📚 Enzymes and Biological Reactions | 酶与生物反应
Enzymes are globular proteins that act as biological catalysts, speeding up metabolic reactions in living organisms without being used up themselves. In the IGCSE Edexcel Biology specification, understanding how enzymes work, the factors affecting their activity, and their roles in processes such as digestion is essential. This article explains the lock‑and‑key model, the effects of temperature, pH and substrate concentration, and how enzymes are involved in key digestive reactions.
酶是球状蛋白质，作为生物催化剂，加快生物体内的代谢反应而自身不被消耗。在 IGCSE Edexcel 生物大纲中，理解酶的作用机制、影响酶活性的因素以及酶在消化等过程中的作用是至关重要的。本文将解释锁钥模型，温度、pH 和底物浓度的影响，以及酶如何参与关键的消化反应。
1. What Are Enzymes? | 什么是酶？
Enzymes are large protein molecules made up of long chains of amino acids folded into a specific three‑dimensional shape. This shape determines the enzyme's function. All enzymes are biological catalysts, meaning they increase the rate of a chemical reaction inside or outside cells without being chemically changed or used up. Even a small amount of enzyme can catalyse the conversion of many substrate molecules into products. Enzymes are highly specific; each enzyme usually catalyses only one type of reaction or acts on a particular substrate.
酶是由氨基酸长链折叠成特定三维形状的大分子蛋白质。这种形状决定了酶的功能。所有的酶都是生物催化剂，这意味着它们能够加快细胞内或细胞外的化学反应速率，而自身不发生化学变化或被消耗。即使极少量的酶也能催化大量底物分子转化为产物。酶具有高度专一性；每种酶通常只催化一种类型的反应或作用于特定的底物。
Enzymes occur in all living organisms and are essential for life. They control processes such as respiration, photosynthesis, protein synthesis and digestion. Without enzymes, these reactions would occur too slowly to sustain life. The names of many enzymes end in '‑ase' and are often derived from the substrate they act on, for example, lipase acts on lipids, and amylase acts on amylose (starch).
酶存在于所有生物体中，对生命至关重要。它们控制着呼吸作用、光合作用、蛋白质合成和消化等过程。如果没有酶，这些反应进行得极慢，不足以维持生命。许多酶的名称以“‑ase”结尾，通常来源于它们所作用的底物，例如脂肪酶作用于脂类，淀粉酶作用于淀粉。
2. Enzymes as Biological Catalysts | 作为生物催化剂的酶
A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Enzymes are biological catalysts because they are produced by living cells. They reduce the amount of activation energy required for a reaction to take place. Activation energy is the minimum energy that reacting particles must possess for a successful collision leading to a reaction. Enzymes provide an alternative reaction pathway with a lower activation energy, so more substrate molecules have enough energy to react at a given temperature. This is why enzyme‑catalysed reactions are much faster than uncatalysed ones.
催化剂是一种能增加化学反应速率而自身不发生永久化学变化的物质。酶之所以被称为生物催化剂，是因为它们由活细胞产生。酶能降低反应发生所需的活化能。活化能是反应粒子发生有效碰撞并导致反应所必须拥有的最低能量。酶提供了一条活化能较低的替代反应途径，因此在给定温度下，具有足够能量进行反应的底物分子数量增多。这就是酶催化反应比非催化反应快得多的原因。
It is important to remember that enzymes do not change the overall energy change (ΔH) of a reaction, nor do they alter the equilibrium position. They only make it easier for the reaction to get started. After the reaction, the enzyme is released unchanged and can be reused to catalyse another reaction cycle with a new substrate molecule.
必须记住，酶不会改变反应的总能量变化 (ΔH)，也不会改变平衡位置。它们只是使反应更容易启动。反应结束后，酶被释放出来，没有发生改变，可以重新用于催化新底物分子的另一个反应循环。
3. The Lock‑and‑Key Model | 锁钥模型
The lock‑and‑key model is a widely accepted explanation for enzyme action. In this model, the enzyme’s active site has a precise shape that is complementary to the shape of the substrate molecule. The substrate fits into the active site like a key fits into a specific lock. The combination forms an enzyme‑substrate complex. Once the reaction occurs, the product or products are released, and the active site remains unchanged, ready to accept another substrate molecule.
锁钥模型是对酶作用机制的一种被广泛接受的解释。在该模型中，酶的活性位点具有精确的形状，与底物分子的形状互补。底物嵌入活性位点，就像钥匙插入特定的锁一样。两者结合形成酶‑底物复合物。一旦发生反应，产物被释放出来，活性位点保持不变，随时准备接受另一个底物分子。
The lock‑and‑key model helps explain why enzymes are so specific. A key will only fit one lock; similarly, a substrate will only fit the active site of one particular enzyme. If the shape of the substrate does not match the active site, the reaction cannot be catalysed. Any change to the shape of the active site, for example due to high temperature or extreme pH, will stop the enzyme from working because the substrate can no longer bind.
锁钥模型有助于解释酶为何如此专一。一把钥匙只能打开一把锁；同样，一种底物只能嵌入特定酶的活性位点。如果底物的形状与活性位点不匹配，反应便无法被催化。活性位点形状的任何改变，例如高温或极端 pH 所致，都会使酶停止工作，因为底物不能再与之结合。
4. The Active Site and Substrate Specificity | 活性位点与底物专一性
The active site of an enzyme is a small region, often a groove or pocket, formed by the folding of the polypeptide chain. The specific arrangement of amino acid residues in this region creates a unique chemical environment that binds the substrate and lowers the activation energy. The binding involves temporary bonds such as hydrogen bonds and ionic interactions. The shape and chemical properties of the active site determine which substrate the enzyme can bind, a property known as substrate specificity.
酶的活性位点是由多肽链折叠形成的一个小区域，通常是一个凹槽或口袋。该区域氨基酸残基的特殊排列创造了独特的化学环境，用于结合底物并降低活化能。这种结合涉及氢键和离子相互作用等暂时的键。活性位点的形状和化学性质决定了酶能够结合哪种底物，这一特性被称为底物专一性。
Substrate specificity means that a particular enzyme will only catalyse reactions involving a substrate with a complementary shape. For example, maltase catalyses the breakdown of maltose into glucose but cannot act on sucrose because sucrose has a different molecular shape. The 'induced‑fit' model, an extension of the lock‑and‑key concept, suggests that the active site can change shape slightly as the substrate binds, ensuring an even tighter fit.
底物专一性意味着某种酶只会催化具有互补形状的底物参与的反应。例如，麦芽糖酶催化麦芽糖分解为葡萄糖，但不能作用于蔗糖，因为蔗糖的分子形状不同。作为对锁钥概念的扩展，“诱导契合”模型提出，当底物结合时，活性位点可略微改变形状，从而确保更紧密的结合。
5. Effect of Temperature on Enzyme Activity | 温度对酶活性的影响
At low temperatures, enzyme‑catalysed reactions occur slowly because the substrate and enzyme molecules have relatively little kinetic energy, so successful collisions are infrequent. As the temperature increases, the molecules move faster, increasing the frequency of collisions and the likelihood of forming enzyme‑substrate complexes. The rate of reaction typically doubles for every 10°C rise in temperature, up to an optimum point. For most human enzymes, the optimum temperature is around 37°C, close to body temperature.
在低温下，酶催化反应进行缓慢，因为底物和酶分子的动能相对较小，有效碰撞不频繁。随着温度升高，分子运动加快，碰撞频率增加，形成酶‑底物复合物的可能性增大。在达到最适温度之前，温度每升高 10°C，反应速率通常加倍。对大多数人体酶而言，最适温度约为 37°C，接近体温。
Above the optimum temperature, the increased kinetic energy begins to break the hydrogen bonds and other interactions that maintain the precise shape of the enzyme's active site. The active site loses its complementary shape, and the enzyme is denatured. Denaturation is a permanent change, so the enzyme can no longer function. The rate of reaction decreases rapidly beyond the optimum, and at very high temperatures the reaction stops completely.
超过最适温度后，增加的动能开始破坏维持酶活性位点精确形状的氢键和其他相互作用。活性位点失去互补形状，酶发生变性。变性是一种不可逆的变化，酶因而无法再发挥作用。超过最适温度后，反应速率急剧下降，而在极高温度下反应完全停止。
6. Effect of pH on Enzyme Activity | pH对酶活性的影响
Each enzyme works best within a narrow pH range, known as its optimum pH. The pH affects the ionisation of the amino acid side chains in the enzyme's active site. These charges are crucial for maintaining the shape of the active site and for binding the substrate. If the pH moves away from the optimum, the charges change, altering the shape of the active site and reducing the enzyme's activity. Most human intracellular enzymes have an optimum pH around 7, but some digestive enzymes work best at much more acidic or alkaline pH values.
每种酶都在一个狭窄的 pH 范围内发挥最佳作用，这个范围称为其最适 pH。pH 会影响酶活性位点中氨基酸侧链的电离状态。这些电荷对于维持活性位点的形状以及结合底物至关重要。如果 pH 偏离最适值，电荷会发生变化，从而改变活性位点形状，降低酶活性。大多数人体细胞内酶的最适 pH 约为 7，但某些消化酶在酸性或碱性更强的 pH 下活性最高。
For example, pepsin, an enzyme that breaks down proteins in the stomach, has an optimum pH of around 2, which matches the acidic environment provided by hydrochloric acid. Trypsin, a protease that works in the small intestine, has an optimum pH of about 8, suited to the slightly alkaline conditions there. If an enzyme is placed in a pH far from its optimum, it may become denatured, and this denaturation can be irreversible.
例如，胃蛋白酶在胃中分解蛋白质，其最适 pH 约为 2，这与盐酸提供的酸性环境相适应。胰蛋白酶是在小肠中发挥作用的一种蛋白酶，最适 pH 约为 8，适合那里微碱性的条件。如果将酶置于远离其最适 pH 的环境中，酶可能会变性，且这种变性有可能是不可逆的。
7. Effect of Substrate Concentration | 底物浓度的影响
If the enzyme concentration is kept constant, increasing the substrate concentration initially increases the rate of reaction. This is because more substrate molecules are available to collide with the active sites of enzyme molecules, so more enzyme‑substrate complexes are formed per unit time. However, this increase in rate only continues up to a certain point. Once all the active sites are occupied, the enzyme is said to be saturated.
如果酶浓度保持不变，最初增加底物浓度会提高反应速率。这是因为有更多的底物分子可以与酶分子的活性位点碰撞，从而在单位时间内形成更多的酶‑底物复合物。然而，这种速率的提升只能持续到一定限度。一旦所有活性位点都被占据，酶就达到了饱和状态。
At the saturation point, the rate of reaction reaches a maximum (Vmax) and becomes constant. Adding more substrate does not further increase the rate because there are no free active sites available to bind additional substrate molecules. The rate can only be increased again by raising the enzyme concentration or by altering other factors such as temperature (if not at the optimum). This relationship is important when designing experiments and understanding metabolic control.
在饱和点时，反应速率达到最大值 (Vmax) 并保持恒定。加入更多的底物不会进一步提高速率，因为没有空闲的活性位点可以与额外的底物分子结合。只有通过增加酶浓度或改变其他因素（如温度，如果尚未达到最适温度）才能再次提高速率。在设计实验和理解代谢调控时，这一关系非常重要。
8. Enzyme Denaturation | 酶的变性
Denaturation is the permanent and irreversible loss of an enzyme's three‑dimensional shape, leading to loss of catalytic function. It occurs when the bonds that hold the protein in its specific conformation are broken. High temperatures and extreme pH values are common causes of denaturation. In the denatured enzyme, the active site is no longer complementary to the substrate, so the enzyme‑substrate complex cannot form.
变性是指酶三维形状的永久性、不可逆的丧失，从而导致催化功能丧失。当维持蛋白质特定构象的键被破坏时，就会发生变性。高温和极端 pH 是变性的常见原因。在变性的酶中，活性位点不再与底物互补，因此无法形成酶‑底物复合物。
It is important to know that denaturation is not the same as simply reducing enzyme activity by lowering temperature. A cooled enzyme will work again when warmed, but a denatured enzyme will not. In industrial processes and medical applications, enzyme denaturation must be avoided to maintain activity; for example, biological washing powders contain enzymes that must work at moderate washing temperatures.
需要了解的是，变性与仅仅通过降低温度来减小酶活性不同。冷却的酶在升温后会重新工作，但变性的酶不会。在工业过程和医疗应用中，必须避免酶变性以保持活性；例如，生物洗衣粉中所含的酶需要在适中的洗涤温度下发挥作用。
9. Enzyme Examples: Digestive Enzymes | 酶的例子：消化酶
Digestive enzymes are produced by specialised cells in the digestive system and are secreted into the gut to break down large, insoluble food molecules into smaller, soluble molecules that can be absorbed into the blood. The three main types of digestive enzymes are amylases, proteases and lipases. Each type acts on a specific nutrient.
消化酶由消化系统中的特化细胞产生，分泌到肠道中，将不溶性大分子食物分解成可被吸收进入血液的小分子可溶性物质。消化酶主要分三类：淀粉酶、蛋白酶和脂肪酶。每种酶作用于特定的营养素。
| Enzyme Type / 酶类型 | Substrate / 底物 | Products / 产物 | Site of Action / 作用部位 |
|---|---|---|---|
| Amylase / 淀粉酶 | Starch / 淀粉 | Maltose / 麦芽糖 | Mouth, small intestine / 口腔、小肠 |
| Protease (e.g. pepsin, trypsin) / 蛋白酶（如胃蛋白酶、胰蛋白酶） | Protein / 蛋白质 | Amino acids / 氨基酸 | Stomach, small intestine / 胃、小肠 |
| Lipase / 脂肪酶 | Lipids (fats and oils) / 脂类（脂肪与油） | Glycerol and fatty acids / 甘油和脂肪酸 | Small intestine / 小肠 |
Bile, produced by the liver and stored in the gall bladder, aids fat digestion by emulsifying large fat droplets into smaller ones, increasing the surface area for lipase to act. This is a mechanical process rather than enzymatic, but it greatly speeds up lipase action.
肝脏分泌并储存在胆囊中的胆汁通过将大脂肪滴乳化成较小的脂肪滴，增大了脂肪酶作用的表面积，从而协助脂肪的消化。这是一个物理过程而非酶促过程，但它大大加速了脂肪酶的作用。
10. Practical Investigation: Amylase and Starch | 实验：淀粉酶与淀粉
To investigate the effect of temperature or pH on enzyme activity, a common practical uses amylase and starch solution. A few drops of amylase solution are added to a starch suspension at a controlled temperature. Samples are withdrawn at regular intervals and mixed with iodine solution on a spotting tile. A blue‑black colour indicates the presence of starch. The time taken for the iodine to remain orange‑brown (showing that all starch has been broken down) is recorded. The shorter the time, the faster the rate of reaction.
为探究温度或 pH 对酶活性的影响，常用的实验方法是使用淀粉酶和淀粉溶液。在受控温度下，向淀粉悬液中加入几滴淀粉酶溶液。定时取样，并在点滴板上与碘液混合。蓝黑色表示淀粉存在。记录碘液保持橙棕色（表明所有淀粉已被分解）所需的时间。时间越短，反应速率越快。
When testing the effect of temperature, water baths at different temperatures (e.g. 20°C, 30°C, 40°C, 50°C, 60°C) can be used. The rate typically increases up to an optimum around 40°C and then falls as the enzyme denatures. It is essential to control variables such as pH, enzyme concentration and substrate concentration to ensure a fair test. A graph of reaction rate against temperature can be plotted, showing a characteristic bell‑shaped curve peaking at the optimum.
在探究温度的影响时，可使用不同温度（如 20°C、30°C、40°C、50°C、60°C）的水浴。反应速率通常在 40°C 左右达到最佳，然后随着酶变性而下降。必须控制 pH、酶浓度和底物浓度等变量，以确保公平测试。可以绘制反应速率对温度的关系图，其显示出在最适温度处达到峰值的典型钟形曲线。
Similarly, the effect of pH can be investigated using buffer solutions at different pH values, while keeping the temperature constant at the optimum. The results show a narrow optimal pH range, with activity falling sharply on either side.
类似地，可以通过使用不同 pH 的缓冲溶液，同时保持温度在最适值，来探究 pH 的影响。结果表明存在一个狭窄的最适 pH 范围，在此范围两侧活性急剧下降。
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