📚 Enzymes: Structure, Function and Applications | 酶：结构、功能与应用
Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed in the process. This article explores the essential concepts of enzyme structure, activity, and practical uses required for the IGCSE Edexcel Science syllabus.
酶是在生物体内加速化学反应而不被消耗的生物催化剂。本文探讨IGCSE Edexcel科学课程中关于酶的结构、活性及实际应用的核心概念。
1. What Are Enzymes? | 什么是酶？
Enzymes are proteins that act as biological catalysts, greatly increasing the rate of chemical reactions by lowering the activation energy.
酶是作为生物催化剂的蛋白质，通过降低活化能来显著提高化学反应速率。
Each enzyme is specific to a particular reaction, and the enzyme itself is not changed or used up at the end of the reaction.
每种酶对特定反应具有专一性，且酶本身在反应结束后不会改变或被消耗。
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
2. Structure of Enzymes | 酶的结构
Enzymes are globular proteins, meaning their polypeptide chains are folded into a compact, roughly spherical shape.
酶是球状蛋白质，其多肽链折叠成紧凑的近似球形形状。
The order of amino acids in the chain determines how the protein folds, giving each enzyme a unique three-dimensional shape.
链中氨基酸的排列顺序决定了蛋白质的折叠方式，使每种酶具有独特的三维形状。
This folded structure contains a special region called the active site, which is essential for catalysis.
这种折叠结构包含一个称为活性位点的特殊区域，对催化至关重要。
3. The Active Site and Specificity | 活性位点与专一性
The active site is the part of the enzyme where the substrate molecule binds and where the chemical reaction takes place.
活性位点是酶分子与底物结合并发生化学反应的区域。
The shapes of the active site and the substrate are complementary, so only substrates with a matching shape can bind.
活性位点与底物的形状互补，因此只有形状匹配的底物才能结合。
When the substrate binds, an enzyme-substrate complex is formed, after which the products are released and the enzyme is free again.
当底物结合时，形成酶-底物复合物，随后产物释放，酶再次恢复自由。
4. Lock and Key vs Induced Fit | 锁钥模型与诱导契合
Two models explain how enzymes interact with substrates: the lock and key model and the induced fit model.
两种模型解释酶与底物的相互作用：锁钥模型和诱导契合模型。
In the lock and key model, the active site has a rigid shape that exactly matches the substrate, like a key fitting into a lock.
在锁钥模型中，活性位点具有刚性的形状，与底物完全匹配，如同钥匙插入锁孔。
In the induced fit model, the active site is flexible and changes shape slightly when the substrate binds, improving the fit and enabling catalysis.
在诱导契合模型中，活性位点具有柔性，当底物结合时会轻微改变形状，以更好地匹配并促进催化。
| Model | Key Idea |
| Lock and Key | Active site shape is fixed; substrate fits exactly. |
| Induced Fit | Active site changes shape to fit substrate. |
The induced fit model is now more widely accepted because it explains how enzymes can stabilise the transition state of a reaction.
诱导契合模型目前被更广泛接受，因为它能解释酶如何稳定反应的过渡态。
5. Effect of Temperature | 温度的影响
Temperature affects enzyme activity by changing the kinetic energy of molecules. As temperature rises, molecules move faster and collide with the active site more frequently.
温度通过改变分子的动能影响酶活性。温度升高时，分子运动加快，与活性位点的碰撞更加频繁。
The rate of reaction increases with temperature up to an optimum point, after which it rapidly decreases.
反应速率随温度升高而增大，直至达到最适温度，之后迅速下降。
For most human enzymes, the optimum temperature is around 37°C, which is body temperature. Above 45°C, enzymes usually become denatured.
大多数人体酶的最适温度约为37°C，即体温。超过45°C时，酶通常会发生变性。
Denaturation means the protein’s three-dimensional structure is permanently changed, especially the active site, so the enzyme can no longer function.
变性意味着蛋白质的三维结构被永久改变，尤其是活性位点，导致酶无法再发挥功能。
6. Effect of pH | pH的影响
Each enzyme has an optimum pH at which its activity is highest. For example, pepsin in the stomach works best at pH 2, while amylase works best at pH 7.
每种酶都有其活性最高的最适pH。例如，胃中的胃蛋白酶在pH 2时活性最佳，而淀粉酶在pH 7时活性最佳。
Changes in pH can alter the hydrogen bonds and ionic bonds that maintain the enzyme’s shape. Very high or very low pH values can cause denaturation.
pH的变化会改变维持酶形状的氢键和离子键。过高或过低的pH值会导致变性。
Enzymes are sensitive to pH changes because the charges on amino acid residues depend on the surrounding pH, affecting the active site’s ability to bind the substrate.
酶对pH变化敏感，因为氨基酸残基的电荷取决于周围pH，这会影响活性位点结合底物的能力。
7. Enzyme Concentration and Substrate Concentration | 酶浓度与底物浓度
Increasing the enzyme concentration while keeping substrate constant provides more active sites, so the rate of reaction increases proportionally.
在底物浓度不变的情况下增加酶浓度，意味着提供更多活性位点，因此反应速率按比例增加。
However, once the substrate is fully occupied, further increases in enzyme concentration have no effect because there are not enough substrate molecules.
然而，一旦底物全部被占据，继续增加酶浓度已无效果，因为没有足够的底物分子。
Increasing substrate concentration with a fixed enzyme concentration also increases the rate, but only up to a plateau where all active sites are constantly occupied.
在酶浓度固定时增加底物浓度，也可提高速率，但会达到平台期，此时所有活性位点一直被占据。
8. Inhibitors | 抑制剂
Inhibitors are substances that reduce or stop enzyme activity. They can be competitive or non-competitive.
抑制剂是降低或阻止酶活性的物质，可分为竞争性抑制剂和非竞争性抑制剂。
Competitive inhibitors have a similar shape to the substrate and compete directly for the active site. Their effect can be reduced by increasing substrate concentration.
竞争性抑制剂具有与底物相似的形状，直接与底物竞争活性位点。通过增加底物浓度可以减少其抑制作用。
Non-competitive inhibitors bind to a different part of the enzyme, changing the shape of the active site so the substrate cannot bind. Increasing substrate concentration does not overcome this effect.
非竞争性抑制剂结合在酶的不同部位，改变活性位点形状，使底物无法结合。增加底物浓度无法消除这种抑制作用。
9. Industrial Uses of Enzymes | 酶的工业应用
Enzymes are widely used in industry due to their specificity and efficiency at moderate temperatures and pressures.
酶因其专一性和在温和温度、压力下的高效性而在工业中得到广泛应用。
Proteases and lipases are used in biological washing powders to break down protein and fat stains into smaller soluble molecules.
蛋白酶和脂肪酶用于生物洗衣粉中，将蛋白质和脂肪污渍分解成较小的可溶性分子。
Pectinase is used in fruit juice production to break down pectin, resulting in a clearer juice and higher yield.
果胶酶用于果汁生产，分解果胶，使果汁更清澈并提高产量。
Other examples include the use of lactase to produce lactose-free milk and amylase in baking and brewing to convert starch into sugars.
其他例子包括使用乳糖酶生产无乳糖牛奶，以及在烘焙和酿造中使用淀粉酶将淀粉转化为糖。
10. Enzymes in Digestion | 消化中的酶
Digestion relies on enzymes to break down large food molecules into smaller, soluble molecules that can be absorbed into the blood.
消化依赖于酶将大的食物分子分解成可吸收进入血液的较小可溶性分子。
Amylase breaks down starch into maltose. It is produced in the salivary glands and the pancreas.
淀粉酶将淀粉分解为麦芽糖，由唾液腺和胰腺分泌。
Proteases, such as pepsin and trypsin, break down proteins into amino acids. Pepsin works in the stomach, while trypsin works in the small intestine.
蛋白酶如胃蛋白酶和胰蛋白酶将蛋白质分解为氨基酸。胃蛋白酶在胃中起作用，而胰蛋白酶在小肠中起作用。
Lipase breaks down fats into fatty acids and glycerol, and is produced in the pancreas and small intestine.
脂肪酶将脂肪分解为脂肪酸和甘油，由胰腺和小肠分泌。
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