Enzymes | 酶

📚 Enzymes | 酶

Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed in the process. They are essential for every metabolic pathway, from digestion to DNA replication. This article covers the key concepts you need for your Edexcel IGCSE Biology exam.

酶是生物催化剂，能够在活生物体内加速化学反应，而自身在反应过程中不被消耗。从消化到 DNA 复制，酶对每条代谢途径都至关重要。本文涵盖你在 Edexcel IGCSE 生物考试中需要掌握的核心概念。

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1. What Are Enzymes? | 什么是酶？

Enzymes are globular proteins made of long chains of amino acids folded into a specific three-dimensional shape. Each enzyme has an active site — a unique region where substrate molecules bind. The shape of the active site is complementary to the shape of the substrate, much like a lock and key.

酶是由氨基酸长链折叠成的特定三维形状的球状蛋白质。每种酶都有一个活性位点——一个底物分子结合的独特区域。活性位点的形状与底物的形状互补，就像锁和钥匙一样。

The reaction occurs when the substrate fits into the active site, forming an enzyme-substrate complex. After the reaction, the product(s) are released, and the enzyme remains unchanged and ready to catalyse another reaction.

当底物嵌入活性位点，形成酶-底物复合物时，反应就会发生。反应结束后，产物被释放，酶本身保持不变，可以继续催化下一个反应。

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

The lock and key model is a simple analogy: the enzyme is the lock, the substrate is the key. Only the correct key (substrate) can fit into the lock (active site). This explains why enzymes are highly specific — each enzyme only catalyses one type of reaction.

锁钥模型是一个简单的类比：酶是锁，底物是钥匙。只有正确的钥匙（底物）才能插入锁（活性位点）中。这解释了酶的高度专一性——每种酶只催化一种类型的反应。

The induced fit model is a more refined version. It suggests that when the substrate binds, the active site changes shape slightly to wrap around the substrate, creating a tighter fit. This stress on the substrate bonds lowers the activation energy needed for the reaction.

诱导契合模型是一个更精细的版本。它提出当底物结合时，活性位点会稍微改变形状来包裹底物，形成更紧密的结合。这种对底物化学键的应力降低了反应所需的活化能。

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3. Enzyme Specificity | 酶的专一性

Enzyme specificity exists at several levels. Absolute specificity means the enzyme catalyses only one reaction. For example, catalase only breaks down hydrogen peroxide into water and oxygen.

酶的专一性存在于多个层面。绝对专一性意味着酶只催化一种反应。例如，过氧化氢酶只将过氧化氢分解为水和氧气。

Group specificity means the enzyme acts on substrates with the same functional group. For instance, protease enzymes break peptide bonds in various proteins. Stereo-specificity means the enzyme only recognises one isomer of a molecule, such as only L-amino acids, not D-amino acids.

基团专一性意味着酶作用于具有相同官能团的底物。例如，蛋白酶可以断裂多种蛋白质中的肽键。立体专一性意味着酶只识别分子的一种异构体，例如只识别 L-氨基酸，而不识别 D-氨基酸。

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4. Characteristics of Enzymes | 酶的特性

• Enzymes are not used up in the reaction — they can be reused repeatedly. 酶在反应中不会被消耗——它们可以反复使用。

• Enzymes are highly specific, catalysing only one reaction or a class of reactions. 酶具有高度专一性，只催化一种或一类反应。

• Enzymes are affected by temperature and pH — each has an optimum condition. 酶受温度和 pH 影响——每种酶都有其最适条件。

• Enzymes lower the activation energy of a reaction, making it happen faster. 酶降低了反应的活化能，使反应进行得更快。

• Enzymes are denatured by high temperatures and extreme pH values. 高温和极端 pH 会导致酶变性失活。

These five characteristics form the foundation of enzyme biology and frequently appear in exam questions.

这五个特性构成了酶生物学的基础，也是考试中经常出现的考点。

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5. Temperature | 温度

As temperature increases from low values, molecules move faster. More collisions occur between enzymes and substrates, so the rate of reaction increases. For most human enzymes, the optimum temperature is around 37 °C (body temperature).

当温度从低值升高时，分子运动加快。酶与底物之间的碰撞增多，反应速率随之上升。对大多数人类酶来说，最适温度在 37 °C 左右（体温）。

Above the optimum temperature, the heat energy breaks the hydrogen bonds and other interactions holding the enzyme’s three-dimensional structure. The active site changes shape and the enzyme is denatured — it can no longer bind the substrate. This is irreversible.

超过最适温度后，热能会破坏维持酶三维结构的氢键和其他相互作用。活性位点形状改变，酶发生变性——无法再与底物结合。这一过程是不可逆的。

Rate of reaction ↑ with temperature up to optimum, then ↓ sharply after denaturation.
反应速率随温度升高而上升至最适点，变性后急剧下降。

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6. pH | pH 值

Each enzyme has an optimum pH at which its activity is highest. Most human enzymes work best at neutral pH (around pH 7). However, pepsin — a digestive enzyme in the stomach — has an optimum pH of about 2, matching the acidic environment of the stomach.

每种酶都有一个最适 pH，在此 pH 下活性最高。大多数人类酶在中性 pH（约 pH 7）下活性最强。然而，胃蛋白酶——胃中的一种消化酶——的最适 pH 约为 2，与胃的酸性环境相匹配。

When pH deviates from the optimum, the concentration of H⁺ or OH⁻ ions disrupts the ionic bonds in the enzyme. The active site changes shape, leading to denaturation. Extreme pH values permanently damage the enzyme.

当 pH 偏离最适值时，H⁺ 或 OH⁻ 离子的浓度会破坏酶内的离子键。活性位点形状改变，导致变性。极端的 pH 值会永久性地损伤酶。

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7. Substrate Concentration | 底物浓度

At a fixed enzyme concentration, increasing the substrate concentration increases the rate of reaction. This is because more substrate molecules are available to collide with active sites.

在固定酶浓度下，增加底物浓度会使反应速率升高。这是因为有更多的底物分子可以与活性位点碰撞。

However, this effect reaches a plateau. When all active sites are occupied (the enzyme is saturated), adding more substrate has no effect on the reaction rate. The enzyme is working at its maximum rate (Vₘₐₓ).

然而，这种效应会达到一个平台期。当所有活性位点都被占据（酶被饱和）时，再增加底物不会影响反应速率。酶正以其最大速率（Vₘₐₓ）工作。

Substrate Concentration 底物浓度	Effect on Rate 对速率的影响
Low 低	Rate increases proportionally 速率按比例增加
High 高	Rate plateaus (saturation) 速率达到平台期（饱和）

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8. Enzyme Concentration | 酶浓度

When substrate concentration is in excess (not limiting), increasing the enzyme concentration increases the rate of reaction proportionally. More active sites are available to catalyse the reaction.

当底物浓度过量（不构成限制）时，增加酶浓度会使反应速率按比例增加。更多的活性位点可用于催化反应。

This linear relationship continues until substrate becomes limiting. In practice, the rate cannot increase indefinitely because reactions are limited by the available substrate molecules.

这种线性关系会一直持续到底物成为限制因素为止。实际上，反应速率不可能无限增加，因为反应受可用底物分子数量的限制。

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9. Enzyme Inhibitors | 酶抑制剂

Inhibitors are molecules that reduce or stop enzyme activity. Competitive inhibitors have a similar shape to the substrate and compete for the active site. Increasing substrate concentration can overcome competitive inhibition.

抑制剂是降低或停止酶活性的分子。竞争性抑制剂的形状与底物相似，会竞争活性位点。增加底物浓度可以克服竞争性抑制。

Non-competitive inhibitors bind elsewhere on the enzyme (allosteric site), changing the enzyme’s shape so the active site no longer works. Increasing substrate concentration cannot overcome this type of inhibition.

非竞争性抑制剂结合在酶的其他位置（别构位点），改变酶的形状，使活性位点不再起作用。增加底物浓度无法克服这种类型的抑制。

In IGCSE Biology, you should be able to interpret graphs showing how inhibitors affect reaction rates and explain their biological significance, such as in regulating metabolic pathways.

在 IGCSE 生物中，你应该能够解读显示抑制剂如何影响反应速率的图表，并解释其生物学意义，例如在代谢途径调控中的作用。

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10. Enzymes in Digestion | 消化中的酶

Digestive enzymes are secreted by various glands and organs. Amylase — produced in the salivary glands and pancreas — breaks starch into maltose. Protease (pepsin in the stomach, trypsin in the small intestine) breaks proteins into amino acids. Lipase — produced in the pancreas — breaks lipids (fats) into fatty acids and glycerol.

消化酶由多种腺体和器官分泌。淀粉酶——由唾液腺和胰腺产生——将淀粉分解为麦芽糖。蛋白酶（胃中的胃蛋白酶、小肠中的胰蛋白酶）将蛋白质分解为氨基酸。脂肪酶——由胰腺产生——将脂质（脂肪）分解为脂肪酸和甘油。

These enzymes work optimally at specific pH levels: amylase works best at neutral pH, pepsin at acidic pH, and lipase at slightly alkaline pH in the small intestine. Bile, produced by the liver, emulsifies fats to increase the surface area for lipase action.

这些酶在特定的 pH 下以最适状态工作：淀粉酶在中性 pH 下活性最高，胃蛋白酶在酸性 pH 下工作，而脂肪酶在小肠中微碱性环境下发挥作用。肝脏产生的胆汁可以将脂肪乳化，增加脂肪酶作用的表面积。

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11. Immobilised Enzymes | 固定化酶

Immobilised enzymes are enzymes attached to an inert, insoluble material such as alginate beads or silica gel. This technique has industrial advantages: the enzyme can be reused, the product is not contaminated with the enzyme, and the process can be continuous.

固定化酶是附着在惰性、不溶性材料（如海藻酸盐珠或硅胶）上的酶。这项技术在工业上具有优势：酶可以重复使用，产物不会被酶污染，而且过程可以是连续式的。

A common exam example is the use of immobilised lactase in bioreactors to break down lactose in milk, producing lactose-free milk for people who are lactose intolerant. Other applications include biosensors, medical diagnostics, and the food industry.

一个常见的考试实例是在生物反应器中使用固定化乳糖酶分解牛奶中的乳糖，为乳糖不耐受人群生产无乳糖牛奶。其他应用包括生物传感器、医学诊断和食品工业。

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12. Exam Tips and Common Mistakes | 考试技巧与常见错误

Students often confuse “denatured” with “inactivated”. Denaturation is permanent — the enzyme’s active site is destroyed and the enzyme can never work again. Inactivation due to low temperature or pH is temporary — when conditions return to optimum, enzyme activity returns.

学生经常混淆”变性”和”失活”。变性是永久的——酶的活性位点被破坏，酶再也无法工作。由于低温或 pH 偏离导致的失活是暂时的——当条件恢复到最适状态时，酶活性会恢复。

When drawing graphs, remember: temperature and pH graphs are bell-shaped curves with a single peak. Substrate concentration graphs rise then plateau. Label axes correctly with units — rate is usually measured as “amount of product formed per unit time” or “amount of substrate used per unit time”.

画图时，请记住：温度和 pH 图是钟形曲线，只有一个峰值。底物浓度图先上升后达到平台期。正确标注坐标轴及单位——速率通常表示为”单位时间内生成的产物量”或”单位时间内消耗的底物量”。

Finally, always state that enzymes are biological catalysts made of protein when asked for a definition — this scores full marks. Use the terms “active site”, “enzyme-substrate complex”, and “specific” in descriptive answers to demonstrate understanding.

最后，当被要求下定义时，务必写出酶是由蛋白质组成的生物催化剂——这样可以拿到满分。在描述性答案中使用”活性位点”、”酶-底物复合物”和”专一性”等术语，以展示你的理解深度。

Published by TutorHao | Biology Revision Series | aleveler.com

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