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

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

Enzymes are biological molecules that speed up chemical reactions in living organisms without being consumed or permanently changed. They are essential for life, regulating everything from digestion to DNA replication.

酶是生物分子，能够在不被消耗或永久改变的情况下加速生物体内的化学反应。它们对生命至关重要，调节着从消化到 DNA 複製的每一个过程。

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1. What Are Enzymes? | 什么是酶？

All enzymes are globular proteins, made of long chains of amino acids folded into a specific three-dimensional shape. This shape is crucial because it creates an active site, a region where substrate molecules bind and react.

所有酶都是球状蛋白质，由长链氨基酸折叠成特定的三维形状。这种形状至关重要，因为它形成了一个活性位点，即底物分子结合并发生反应的结构区域。

• Enzymes act as catalysts, lowering the activation energy required for a reaction.
• They are specific: each enzyme only works with one type of substrate (or a few closely related ones).
• They remain unchanged after the reaction and can be used again.

• 酶作为催化剂，降低反应所需的活化能。
• 它们具有专一性：每种酶通常只与一种底物（或少数几种密切相关的底物）结合。
• 它们在反应后不会改变，可以重复使用。

Enzyme + Substrate → Enzyme–Substrate Complex → Enzyme + Product

酶 + 底物 → 酶–底物复合物 → 酶 + 产物

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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’. In this model, the enzyme is the lock and the substrate is the key. Only the correctly shaped substrate can fit into the active site, triggering the reaction.

为了解释酶的专一性，科学家使用“锁钥模型”。在这个模型中，酶相当于锁，底物相当于钥匙。只有形状匹配的底物才能进入活性位点，从而触发反应。

• The active site has a fixed, rigid shape in this model.
• The substrate must fit perfectly, like a key into a lock.
• If the substrate is the wrong shape, no reaction occurs.

• 在该模型中，活性位点的形状是固定且刚性的。
• 底物必须完美匹配，就像钥匙插入锁中。
• 如果底物形状不对，反应就不会发生。

In reality, some enzymes are better described by the induced-fit model, where the active site changes shape slightly to accommodate the substrate. However, the lock-and-key model is a useful starting point for IGCSE.

实际上，有些酶用“诱导契合模型”描述更准确，即活性位点会稍微改变形状以容纳底物。但对于 IGCSE 而言，锁钥模型是一个很好的起点。

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3. Factors Affecting Enzyme Activity | 影响酶活性的因素

Enzyme-controlled reactions are affected by several factors: temperature, pH, substrate concentration, and enzyme concentration. Changing any of these can increase or decrease the rate of reaction.

酶促反应受多种因素影响：温度、pH、底物浓度和酶浓度。改变这些因素中的任何一个，都能提高或降低反应速率。

Factor	Effect on Rate
Temperature	Increases until optimum, then decreases sharply.
pH	Optimum pH gives maximum rate; deviation reduces activity.
Substrate concentration	Increases rate up to a plateau.
Enzyme concentration	Increases rate up to a plateau (if substrate is limited).

因素	对速率的影响
温度	在达到最适温度前速率上升，之后急剧下降。
pH	最适 pH 下速率最大；偏离后活性降低。
底物浓度	速率随浓度升高而增加，最终达到平台期。
酶浓度	速率随酶浓度升高而增加，最终达到平台期（当底物有限时）。

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4. Temperature and Enzymes | 温度与酶

At low temperatures, molecules move slowly and collisions between enzyme and substrate are rare, so the rate is low. As temperature rises, particles gain kinetic energy, more collisions occur, and the rate increases.

在低温下，分子运动缓慢，酶与底物之间的碰撞很少，因此反应速率较低。随着温度升高，粒子获得动能，碰撞次数增多，速率增加。

At the optimum temperature, usually around 37 °C for human enzymes, the rate reaches its maximum. When temperature exceeds the optimum, bonds holding the enzyme together begin to break, causing the active site to lose its shape. Enzyme activity rapidly decreases, and the enzyme is said to be denatured.

在最适温度下（人体酶通常约为 37 °C），反应速率达到最大值。当温度超过最适温度时，维持酶结构的键开始断裂，导致活性位点失去形状。酶活性迅速下降，此时称酶已变性。

Rate of reaction ↑ with temperature until optimum, then ↓ sharply due to denaturation.

反应速率随温度升高而上升，至最适温度后因变性而急剧下降。

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5. pH and Enzymes | pH 与酶

Each enzyme has an optimum pH at which its activity is highest. For example, pepsin in the stomach works best at pH 2, while trypsin in the small intestine works best at pH 8.5.

每种酶都有其最适 pH，在该 pH 下活性最高。例如，胃中的胃蛋白酶在 pH 2 时作用最佳，而小肠中的胰蛋白酶在 pH 8.5 时作用最佳。

If the pH is too high or too low, the hydrogen bond structure of the enzyme becomes disrupted, changing the shape of the active site. This leads to denaturation, just as with high temperature.

如果 pH 过高或过低，酶的氢键结构会被破坏，从而改变活性位点的形状。这同样会导致变性，与高温导致的情况相似。

• Most enzymes in the human body work best at neutral pH (around 7).
• Enzymes in the stomach have acidic optima; enzymes in the intestine have alkaline optima.

• 人体中大多数酶在中性 pH（约 7）下作用最佳。
• 胃中的酶具有酸性最适 pH；小肠中的酶具有碱性最适 pH。

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6. Substrate and Enzyme Concentration | 底物浓度和酶浓度

As substrate concentration increases, the rate of reaction increases because more substrate molecules are available to bind to active sites. However, once all active sites are occupied, the reaction reaches its maximum rate (Vmax). Adding more substrate will not increase the rate further.

随着底物浓度升高，反应速率增加，因为更多底物分子可以结合到活性位点。然而，当所有活性位点都被占据后，反应达到最大速率（Vmax）。再增加底物不会进一步提高速率。

Similarly, when substrate is in excess, increasing enzyme concentration proportionally increases the rate of reaction. More enzymes mean more active sites available to process substrate.

类似地，当底物过量时，增加酶浓度会使反应速率按比例提高。酶越多，可处理底物的活性位点就越多。

Rate ∝ enzyme concentration (when substrate is in excess). Rate rises with substrate concentration until Vmax.

速率与酶浓度成正比（当底物过量时）。速率随底物浓度升高直至 Vmax。

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7. Enzymes and Metabolism | 酶与代谢

Metabolism is the sum of all enzyme-controlled reactions in an organism. These reactions are classified into two types: catabolic and anabolic.

代谢是生物体内所有酶促反应的总和。这些反应分为两类：分解代谢和合成代谢。

• Catabolic reactions break down large molecules into smaller ones, releasing energy. Example: respiration breaks down glucose into CO₂ and H₂O.
• Anabolic reactions build large molecules from smaller ones, using energy. Example: photosynthesis builds glucose from CO₂ and H₂O.

• 分解代谢反应将大分子分解为小分子，释放能量。例如：呼吸作用将葡萄糖分解为 CO₂ 和 H₂O。
• 合成代谢反应利用能量从小分子合成大分子。例如：光合作用从 CO₂ 和 H₂O 合成葡萄糖。

Enzymes control every step of these pathways, ensuring that reactions occur quickly at normal body temperatures.

酶控制着这些代谢途径的每一步，确保反应在正常体温下快速进行。

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8. Examples of Enzymes | 酶的实例

Several enzymes are frequently studied in IGCSE Biology. recognising them and their substrates/products is essential.

在 IGCSE 生物中，有一些酶是经常考查的。识别它们及其底物/产物至关重要。

Enzyme	Substrate	Product
Amylase	Starch	Maltose
Protease	Protein	Amino acids
Lipase	Fats (lipids)	Fatty acids + glycerol
Catalase	Hydrogen peroxide	Water + oxygen

酶	底物	产物
淀粉酶	淀粉	麦芽糖
蛋白酶	蛋白质	氨基酸
脂肪酶	脂肪（脂质）	脂肪酸 + 甘油
过氧化氢酶	过氧化氢	水 + 氧气

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9. Uses of Enzymes in Biotechnology | 酶在生物技术中的应用

Enzymes are widely used in industry, medicine, and everyday products due to their efficiency and specificity.

由于酶的高效性和专一性，它们在工业、医学和日常产品中得到广泛应用。

• Biological detergents contain proteases and lipases to break down protein and fat stains at low temperatures.
• Food industry uses invertase to make syrup, and amylase to clarify fruit juices.
• Medicine uses enzymes such as streptokinase to dissolve blood clots.

• 生物洗涤剂含有蛋白酶和脂肪酶，可在低温下分解蛋白质和脂肪污渍。
• 食品工业使用蔗糖酶制造糖浆，使用淀粉酶澄清果汁。
• 医学使用链激酶等酶溶解血栓。

In IGCSE, you should also be able to describe a simple practical experiment investigating the effect of temperature or pH on enzyme activity, using iodine solution or a pH probe.

在 IGCSE 中，你还应该能够描述一个简单实验，探究温度或 pH 对酶活性的影响，例如使用碘液或 pH 探针来监测反应进程。

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Published by TutorHao | Biology Revision Series | aleveler.com

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