Enzymes: The Biological Catalysts | 酶：生物催化剂

📚 Enzymes: The Biological Catalysts | 酶：生物催化剂

Enzymes are among the most important molecules in living organisms. They control every metabolic reaction, from the breakdown of food in your gut to the synthesis of DNA in your cells. In the Edexcel IGCSE Science specification, enzymes are examined in both Biology and Chemistry contexts, so a clear understanding of their structure, function and the factors that affect them is essential for exam success.

酶是生物体内最重要的分子之一。它们控制着每一个代谢反应，从消化道中食物的分解到细胞中DNA的合成。在Edexcel IGCSE科学大纲中，酶在生物和化学两个背景下都会被考查，因此清晰理解酶的结构、功能以及影响它们活性的因素，对于考试取得好成绩至关重要。

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1. What Are Enzymes? | 什么是酶？

Enzymes are biological catalysts. A catalyst is a substance that speeds up the rate of a chemical reaction without being used up or changed in the process. In living organisms, almost every biochemical reaction is controlled by an enzyme.

酶是生物催化剂。催化剂是一种能够加快化学反应速率，但自身在反应过程中不会被消耗或改变的物质。在生物体内，几乎每一个生化反应都由酶控制。

All enzymes are proteins, made from long chains of amino acids folded into a specific three-dimensional shape. The shape of the enzyme is vital: it creates a region called the active site, where the reactant molecules, known as substrates, bind.

所有酶都是蛋白质，由长链氨基酸折叠成特定的三维形状。酶的形状至关重要：它形成一个称为活性位点的区域，反应物分子（即底物）在此结合。

The key features of enzymes to remember are:

需要牢记的酶的关键特征是：

• Enzymes remain unchanged after the reaction, so they can be reused. | 反应后酶不发生改变，因此可以重复使用。

• Enzymes are highly specific: each enzyme only catalyses one type of reaction. | 酶具有高度的专一性：每种酶只催化一种类型的反应。

• Enzymes lower the activation energy needed for a reaction, allowing it to proceed faster. | 酶降低反应所需的活化能，使反应进行得更快。

A classic example used in Edexcel practicals is catalase, which speeds up the breakdown of hydrogen peroxide into water and oxygen.

Edexcel实验中一个经典例子是过氧化氢酶，它加速过氧化氢分解为水和氧气。

2H₂O₂ → 2H₂O + O₂ (catalysed by catalase | 由过氧化氢酶催化)

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2. The Lock and Key Model | 锁钥模型

To explain enzyme specificity, scientists use the lock and key model. The enzyme is the lock, and the substrate is the key. The active site has a fixed shape that is complementary to the shape of the substrate.

为了解释酶的专一性，科学家使用锁钥模型。酶是锁，底物是钥匙。活性位点具有与底物形状互补的固定形状。

When the substrate fits into the active site, an enzyme-substrate complex is formed. The reaction then takes place, and the products are released, leaving the enzyme unchanged and ready for the next substrate molecule.

当底物嵌入活性位点时，形成酶-底物复合物。随后反应发生，产物被释放，酶保持原样并准备接受下一个底物分子。

A more refined version, the induced fit model, suggests that the active site changes shape slightly when the substrate binds, making the fit even tighter. Some exam questions ask you to compare these two models, so learn both descriptions.

一个更精细的版本——诱导契合模型——认为底物结合时活性位点会轻微改变形状，使结合更加紧密。有些考题要求你比较这两种模型，所以请同时记住这两种描述。

Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product | 酶 + 底物 → 酶-底物复合物 → 酶 + 产物

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

Temperature has a dramatic effect on enzyme activity. As temperature increases from cold, the rate of reaction rises because molecules gain kinetic energy and collide more frequently with the active site.

温度对酶活性有显著影响。当温度从较低值升高时，反应速率上升，因为分子获得动能，与活性位点碰撞更加频繁。

However, this increase only continues up to a certain point, called the optimum temperature. For most human enzymes, the optimum temperature is around 37 °C, which is body temperature.

然而，这种上升只持续到某一点，即最适温度。对于大多数人体酶来说，最适温度大约为37 °C，即体温。

Above the optimum temperature, the enzyme molecules vibrate so violently that the bonds maintaining the three-dimensional structure begin to break. The shape of the active site is destroyed, and the enzyme is said to be denatured. Denaturation is permanent: the enzyme can no longer catalyse the reaction.

超过最适温度后，酶分子振动过于剧烈，维持三维结构的化学键开始断裂。活性位点的形状被破坏，我们说酶发生了变性。变性是不可逆的：酶不能再催化反应。

Temperature rises → rate increases to optimum (37 °C) → denaturation → rate falls to zero | 温度升高 → 速率升至最适点（37 °C）→ 变性 → 速率降为零

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4. pH and Enzyme Activity | pH与酶活性

Each enzyme also has an optimum pH. Small changes in pH alter the concentration of hydrogen ions (H⁺) in the solution, which can disrupt the ionic and hydrogen bonds that hold the enzyme in its precise shape.

每种酶也有其最适pH。pH的微小变化会改变溶液中氢离子（H⁺）的浓度，从而破坏维持酶精确形状的离子键和氢键。

At the optimum pH, the rate of reaction is maximum. For most enzymes in the human body, this is around pH 7. A notable exception is pepsin, a digestive enzyme in the stomach, which works best at pH 2 because the stomach contains hydrochloric acid.

在最适pH下，反应速率最大。人体内大多数酶的最适pH约为7。一个著名的例外是胃蛋白酶，它是胃中的消化酶，由于胃中含有盐酸，它在pH 2时活性最高。

Extreme pH values, either very acidic or very alkaline, cause denaturation in the same way as high temperature: the active site loses its shape and the enzyme stops working.

极端pH值，无论过酸还是过碱，都会像高温一样导致变性：活性位点失去形状，酶停止工作。

Enzyme 酶	Optimum pH 最适pH	Location 位置
Pepsin 胃蛋白酶	2	Stomach 胃
Amylase 淀粉酶	7	Saliva / Pancreas 唾液 / 胰脏
Catalase 过氧化氢酶	7	Cells 细胞

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5. Enzyme Concentration | 酶浓度

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