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تسجيل مستخدم جديدDiffusion and steady state distributions of flexible chemotactic enzymes
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Many experiments in recent years have reported that, when exposed to their corresponding substrate, catalytic enzymes undergo enhanced diffusion as well as chemotaxis (biased motion in the direction of a substrate gradient). Among other possible mechanisms, in a number of recent works we have explored several passive mechanisms for enhanced diffusion and chemotaxis, in the sense that they require only binding and unbinding of the enzyme to the substrate rather than the catalytic reaction itself. These mechanisms rely on conformational changes of the enzyme due to binding, as well as on phoresis due to non-contact interactions between enzyme and substrate. Here, after reviewing and generalizing our previous findings, we extend them in two different ways. In the case of enhanced diffusion, we show that an exact result for the long-time diffusion coefficient of the enzyme can be obtained using generalized Taylor dispersion theory, which results in much simpler and transparent analytical expressions for the diffusion enhancement. In the case of chemotaxis, we show that the competition between phoresis and binding-induced changes in diffusion results in non-trivial steady state distributions for the enzyme, which can either accumulate in or be depleted from regions with a specific substrate concentration.
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In many biological situations, a species arriving from a remote source diffuses in a domain confined between two parallel surfaces until it finds a binding partner. Since such a geometric shape falls in between two- and three-dimensional settings, th
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th generations of accumulated knowledge about diffusion at equilibrium. To test the limits of this idea, a critical testbed is mobility of catalytically active enzymes. Sentiment is divided about reality of enhanced enzyme diffusion with evidence for and against. Here a master curve shows that enzyme diffusion coefficient increases in proportion to the energy release rate, the product of Michaelis-Menten reaction rate and Gibbs free energy change with the highly satisfactory correlation coefficient of 0.97. For ten catalytic enzymes (urease, acetylcholinesterase, seven enzymes from the glucose cascade cycle, and another), our measurements span from roughly 40% enhanced diffusion coefficient at high turnover rate and negative Gibbs free energy to no enhancement at slow turnover rate and positive Gibbs free energy. Moreover, two independent measures of mobility show consistency, provided that one avoids undesirable fluorescence photophysics. The master curve presented here quantifies the limits of both ideas, that enzymes display enhanced diffusion and that they do not within instrumental resolution, and has possible implications for understanding enzyme mobility in cellular environments. The striking linear dependence for the exergonic enzymes (negative Gibbs free energy) together with the vanishing effect for endergonic enzyme (positive Gibbs free energy) are consistent with a physical picture where the mechanism boosting the diffusion is an active one, utilizing the available work from the chemical reaction.
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