Enzymes | 酶

📚 Enzymes | 酶

Enzymes are biological catalysts that speed up chemical reactions in living organisms without being used up themselves. They are crucial for life because nearly all metabolic processes in cells require enzymes to occur at a rate fast enough to sustain the organism. This article covers the key concepts of enzymes as required by the CIE IGCSE Biology syllabus, including their structure, mechanism of action, factors affecting activity, and practical applications.

酶是生物催化剂，能加速生物体内的化学反应而自身不被消耗。它们对生命至关重要，因为细胞中几乎所有的代谢过程都需要酶的参与，才能以足够快的速度维持生命活动。本文按 CIE IGCSE 生物教学大纲要求，梳理酶的核心概念，涵盖其结构、作用机制、影响活性的因素及实际应用。

1. Definition and Characteristics of Enzymes | 酶的定义与特征

An enzyme is a protein that functions as a biological catalyst. Catalysts lower the activation energy needed for a reaction to proceed, thereby increasing the reaction rate. Enzymes are highly specific, meaning each enzyme only catalyses one particular reaction or a group of very similar reactions. They remain chemically unchanged at the end of the reaction and can be reused many times.

酶是一种起生物催化剂作用的蛋白质。催化剂能降低反应所需的活化能，从而提高反应速率。酶具有高度专一性，即每种酶只催化一种特定的反应或一组非常相似的反应。反应结束时酶本身不发生化学变化，可被重复使用多次。

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2. Structure and Active Site | 酶的结构与活性位点

Being proteins, enzymes are made of long chains of amino acids folded into a specific three-dimensional shape. This shape creates a special region called the active site, which is complementary in shape and chemical properties to the substrate – the molecule on which the enzyme acts. The precise folding determines the shape of the active site, and even a slight alteration can prevent the enzyme from functioning.

酶的本质是蛋白质，由长链氨基酸折叠成特定的三维结构。这种结构形成一个特殊区域，称为活性位点，其形状和化学性质与底物（酶所作用的分子）互补。精确的折叠决定了活性位点的形状，哪怕微小的改变都可能导致酶失去功能。

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3. Mechanism of Enzyme Action | 酶的作用机制

The generally accepted model for enzyme action is the lock-and-key model. In this model, the substrate fits into the enzyme’s active site like a key into a lock. The enzyme-substrate complex forms, and the reaction takes place rapidly. The products are then released, leaving the enzyme free to bind with another substrate molecule. An extension of this idea is the induced-fit model, where the active site changes shape slightly to accommodate the substrate, but for IGCSE, the lock-and-key model is sufficient.

普遍接受的酶作用模型是锁钥模型。在该模型中，底物像钥匙插入锁孔一样嵌入酶的活性位点，形成酶-底物复合物，反应迅速发生。产物随后被释放，酶则恢复原状，可以与另一个底物分子结合。这一模型的延伸是诱导契合模型，即活性位点略微改变形状以容纳底物，但在 IGCSE 阶段，掌握锁钥模型即可。

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4. Activation Energy and Reaction Rate | 活化能与反应速率

All chemical reactions require a certain amount of energy to get started – this is the activation energy. Enzymes lower the activation energy by providing an alternative reaction pathway that requires less energy. As a result, more substrate molecules have enough energy to react when they collide, and the reaction rate increases dramatically without raising the temperature.

所有化学反应都需要一定的能量才能启动，这就是活化能。酶通过提供一条所需能量更低的替代反应途径来降低活化能。这样一来，更多的底物分子在碰撞时具有足够的能量进行反应，反应速率大幅提高，而无需升高温度。

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5. Effect of Temperature on Enzyme Activity | 温度对酶活性的影响

As temperature rises, enzyme activity increases because the kinetic energy of molecules increases, leading to more frequent and energetic collisions between enzyme and substrate. However, this only continues up to the optimum temperature (around 37 °C for many human enzymes). Beyond this, the enzyme molecules vibrate too violently, breaking the weak bonds that maintain the three-dimensional shape. The active site becomes denatured and the enzyme loses its catalytic function irreversibly.

温度升高时，酶活性随之增强，因为分子动能增加，酶与底物之间的碰撞更频繁、更有力。但这种趋势只持续到最适温度（对人类许多酶而言约为 37 °C）。超过最适温度后，酶分子剧烈振动，破坏了维持三维结构的弱键。活性位点变性，酶不可逆地丧失催化功能。

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6. Effect of pH on Enzyme Activity | pH 对酶活性的影响

Each enzyme has an optimum pH at which it works best. The pH affects the charges on the amino acid side chains that make up the active site. If the pH moves too far from the optimum, the active site’s shape and chemical properties are altered, reducing enzyme activity. Extreme pH values cause denaturation, which may be irreversible. For example, pepsin in the stomach works best at pH 2, while trypsin in the small intestine has an optimum around pH 8.

每种酶都有其工作最适 pH 值。pH 影响构成活性位点的氨基酸侧链所带电荷。如果 pH 偏离最适值过多，活性位点的形状和化学性质就会改变，酶活性下降。极端的 pH 值会导致变性，且可能是不可逆的。例如，胃中的胃蛋白酶在 pH 2 时活性最高，而小肠中的胰蛋白酶最适 pH 约为 8。

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7. Enzyme Inhibitors | 酶的抑制剂

Substances that reduce or stop enzyme activity are called inhibitors. Competitive inhibitors have a shape similar to the substrate and bind to the active site, blocking the actual substrate from binding. Their effect can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to a different part of the enzyme (allosteric site), changing the shape of the active site so the substrate can no longer bind. This effect is not reversed by adding more substrate. Inhibitors are important in regulating metabolic pathways and are also the basis of many drugs and poisons.

能降低或终止酶活性的物质称为抑制剂。竞争性抑制剂的形状与底物相似，会占据活性位点，阻止真正的底物结合；其作用可通过增加底物浓度来克服。非竞争性抑制剂结合在酶的其他部位（别构位点），改变活性位点形状，使底物无法结合；增加底物浓度无法逆转此效应。抑制剂在调节代谢途径中起重要作用，也是许多药物和毒物的作用原理。

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8. Cofactors and Coenzymes | 辅因子与辅酶

Some enzymes require additional non-protein helpers to function properly. Inorganic cofactors include metal ions like iron (Fe²⁺), magnesium (Mg²⁺), and zinc (Zn²⁺). Organic cofactors are called coenzymes; many are derived from vitamins. For example, NAD (nicotinamide adenine dinucleotide) is a coenzyme that carries hydrogen atoms in respiration. These helpers may be tightly bound to the enzyme (prosthetic groups) or may associate loosely during the reaction.

某些酶需要额外的非蛋白质辅助因子才能正常发挥作用。无机辅因子包括铁 (Fe²⁺)、镁 (Mg²⁺)、锌 (Zn²⁺) 等金属离子。有机辅因子称为辅酶，许多源自维生素。例如，NAD（烟酰胺腺嘌呤二核苷酸）是在呼吸作用中传递氢原子的辅酶。这些辅助因子可能紧密结合在酶上（辅基），也可能在反应时临时结合。

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9. Enzymes in Digestion | 酶在消化中的作用

Digestive enzymes break down large insoluble food molecules into smaller, soluble ones that can be absorbed into the blood. Amylase, produced in the salivary glands and pancreas, breaks down starch into maltose. Proteases, such as pepsin in the stomach and trypsin from the pancreas, break down proteins into peptides and amino acids. Lipase, secreted by the pancreas, digests fats into fatty acids and glycerol. The specificity of each enzyme ensures that digestion proceeds in an orderly manner along the alimentary canal.

消化酶将大分子不溶性食物分解为可吸收的小分子可溶性物质。唾液腺和胰腺产生的淀粉酶将淀粉分解为麦芽糖。蛋白酶如胃中的胃蛋白酶和胰脏分泌的胰蛋白酶，将蛋白质分解为肽和氨基酸。胰腺分泌的脂肪酶则将脂肪消化为脂肪酸和甘油。每种酶的专一性保证了消化过程在消化道中有序进行。

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10. Enzymes in Biological Washing Powders | 酶在生物洗衣粉中的应用

Many modern washing powders contain enzymes, such as proteases and lipases, to remove stains like blood, egg, and grease. These enzymes work at relatively low temperatures (30–40 °C), which saves energy. However, care must be taken because some people may develop allergies if enzyme dust is inhaled, and silk or wool (which are proteins) can be damaged by proteases. This example illustrates how understanding enzyme properties leads to practical industrial applications.

许多现代洗衣粉含有蛋白酶和脂肪酶等酶，用于去除血渍、蛋渍和油脂等污渍。这些酶在较低温度（30–40 °C）下发挥作用，从而节约能源。但需注意，一些人吸入酶粉尘可能引发过敏，而丝绸或羊毛（由蛋白质构成）可能被蛋白酶损坏。这一例子说明了理解酶的性质如何推动实际工业应用。

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11. Investigation of Enzyme Activity | 酶活性的实验研究

A typical IGCSE experiment investigates the effect of temperature on amylase activity by mixing starch solution with amylase at different temperatures and testing samples with iodine solution at regular intervals. The time taken for the iodine to stop turning blue-black indicates the rate of starch breakdown. Variables such as pH, substrate concentration, and enzyme concentration must be controlled. Students should be able to describe how to plan such an investigation, identify independent, dependent, and control variables, and interpret results to determine the optimum temperature.

一个典型的 IGCSE 实验是研究温度对淀粉酶活性的影响：将淀粉溶液与淀粉酶在不同温度下混合，定时取样并用碘液检测。碘液不再变为蓝黑色的时间长短反映淀粉分解的速率。pH、底物浓度和酶浓度等变量必须加以控制。学生应能描述如何设计此类实验，识别自变量、因变量和控制变量，并能分析结果以确定最适温度。

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12. Summary and Key Points for Revision | 复习要点总结

Enzymes are proteins that catalyse metabolic reactions by lowering activation energy. They are specific due to the unique shape of their active site, explained by the lock-and-key model. Temperature and pH affect enzyme activity, with extremes causing denaturation. Inhibitors can reduce activity competitively or non-competitively. Digestive enzymes illustrate biological roles, while biological washing powders show applied uses. Designing controlled experiments to measure enzyme activity is a core practical skill. Mastering these concepts will support success in the IGCSE Biology exam.

酶是蛋白质，通过降低活化能催化代谢反应。由于活性位点独特形状而具有专一性，可用锁钥模型解释。温度和 pH 影响酶活性，极端条件可导致变性。抑制剂通过竞争或非竞争方式降低活性。消化酶体现了酶的生物学功能，生物洗衣粉则展示了其实际应用。设计对照实验测定酶活性是核心实验技能。掌握这些概念将有助于在 IGCSE 生物考试中取得成功。

Published by TutorHao | Biology Revision Series | aleveler.com

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