📚 Enzymes: Structure and Function | 酶：结构与功能
Enzymes are biological catalysts that speed up chemical reactions in living organisms without being used up themselves. They are essential for digestion, respiration, and many metabolic pathways studied in the Edexcel IGCSE Science course.
酶是生物催化剂，能在生物体内加速化学反应，而自身不会被消耗。它们对消化、呼吸以及 Edexcel IGCSE 科学课程中研究的许多代谢途径至关重要。
1. What Are Enzymes? | 什么是酶？
Enzymes are proteins made of long chains of amino acids folded into a unique three-dimensional shape. This shape is critical for their function.
酶是由氨基酸长链折叠成独特三维形状的蛋白质。这种形状对其功能至关重要。
They lower the activation energy of reactions, allowing them to proceed faster at body temperature.
它们降低反应的活化能，使其在体温下能更快进行。
They are specific: each enzyme catalyzes only one type of reaction.
它们具有专一性：每种酶只催化一种类型的反应。
They are reusable and are not altered by the reaction they catalyze.
它们可重复使用，且不会被其所催化的反应改变。
2. The Active Site | 活性位点
The active site is a specific region of the enzyme where substrate molecules bind. Its shape is complementary to the substrate.
活性位点是酶上一个特定的区域，底物分子在此结合。其形状与底物互补。
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
酶 + 底物 → 酶-底物复合物 → 酶 + 产物
If the shape of the active site changes, the enzyme loses its catalytic ability. This is known as denaturation.
如果活性位点形状改变，酶就会失去催化能力，这称为变性。
3. Lock and Key vs. Induced Fit | 锁钥模型与诱导契合
The lock and key model suggests that the substrate fits perfectly into the active site like a key into a lock, with no change in shape.
锁钥模型认为底物完美契合活性位点，就像钥匙插入锁中，形状不发生变化。
The induced fit model is more modern: the active site changes shape slightly to enclose the substrate, making the binding stronger and facilitating the reaction.
诱导契合模型更为现代：活性位点会略微改变形状以包裹底物，使结合更牢固并促进反应。
| Model | 模型 | Shape Change | 形状变化 |
| Lock and key | 锁钥 | No change | 无变化 |
| Induced fit | 诱导契合 | Active site changes | 活性位点改变 |
4. Effect of Temperature | 温度的影响
As temperature increases, kinetic energy increases, so molecules collide more often and with more energy, increasing the rate of reaction up to an optimum temperature (usually 37 °C in humans).
随着温度升高，动能增加，分子碰撞更频繁且能量更大，反应速率升高，直至最适温度（人体内通常为 37 °C）。
Above the optimum, the enzyme's bonds break, the active site distorts, and the enzyme becomes denatured.
超过最适温度后，酶的化学键断裂，活性位点变形，酶发生变性。
| Temperature (°C) | 温度（°C） | Effect | 影响 |
| 0–37 | Rate increases | 反应速率加快 |
| 37 | Optimum activity | 活性最高 |
| >45 | Denaturation | 变性失活 |
5. Effect of pH | pH 的影响
Each enzyme has an optimum pH. For example, pepsin works best at pH 2 in the stomach, while amylase works best at pH 7 in the mouth and small intestine.
每种酶都有最适 pH。例如，胃蛋白酶在胃内 pH 2 时活性最高，而唾液淀粉酶在口腔和小肠的 pH 7 时活性最高。
Extreme pH values alter the ionic bonds and hydrogen bonds that maintain the enzyme's shape, leading to denaturation.
极端 pH 会改变维持酶形状的离子键和氢键，导致酶变性。
Optimum pH = pH at which the rate of reaction is maximum | 最适 pH = 反应速率最大时的 pH
6. Effect of Substrate Concentration | 底物浓度的影响
At constant enzyme concentration, increasing substrate concentration increases the rate of reaction initially because more substrate molecules are available to bind to active sites.
在酶浓度不变时，增加底物浓度起初会加快反应速率，因为更多底物分子可供活性位点结合。
However, when all active sites are occupied, the reaction reaches its maximum velocity (Vmax) and adding more substrate has no further effect.
然而，当所有活性位点都被占用时，反应达到最大速度（Vmax），再增加底物也不会提高速率。
This is called saturation. | 这称为饱和。
7. Effect of Enzyme Concentration | 酶浓度的影响
If substrate is in excess, increasing enzyme concentration will proportionally increase the rate of reaction, because there are more active sites available.
当底物过量时，增加酶浓度会成比例地加快反应速率，因为可用的活性位点更多。
This linear relationship continues until other limiting factors, such as substrate supply, become limiting.
这种线性关系会持续到其他限制因素（如底物供应）成为限制因素为止。
This is an important concept in industrial biotechnology where enzyme concentration is controlled.
这是工业生物技术中控制酶浓度的重要概念。
8. Inhibitors | 抑制剂
Inhibitors are substances that reduce enzyme activity. They can be competitive or non-competitive.
抑制剂是降低酶活性的物质，可分为竞争性和非竞争性两类。
Competitive inhibitors: similar shape to the substrate, they compete for the active site. Increasing substrate concentration can overcome their effect.
竞争性抑制剂：形状类似底物，与底物竞争活性位点。增加底物浓度可消除其影响。
Non-competitive inhibitors: bind to another site on the enzyme, changing the active site shape. Increasing substrate concentration does not help.
非竞争性抑制剂：结合在酶的其他位点，改变活性位点形状，增加底物浓度无效。
Heavy metals and temperature can act as non-specific inhibitors by denaturing enzymes.
重金属和温度可通过使酶变性而成为非特异性抑制剂。
9. Uses of Enzymes in Biotechnology | 酶在生物技术中的应用
Enzymes are widely used in industry due to their specificity and efficiency.
酶因其专一性和高效性而广泛用于工业。
Biological detergents contain proteases and lipases to break down protein and fat stains.
生物洗涤剂含蛋白酶和脂肪酶，用于分解蛋白质和脂肪污渍。
In food production, invertase is used to make syrups, and glucose isomerase makes high-fructose corn syrup.
在食品生产中，蔗糖酶用于制造糖浆，葡萄糖异构酶用于生产高果糖玉米糖浆。
Lactase is added to milk to produce lactose-free milk for lactose-intolerant people.
乳糖酶被添加到牛奶中以生产无乳糖牛奶，供乳糖不耐受人群食用。
These enzymes are often immobilised to be reused and to keep the product pure.
这些酶常被固定化以便重复使用并保持产物纯净。
10. Enzymes in Digestion | 消化中的酶
Digestive enzymes are produced by the salivary glands, stomach, pancreas, and small intestine.
消化酶由唾液腺、胃、胰腺和小肠产生。
| Enzyme | 酶 | Substrate | 底物 | Product | 产物 |
| Amylase | 淀粉酶 | Starch | 淀粉 | Maltose | 麦芽糖 |
| Protease | 蛋白酶 | Protein | 蛋白质 | Amino acids | 氨基酸 |
| Lipase | 脂肪酶 | Lipids | 脂肪 | Fatty acids + glycerol | 脂肪酸 + 甘油 |
These enzymes work optimally at body temperature and specific pH conditions maintained by the digestive system.
这些酶在体温和消化系统维持的特定 pH 条件下发挥最佳作用。
Understanding enzyme structure and function is essential for explaining metabolic processes, industrial applications, and diseases caused by enzyme malfunction.
理解酶的结构和功能对于解释代谢过程、工业应用及酶功能障碍引起的疾病至关重要。
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