Enzymes: A-Level CIE Biology Key Concept Review | A-Level CIE 生物：酶 考点精讲

📚 Enzymes: A-Level CIE Biology Key Concept Review | A-Level CIE 生物：酶 考点精讲

Enzymes are fundamental to all biochemical processes. In CIE A-Level Biology, you must understand their structure, specificity, kinetics, and the factors that influence their activity. This revision guide covers every essential point — from the induced-fit model to the industrial use of immobilised enzymes — so you can tackle exam questions with confidence.

酶是所有生化反应的基础。在 CIE A-Level 生物考试中，你需要掌握酶的结构、特异性、动力学以及影响其活性的各种因素。本文梳理了所有核心考点——从诱导契合模型到固定化酶的工业应用——帮助你有信心地应对考题。

1. What Are Enzymes? | 什么是酶？

Enzymes are globular proteins that act as biological catalysts. They speed up metabolic reactions by lowering the activation energy required, without being used up or permanently changed in the process. Most enzymes are highly specific, catalysing only one reaction or a group of closely related reactions. Their active site has a precise three-dimensional shape that is complementary to the substrate. Some enzymes require cofactors – such as metal ions or coenzymes (e.g. NAD⁺) – to function.

酶是球状蛋白质，作为生物催化剂发挥作用。它们通过降低反应所需的活化能来加速代谢反应，自身在过程中不被消耗或永久改变。大多数酶具有高度特异性，仅催化一种或一类密切相关的反应。它们的活性位点具有与底物互补的精确三维形状。一些酶需要辅助因子——如金属离子或辅酶（如 NAD⁺）——才能发挥作用。

Enzymes are not living; they are molecules. The name of an enzyme usually ends in ‘-ase’ and often derives from its substrate or the reaction it catalyses, e.g. catalase breaks down hydrogen peroxide.

酶并非生命体，而是分子。酶的名称通常以“-ase”结尾，往往源自其底物或催化的反应，例如过氧化氢酶（catalase）分解过氧化氢。

2. Mechanism of Enzyme Action: Lock and Key vs Induced Fit | 酶的作用机制：锁钥模型与诱导契合模型

The lock-and-key model proposes that the active site of the enzyme is already perfectly complementary in shape to the substrate. The substrate fits into the active site just as a key fits a lock. This model explains enzyme specificity but does not account for the transition state stabilisation.

锁钥模型认为，酶的活性位点在形状上早已与底物完美互补。底物嵌入活性位点，就像钥匙插入锁孔一样。该模型解释了酶的特异性，但无法解释过渡态的稳定化。

The more accurate induced-fit model states that the active site is flexible. When the substrate binds, the active site changes its shape slightly, moulding around the substrate. This conformational change strains particular bonds in the substrate, lowering the activation energy more effectively and stabilising the transition state. The enzyme–substrate complex is formed, followed by the enzyme–product complex, and then products are released, leaving the enzyme unchanged.

更为准确的诱导契合模型指出，活性位点具有柔性。当底物结合时，活性位点的形状发生轻微改变，包绕底物。这种构象变化使底物中的特定化学键发生扭曲，从而更有效地降低活化能，并稳定过渡态。形成酶-底物复合物后，再转化为酶-产物复合物，最终释放产物，酶恢复原状。

Exam tip: CIE often wants you to compare the two models and state why the induced-fit model is preferred – because it explains how the enzyme reduces activation energy by stressing bonds.

考试提示：CIE 常要求你比较这两种模型，并说明为何诱导契合模型更优——因为它解释了酶如何通过应力作用于化学键来降低活化能。

3. Temperature and Enzyme Activity | 温度与酶活性

Increasing temperature increases the kinetic energy of molecules. Both enzyme and substrate molecules move faster, leading to more frequent successful collisions and more enzyme–substrate complexes formed per unit time. As a result, the rate of reaction increases, typically doubling for every 10 °C rise (a Q₁₀ coefficient of about 2) until an optimum temperature is reached.

升高温度会增加分子的动能。酶和底物分子运动加快，导致单位时间内成功碰撞次数增多，形成更多的酶-底物复合物。因此，反应速率随温度升高而加快，通常每升高 10 °C 反应速率加倍（Q₁₀ 系数约为 2），直至达到最适温度。

Beyond the optimum temperature, the high thermal energy breaks the hydrogen bonds, ionic bonds and hydrophobic interactions that maintain the enzyme’s tertiary structure. The active site becomes denatured and no longer complements the substrate. Denaturation is irreversible once these bonds are broken; the enzyme loses its catalytic function. The rate of reaction drops sharply.

超过最适温度后，高热能使维持酶三级结构的氢键、离子键和疏水相互作用断裂。活性位点变性，无法再与底物互补。这些键一旦断裂，变性就是不可逆的；酶失去催化功能，反应速率急剧下降。

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