📚 Enzymes: Biological Catalysts | 酶：生物催化剂
Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed in the process. Virtually every metabolic reaction in the body is controlled by a specific enzyme, making them essential for life. From digesting food to synthesising DNA, enzymes lower the activation energy required for reactions, enabling them to occur rapidly at body temperature.
酶是生物催化剂，能够加速生物体内的化学反应而自身不被消耗。人体几乎每一个代谢反应都由特定的酶控制，因而酶对生命至关重要。从消化食物到合成DNA，酶降低了反应所需的活化能，使反应在体温下就能快速进行。
1. Enzyme Structure and Active Site | 酶的结构与活性位点
Enzymes are globular proteins with a unique three‑dimensional shape. A region on the enzyme surface called the active site has a specific shape complementary to the substrate molecule. The active site is formed by a small number of amino acids, and its precise arrangement is critical for catalytic function.
酶是具有独特三维结构的球状蛋白质。酶表面有一个称为活性位点的区域，其形状与底物分子互补。活性位点由少量氨基酸构成，其精确的排列对催化功能至关重要。
If the shape of the active site is altered – for example, by high temperature or extreme pH – the enzyme can no longer bind to its substrate, a condition known as denaturation. Denaturation is often irreversible and destroys enzyme activity.
如果活性位点的形状因高温或极端pH等因素发生改变，酶就无法再与底物结合，这种情况称为变性。变性通常是不可逆的，会破坏酶的活性。
2. Lock‑and‑Key Model | 锁钥模型
The lock‑and‑key model explains enzyme action by imagining the substrate as a key that fits exactly into the enzyme’s active site, which acts as the lock. Once bound, the enzyme–substrate complex forms, and the reaction proceeds, converting substrate into product.
锁钥模型将底物想象成一把钥匙，恰好能插入充当锁的酶活性位点。二者结合后形成酶–底物复合物，反应随之进行，底物转化为产物。
This model emphasises the specificity of enzymes – each enzyme can only catalyse one particular reaction or type of reaction. For example, the enzyme sucrase only acts on sucrose, not on other disaccharides like maltose.
该模型强调了酶的专一性——每种酶只能催化一种特定的反应或反应类型。例如，蔗糖酶只作用于蔗糖，而不作用于麦芽糖等其他二糖。
3. Induced‑Fit Model | 诱导契合模型
A more refined explanation is the induced‑fit model. Instead of a rigid active site, the enzyme’s active site slightly changes shape when the substrate approaches. This conformational change allows an even tighter fit, putting strain on substrate bonds and lowering the activation energy more effectively.
更为精确的解释是诱导契合模型。该模型认为酶的活性位点并非刚性不变，而是当底物靠近时，活性位点形状略微改变。这种构象变化能使结合更紧密，对底物分子键施加张力，更有效地降低活化能。
This model helps explain why some enzymes are able to catalyse reactions involving more than one substrate or why inhibitors can sometimes bind without perfect shape complementarity.
这个模型有助于解释为何某些酶能催化涉及不止一种底物的反应，或抑制剂为何有时即使形状不完全互补也能结合。
4. Factors Affecting Enzyme Activity: Temperature | 影响酶活性的因素：温度
As temperature increases, kinetic energy of molecules rises, leading to more frequent collisions between enzyme and substrate. Consequently, the rate of reaction increases up to an optimum temperature, typically around 37 °C for human enzymes.
温度升高时，分子的动能增加，酶与底物之间的碰撞更加频繁。因此，反应速率会随温度升高而增加，直至达到最适温度——人体酶的最适温度通常在37 °C左右。
Beyond the optimum, the rapidly vibrating molecules disrupt the weak bonds maintaining the enzyme’s tertiary structure. The active site loses its shape, the enzyme denatures, and the reaction rate falls steeply.
超过最适温度后，剧烈振动的分子会破坏维持酶三级结构的弱键。活性位点形状丧失，酶变性，反应速率急剧下降。
We can summarise the temperature effect in a simple table:
| Temperature Range | Observation |
| 0–10 °C | Very low activity; molecules move slowly |
| 20–37 °C | Increasing activity; more collisions |
| 37 °C (optimum) | Maximum rate |
| Above 50 °C | Rapid denaturation; activity lost |
温度范围总结如下：
| 温度范围 | 观察结果 |
| 0–10 °C | 活性极低；分子运动缓慢 |
| 20–37 °C | 活性升高；碰撞次数增加 |
| 37 °C (最适) | 最大速率 |
| 超过50 °C | 快速变性；活性丧失 |
5. Factors Affecting Enzyme Activity: pH | 影响酶活性的因素：pH
Each enzyme has an optimum pH at which it functions most efficiently. The charges on acidic and basic amino acid side chains in the active site depend on the hydrogen ion concentration. Deviating from the optimum pH disrupts these charges, alters ionic bonds, and changes the shape of the active site, leading to denaturation.
每种酶都有其最适pH，在此pH下酶的功能最强。活性位点中酸性及碱性氨基酸侧链上的电荷取决于氢离子浓度。偏离最适pH会扰乱这些电荷，破坏离子键，改变活性位点形状，从而导致变性。
For example, pepsin, a digestive enzyme in the stomach, works best at pH 2, whereas trypsin, operating in the small intestine, has an optimum pH around 8. This ensures that different enzymes are active in their specific locations.
例如，胃蛋白酶在胃中发挥消化作用，最适pH为2；而胰蛋白酶在小肠中工作，最适pH约为8。这保证了不同的酶在各自特定环境中保持活性。
6. Substrate Concentration and Saturation | 底物浓度与饱和效应
At low substrate concentrations, the active sites of enzyme molecules are mostly empty, so increasing substrate concentration increases the rate of reaction proportionally. As more substrate is added, active sites become occupied, and the rate of increase slows down.
在低底物浓度下，酶分子的活性位点大部分空缺，因此增加底物浓度会使反应速率成比例上升。随着更多底物加入，活性位点逐渐被占据，速率增长开始放缓。
Eventually, when all active sites are occupied, the enzyme is saturated. Adding further substrate does not increase the rate, as the maximum turnover number per enzyme molecule has been reached. The reaction rate reaches a plateau known as Vmax.
最终，当所有活性位点都被占据时，酶达到饱和。继续增加底物浓度不会提高速率，因为每个酶分子的周转数已达到上限。反应速率达到一个平台值，称为Vmax。
7. Enzyme Inhibitors | 酶抑制剂
Inhibitors are molecules that reduce enzyme activity. They can be classified as competitive or non‑competitive. Competitive inhibitors have a shape similar to the substrate and compete for the active site. Adding more substrate can overcome the inhibition, so Vmax remains unchanged but the apparent affinity (Km) increases.
抑制剂是降低酶活性的分子，可分为竞争性抑制剂和非竞争性抑制剂。竞争性抑制剂形状与底物相似，会竞争结合活性位点。增加底物浓度可以克服抑制，因此Vmax不变，但表观亲和力(Km)增大。
Non‑competitive inhibitors bind to a different site on the enzyme (allosteric site), altering the shape of the active site so the substrate can no longer bind. In this case, Vmax decreases because the number of functional enzyme molecules is reduced, while Km may stay the same.
非竞争性抑制剂与酶上的另一个位点（别构位点）结合，改变活性位点形状，使底物无法结合。这种情况下，Vmax降低，因为有功能的酶分子数量减少了，而Km可能保持不变。
8. Enzyme Specificity and Metabolic Pathways | 酶专一性与代谢途径
Enzymes exhibit absolute specificity, meaning each enzyme catalyses only one specific reaction. This ensures order in metabolism: cells can control pathways by regulating the activity of key enzymes. Enzyme specificity arises from the precise shape and chemical environment of the active site, which only accommodates one particular substrate or a very limited group of substrates.
酶表现出绝对专一性，即每种酶只催化一个特定的反应。这保证了代谢活动的有序性：细胞可以通过调控关键酶的活性来控制整个代谢途径。酶专一性的根源在于活性位点精确的形状和化学环境，只能容纳一个特定的底物，或极为有限的几个底物。
Metabolic pathways are sequences of enzyme‑catalysed reactions. For instance, the breakdown of glucose during respiration involves a series of enzymes, each performing a single step. This allows fine regulation through feedback inhibition, where the end product inhibits the first committed step enzyme.
代谢途径是一系列酶催化反应的序列。例如，呼吸作用中葡萄糖的分解就涉及一系列酶，每个酶只执行一步反应。这通过反馈抑制实现了精细调节，即终产物抑制第一步关键酶。
9. Enzymes in Digestion | 消化酶
Digestive enzymes break down large, insoluble food molecules into small, soluble ones that can be absorbed into the blood. Amylase, produced by the salivary glands and pancreas, catalyses the hydrolysis of starch into maltose:
Starch → Maltose
消化酶将大分子不溶性食物分解为可被吸收进入血液的小分子可溶性物质。由唾液腺和胰腺分泌的淀粉酶催化淀粉水解为麦芽糖：
淀粉 → 麦芽糖
Proteases, such as pepsin in the stomach and trypsin in the small intestine, hydrolyse proteins into amino acids. Lipases break down lipids into fatty acids and glycerol. The coordinated action of these enzymes ensures efficient digestion.
蛋白酶（如胃中的胃蛋白酶和小肠中的胰蛋白酶）将蛋白质水解为氨基酸。脂肪酶将脂质分解为脂肪酸和甘油。这些酶的协同作用保证了消化过程的高效进行。
10. Industrial Uses of Enzymes | 酶的工业应用
Enzymes are widely employed in industry because they operate at moderate temperatures and pressures, are biodegradable, and catalyse specific reactions with high efficiency. Biological washing powders contain proteases and lipases to remove protein‑based and fatty stains from clothes at low temperatures.
酶在工业中应用广泛，因为它们能在温和的温度和压力下工作，可生物降解，并能高效催化特定反应。生物洗衣粉含有蛋白酶和脂肪酶，能在低温下去除衣物上的蛋白质类和脂肪类污渍。
In food processing, the enzyme pectinase is used to clarify fruit juices by breaking down pectin, while lactase converts lactose into glucose and galactose, producing lactose‑free milk for lactose‑intolerant individuals. In biotechnology, DNA polymerase is essential for the polymerase chain reaction (PCR).
在食品加工中，果胶酶通过分解果胶来澄清果汁；乳糖酶将乳糖转化为葡萄糖和半乳糖，用于生产适合乳糖不耐受人群的零乳糖牛奶。在生物技术中，DNA聚合酶在聚合酶链式反应(PCR)中至关重要。
11. Enzymes and Disease | 酶与疾病
Many diseases are linked to malfunctioning or deficient enzymes. For instance, the genetic disorder phenylketonuria (PKU) results from the absence of the enzyme phenylalanine hydroxylase, leading to a toxic buildup of phenylalanine. Similarly, lactose intolerance arises from insufficient lactase activity in the small intestine.
许多疾病与酶的功能失常或缺乏有关。例如，遗传病苯丙酮尿症(PKU)是由于缺乏苯丙氨酸羟化酶，导致苯丙氨酸有毒积累。同样，乳糖不耐受则是由于小肠中乳糖酶活性不足所致。
Some medicines act as enzyme inhibitors. Statins, used to lower cholesterol, inhibit the enzyme involved in cholesterol synthesis. Aspirin irreversibly inhibits the cyclooxygenase enzyme, reducing inflammation and pain. Understanding enzyme kinetics is therefore crucial for drug design.
有些药物本身就是酶抑制剂。用于降低胆固醇的他汀类药物会抑制参与胆固醇合成的酶。阿司匹林不可逆地抑制环加氧酶，从而减轻炎症和疼痛。因此，理解酶动力学对药物设计至关重要。
12. Investigative Skills: Designing an Enzyme Experiment | 探究技能：设计酶实验
IGCSE students often plan investigations to measure enzyme activity, for example studying the effect of pH on catalase activity using hydrogen peroxide. Variables must be carefully controlled: temperature (use a water bath), substrate concentration, and enzyme volume. The rate can be followed by measuring the volume of oxygen produced per unit time.
IGCSE学生经常设计实验来测量酶活性，例如用过氧化氢研究pH对过氧化氢酶活性的影响。必须仔细控制变量：温度（使用水浴）、底物浓度和酶体积。可以通过测量单位时间产生的氧气体积来跟踪反应速率。
Safe and reliable results depend on repeating measurements and excluding the first few seconds if the reaction mixture is still stabilising. A tangent to the early linear part of the curve yields the initial rate, which serves as a reliable indicator of enzyme activity.
要获得安全可靠的结果，需重复测量，并在反应混合物初始稳定阶段排除头几秒的数据。在曲线的早期线性部分作切线，可求得初始速率，这是衡量酶活性的可靠指标。
Published by TutorHao | IGCSE Biology/Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图