrium" reactions shows that they can remain far from equilibrium even though the ratio of enzyme flux to steady state flux through the overall pathway is high. These calculations show that a reaction's proximity to equilibrium depends on the overall flux through the enzyme locus as well as on the kinetic parameters of the other enzymes in the pathway. Thus, combinations of kinetic parameters may exist that allow certain reactions to approach equilibrium but these conditions are not universal.Key words: equilibria, theoretical kinetics, metabolic control.

scholarly journals The effect of feedback on pathway transient response

1986 ◽  
Vol 233 (3) ◽  
pp. 871-875 ◽  
Author(s):  
J S Easterby
Keyword(s):  
Steady State ◽  
Metabolic Pathways ◽  
Transient Analysis ◽  
Steady States ◽  
Transient Time ◽  
Transient Behaviour ◽  
Steady State Flux ◽  
The Times ◽  
Time Required ◽  
State Functions

The effect of variation of the rate of input of material on the transient behaviour of metabolic pathways is examined. This reveals the existence of three transient times which make up the overall pathway transient. Two of these have been described previously and represent the times required for the accumulation of the free intermediate pool and the pool of enzyme-bound intermediate. They are state functions and as such are independent of the way in which the steady state was reached. The third is attributable to the variation in the rate of input of material to the pathway. It is dependent on three further factors. These are (a) the time required for the initial enzyme to reach its own steady state, (b) substrate depletion and (c) feedback. The description of the transient is: (Formula: see text) where V0 represents the rate of input and Vss represents the steady-state flux. The transient time associated with the transition between steady-states is shown to be a simple function of the transients for the establishment of each steady state from rest and may be expressed as: tau = tau b-Va/Vb . tau a where Va and Vb refer to the fluxes in the two steady states and tau a and tau b represent the transient times for the establishment of each of the steady-states from rest. The total pathway transient may now be completely defined as: (formula: see text) where summation over all intermediates, I, is implied. The significance of this to the analysis of pathway behaviour is discussed with more general examples of pathway transient analysis.

scholarly journals Time-Dependent Reliability Analysis of Reinforced Concrete Beams Subjected to Uniform and Pitting Corrosion and Brittle Fracture

Materials ◽  
2021 ◽  
Vol 14 (8) ◽  
pp. 1820
Author(s):  
Mohamed El Amine Ben Seghier ◽  
Behrooz Keshtegar ◽  
Hussam Mahmoud
Keyword(s):  
Reinforced Concrete ◽  
Brittle Fracture ◽  
Reliability Analysis ◽  
Pitting Corrosion ◽  
Structural Reliability ◽  
Time Dependent ◽  
State Function ◽  
Rc Beams ◽  
Highly Nonlinear ◽  
Reliability Method

Reinforced concrete (RC) beams are basic elements used in the construction of various structures and infrastructural systems. When exposed to harsh environmental conditions, the integrity of RC beams could be compromised as a result of various deterioration mechanisms. One of the most common deterioration mechanisms is the formation of different types of corrosion in the steel reinforcements of the beams, which could impact the overall reliability of the beam. Existing classical reliability analysis methods have shown unstable results when used for the assessment of highly nonlinear problems, such as corroded RC beams. To that end, the main purpose of this paper is to explore the use of a structural reliability method for the multi-state assessment of corroded RC beams. To do so, an improved reliability method, namely the three-term conjugate map (TCM) based on the first order reliability method (FORM), is used. The application of the TCM method to identify the multi-state failure of RC beams is validated against various well-known structural reliability-based FORM formulations. The limit state function (LSF) for corroded RC beams is formulated in accordance with two corrosion types, namely uniform and pitting corrosion, and with consideration of brittle fracture due to the pit-to-crack transition probability. The time-dependent reliability analyses conducted in this study are also used to assess the influence of various parameters on the resulting failure probability of the corroded beams. The results show that the nominal bar diameter, corrosion initiation rate, and the external loads have an important influence on the safety of these structures. In addition, the proposed method is shown to outperform other reliability-based FORM formulations in predicting the level of reliability in RC beams.

scholarly journals Ground- and excited-state characteristics in photovoltaic polymer N2200

RSC Advances ◽  
2021 ◽  
Author(s):  
Guanzhao Wen ◽  
Xianshao Zou ◽  
Rong Hu ◽  
Jun Peng ◽  
Zhifeng Chen ◽  
...  
Keyword(s):  
Density Functional Theory ◽  
Excited State ◽  
Steady State ◽  
Excited States ◽  
Density Functional ◽  
Density Functional Theory Calculations ◽  
Time Dependent ◽  
Functional Theory ◽  
Time Resolved ◽  
Dependent Density

Ground- and excited-states properties of N2200 have been studied by steady-state and time-resolved spectroscopies as well as time-dependent density functional theory calculations.

scholarly journals Determinants of mRNA stability in Dictyostelium discoideum amoebae: differences in poly(A) tail length, ribosome loading, and mRNA size cannot account for the heterogeneity of mRNA decay rates.

1988 ◽  
Vol 8 (5) ◽  
pp. 1957-1969 ◽  
Author(s):  
R A Shapiro ◽  
D Herrick ◽  
R E Manrow ◽  
D Blinder ◽  
A Jacobson
Keyword(s):  
Steady State ◽  
Dictyostelium Discoideum ◽  
Mrna Decay ◽  
Decay Rates ◽  
Time Dependent ◽  
Translational Efficiency ◽  
Cdna Clones ◽  
Dependent Manner ◽  
Significant Difference ◽  
Kinetics Of

As an approach to understanding the structures and mechanisms which determine mRNA decay rates, we have cloned and begun to characterize cDNAs which encode mRNAs representative of the stability extremes in the poly(A)+ RNA population of Dictyostelium discoideum amoebae. The cDNA clones were identified in a screening procedure which was based on the occurrence of poly(A) shortening during mRNA aging. mRNA half-lives were determined by hybridization of poly(A)+ RNA, isolated from cells labeled in a 32PO4 pulse-chase, to dots of excess cloned DNA. Individual mRNAs decayed with unique first-order decay rates ranging from 0.9 to 9.6 h, indicating that the complex decay kinetics of total poly(A)+ RNA in D. discoideum amoebae reflect the sum of the decay rates of individual mRNAs. Using specific probes derived from these cDNA clones, we have compared the sizes, extents of ribosome loading, and poly(A) tail lengths of stable, moderately stable, and unstable mRNAs. We found (i) no correlation between mRNA size and decay rate; (ii) no significant difference in the number of ribosomes per unit length of stable versus unstable mRNAs, and (iii) a general inverse relationship between mRNA decay rates and poly(A) tail lengths. Collectively, these observations indicate that mRNA decay in D. discoideum amoebae cannot be explained in terms of random nucleolytic events. The possibility that specific 3'-structural determinants can confer mRNA instability is suggested by a comparison of the labeling and turnover kinetics of different actin mRNAs. A correlation was observed between the steady-state percentage of a given mRNA found in polysomes and its degree of instability; i.e., unstable mRNAs were more efficiently recruited into polysomes than stable mRNAs. Since stable mRNAs are, on average, "older" than unstable mRNAs, this correlation may reflect a translational role for mRNA modifications that change in a time-dependent manner. Our previous studies have demonstrated both a time-dependent shortening and a possible translational role for the 3' poly(A) tracts of mRNA. We suggest, therefore, that the observed differences in the translational efficiency of stable and unstable mRNAs may, in part, be attributable to differences in steady-state poly(A) tail lengths.

Tetanic hyperpolarization of single medullated nerve fibers in sodium and lithium

1976 ◽  
Vol 231 (4) ◽  
pp. 1033-1038 ◽  
Author(s):  
GM Schoepfle
Keyword(s):  
Steady State ◽  
Interspike Interval ◽  
Nerve Fibers ◽  
Repetitive Stimulation ◽  
Time Dependent ◽  
Potassium Ions ◽  
Current Densities ◽  
Medullated Nerve Fiber ◽  
Sodium And Potassium ◽  
Stimulation Of

Repetitive stimulation of a single medullated nerve fiber of Xenopus yields a succession of postspike voltage-time curves which are nearly coincident until attainment of a voltage that corresponds to that of the maximum attained by the normal postspike undershoot. Initially the interspike potential returns toward a resting level after this brief phase of hyperpolarization. However, as tetanization proceeds, a pattern of hyperpolarization develops with the result that, in the tetanic steady state, there exists a progressive hyperpolarization throughout each interspike interval. Extent of postspike hyperpolarization in terms of a deviation deltaVm from the resting level of membrane potential is approximated by the variation deltaVm = delta[MNa + MK]/[GNa + GK] where MNa and MK are current densities associated with active pumping of sodium and potassium ions and GNa and GK are corresponding time-dependent leak conductances. Tetanic hyperpolarization is reversibly abolished by cyanide and by exposure to lithium Ringer. Eventual reappearance of tetanic hyperpolarization in the presence of lithium Ringer suggests lithium pumping.

scholarly journals Symmetry analysis of rotating fluid

2005 ◽  
Vol 47 (1) ◽  
pp. 65-74 ◽  
Author(s):  
K. Fakhar ◽  
Zu-Chi Chen ◽  
Xiaoda Ji
Keyword(s):  
Steady State ◽  
Infinite Number ◽  
Special Function ◽  
Equations Of Motion ◽  
Time Dependent ◽  
Lie Theory ◽  
Rotating Fluid ◽  
Type Solution ◽  
Function Type ◽  
Basic Equations

AbstractThe machinery of Lie theory (groups and algebras) is applied to the unsteady equations of motion of rotating fluid. A special-function type solution for the steady state is derived. It is then shown how the solution generates an infinite number of time-dependent solutions via three arbitrary functions of time. This algebraic structure also provides the mechanism to search for other solutions since its character is inferred from the basic equations.

Numerical Experiments of Coolant Mixing in a Lower Plenum of PWR Under Asymmetric Thermal- Hydraulics Conditions

2006 ◽  
Author(s):  
Masanori Ohtani ◽  
Akito Kozuru ◽  
Yasuyuki Kashimoto ◽  
Mitsuto Montani ◽  
Koutaro Takeda ◽  
...  
Keyword(s):  
Steady State ◽  
Temperature Distribution ◽  
Turbulent Mixing ◽  
Numerical Experiments ◽  
Time Dependent ◽  
Inlet Temperature ◽  
Velocity Fluctuations ◽  
The Core ◽  
State Calculation ◽  
Lower Temperature

Asymmetric thermal-hydraulic conditions among primary loops during a postulated steam line break (SLB) induce a non-uniform temperature distribution at a core inlet. When coolant of lower temperature intrudes into a part of core, it leads to a reactivity insertion and a local power increase. Therefore, an appropriate model for the core inlet temperature distribution is required for a realistic SLB analysis. In this study, numerical experiments were conducted to examine the core inlet temperature distribution under the asymmetric thermal-hydraulic coolant conditions among primary loops. 3D steady-state calculations were carried out for Japanese standard Pressurized Water Reactor (PWR) such as 2, 3, 4 loop types and an advanced PWR. Since the flow in a reactor vessel involves time-dependent velocity fluctuations due to a high Reynolds number condition and a complicated geometry of flow path, the turbulent mixing might be enhanced. Hence, the turbulent thermal diffusivity for the steady-state calculation was examined based on experimental results and another transient calculation. As a result, it was confirmed that (1) the turbulent mixing in a downcomer and a lower plenum were enhanced due to time-dependent velocity fluctuations and therefore the turbulent thermal diffusivity for steady-state calculation was specified to be greater, (2) the core inlet temperature distribution predicted by a steady-state calculation reasonably agreed with a experimental data, (3) the patterns of core inlet temperature distribution were comprehended to be dependent on the plant type, i.e. the number of primary loop and (4) under a low flow rate condition, the coolant of lower temperature appeared on the opposite side of the affected loop due to the effect of a natural convection.

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