酶的趋化性理论：实验与模型的比较
A Theory of Enzyme Chemotaxis: Comparison Between Experiment and Model
摘要：
酶在溶液中有底物存在时表现出两种不同的转运行为。首先，它们的扩散性随着衬底浓度的增加而增强。此外，酶向底物浓度高的区域进行定向运动，称为趋化性。虽然各种酶已被证明经历趋化性，仍然缺乏定量的理解的现象。在这里，我们提供了酶在其底物的浓度梯度中的主动运动的一般表达式。该模型考虑了底物结合和催化周转步骤，以及增强的扩散效应。我们已经通过实验测量了一个快速和缓慢的酶的趋化性：在催化条件下的尿素酶，和己糖激酶的充分催化和简单的非催化底物结合。所提出的模型与实验结果吻合较好。该模型是通用的，没有可调参数，只需要三个实验定义的常数来量化趋化性：酶-底物结合亲和力（Kd），Michaelis-Menten常数（KM）和相关底物的扩散增强水平（α）。
英文摘要：
Enzymes show two distinct transport behaviors in the presence of their substrates in solution. First, their diffusivity enhances with increasing substrate concentration. In addition, enzymes perform directional motion toward regions with high substrate concentration, termed chemotaxis. While a variety of enzymes has been shown to undergo chemotaxis, there remains a lack of quantitative understanding of the phenomenon. Here, we provide a general expression for the active movement of an enzyme in a concentration gradient of its substrate. The proposed model takes into account both the substrate-binding and catalytic turnover step, as well as the enhanced diffusion effect. We have experimentally measured the chemotaxis of a fast and a slow enzyme: urease under catalytic conditions, and hexokinase for both full catalysis and for simple non-catalytic substrate binding. There is good agreement between the proposed model and the experiments. The model is general, has no adjustable parameters, and only requires three experimentally defined constants to quantify chemotaxis: enzyme-substrate binding affinity (Kd), Michaelis-Menten constant (KM) and level of diffusion enhancement in the associated substrate (α).
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