📚 Enzymes: Biological Catalysts | 酶：生物催化剂
Enzymes are globular proteins that act as biological catalysts, speeding up chemical reactions in living organisms without being consumed in the process. This article covers the structure, mechanism, factors affecting activity, inhibition, and applications of enzymes, aligned with the Biology 2.4 Enzymes syllabus.
酶是一类球状蛋白质，作为生物催化剂，能够在不被消耗的情况下加速生物体内的化学反应。本文涵盖酶的结构、作用机制、影响活性的因素、抑制作用及其应用，与生物 2.4 酶的教学大纲保持一致。
1. Introduction to Enzymes | 酶简介
Enzymes are essential for life because most metabolic reactions would occur too slowly at normal body temperatures without them. They lower the activation energy required for a reaction, allowing metabolic pathways to proceed rapidly and under mild conditions.
酶对生命至关重要，因为如果没有酶，大多数代谢反应在正常体温下进行得太慢。酶降低了反应所需的活化能，使代谢途径能够在温和条件下快速进行。
Enzymes are highly specific, meaning each enzyme typically catalyses only one type of reaction or acts on a single substrate. This specificity arises from the unique three-dimensional shape of the enzyme’s active site.
酶具有高度特异性，意味着每种酶通常只催化一种类型的反应或作用于单一底物。这种特异性源于酶活性位点独特的三维形状。
2. Enzyme Structure and Active Site | 酶的结构与活性位点
Enzymes are proteins made of long chains of amino acids folded into a specific three-dimensional structure. The active site is a groove or pocket on the enzyme surface where the substrate binds and the reaction occurs.
酶是由长链氨基酸折叠成特定三维结构的蛋白质。活性位点是酶表面的凹槽或口袋，底物在此结合并发生反应。
The active site is formed by a few amino acid residues whose side chains create a unique chemical environment. The rest of the protein maintains the overall shape, ensuring the active site remains functional.
活性位点由少数氨基酸残基形成，其侧链创造了独特的化学环境。蛋白质其余部分维持整体形状，确保活性位点保持功能。
The substrate binds to the active site through weak interactions such as hydrogen bonds, ionic bonds, and hydrophobic interactions. After the reaction, the products are released, and the enzyme returns to its original state.
底物通过氢键、离子键和疏水相互作用等弱相互作用与活性位点结合。反应后，产物被释放，酶恢复其原始状态。
3. Mechanism of Enzyme Action: Lock-and-Key and Induced Fit | 酶作用机制：锁钥模型与诱导契合
The lock-and-key model proposes that the active site is a rigid shape exactly complementary to the substrate, like a key fitting a lock. This model explains enzyme specificity but does not account for the flexibility of proteins.
锁钥模型认为活性位点是一个刚性的形状，与底物完全互补，就像钥匙插入锁中。该模型解释了酶的特异性，但没有考虑蛋白质的柔性。
The induced fit model is more widely accepted. It states that the active site changes shape slightly when the substrate binds, moulding around the substrate to form a more precise fit. This conformational change can strain substrate bonds, lowering the activation energy.
诱导契合模型被更广泛接受。它指出当底物结合时，活性位点会稍微改变形状，围绕底物形成更精确的匹配。这种构象变化可以拉伸底物键，降低活化能。
Both models agree that the enzyme-substrate complex is a temporary intermediate. The activation energy is reduced because the enzyme provides an alternative reaction pathway with a lower energy barrier.
两种模型都认为酶-底物复合物是一种暂时的中间体。活化能之所以降低，是因为酶提供了一条具有较低能垒的替代反应途径。
4. Factors Affecting Enzyme Activity: Temperature | 影响酶活性的因素：温度
As temperature increases, enzyme activity generally rises because molecules have more kinetic energy, leading to more frequent collisions between enzyme and substrate. The rate of reaction typically doubles for every 10 °C rise up to an optimum temperature.
随着温度升高，酶活性通常增加，因为分子具有更多动能，导致酶与底物之间的碰撞更频繁。在达到最适温度之前，反应速率通常每升高 10 °C 翻倍。
Above the optimum temperature, the increased thermal energy disrupts the weak bonds maintaining the enzyme’s tertiary structure. The active site loses its shape, and the enzyme is denatured. Denaturation is often irreversible, and activity drops sharply.
超过最适温度后，增加的热能破坏了维持酶三级结构的弱键。活性位点失去形状，酶变性。变性通常是不可逆的，活性急剧下降。
Most human enzymes have an optimum temperature around 37 °C. Thermophilic bacteria have enzymes with optimum temperatures above 70 °C, adapted for hot environments.
大多数人类酶的最适温度约为 37 °C。嗜热细菌的酶最适温度高于 70 °C，适应高温环境。
5. Factors Affecting Enzyme Activity: pH | 影响酶活性的因素：pH
Each enzyme has an optimum pH at which it works best. Changes in pH alter the charges on amino acid side chains in the active site, affecting substrate binding and catalysis.
每种酶都有其最适 pH，在该 pH 下活性最高。pH 的变化会改变活性位点内氨基酸侧链的电荷，影响底物结合和催化作用。
Extreme pH levels disrupt the ionic and hydrogen bonds that stabilise the enzyme’s tertiary structure, causing denaturation. For example, pepsin in the stomach has an optimum pH around 2, while trypsin in the small intestine works best at pH 8.
极端的 pH 水平会破坏稳定酶三级结构的离子键和氢键，导致变性。例如，胃中的胃蛋白酶最适 pH 约为 2，而小肠中的胰蛋白酶在 pH 8 时活性最佳。
pH curves are typically bell-shaped, showing a narrow range of high activity around the optimum. Unlike temperature, pH denaturation is more gradual but can still be irreversible.
pH 曲线通常呈钟形，显示最适 pH 附近存在一个较窄的高活性范围。与温度不同，pH 变性更渐进，但同样可能不可逆。
6. Enzyme Concentration and Substrate Concentration | 酶浓度与底物浓度
At a fixed substrate concentration, increasing enzyme concentration increases the rate of reaction linearly, provided there is enough substrate to saturate all enzyme active sites. The rate becomes limited only by the number of enzyme molecules.
在底物浓度固定的情况下，增加酶浓度会线性提高反应速率，前提是有足够的底物来饱和所有酶活性位点。此时速率仅受酶分子数量的限制。
When enzyme concentration is fixed and substrate concentration increases, the reaction rate rises initially but eventually reaches a maximum velocity (Vmax). At Vmax, all active sites are occupied, and adding more substrate has no effect.
当酶浓度固定而底物浓度增加时，反应速率最初上升，但最终达到最大速率（Vmax）。在 Vmax 时，所有活性位点都被占据，再加入更多底物也没有效果。
The Michaelis constant (Kₘ) is the substrate concentration at which the reaction rate is half of Vmax. A low Kₘ indicates high affinity between enzyme and substrate.
米氏常数（Kₘ）是反应速率为 Vmax 一半时的底物浓度。Kₘ 值低表示酶与底物之间的亲和力高。
7. Enzyme Inhibition: Competitive and Non-competitive | 酶抑制作用：竞争性与非竞争性
Competitive inhibitors have a shape similar to the substrate and compete for the active site. They bind reversibly to the active site, preventing the substrate from binding. Increasing substrate concentration can overcome competitive inhibition.
竞争性抑制剂具有与底物相似的形状，并与底物竞争活性位点。它们可逆地与活性位点结合，阻止底物结合。增加底物浓度可以克服竞争性抑制。
Non-competitive inhibitors bind to a site other than the active site, called the allosteric site. This binding changes the shape of the active site so the substrate can no longer bind effectively. Increasing substrate concentration cannot overcome this type of inhibition.
非竞争性抑制剂结合在活性位点以外的位点，称为别构位点。这种结合改变了活性位点的形状，使底物无法有效结合。增加底物浓度无法克服这类抑制。
Enzyme inhibitors are important in medicine. For example, aspirin irreversibly inhibits the enzyme cyclooxygenase, reducing inflammation and pain. Many drugs act as enzyme inhibitors to regulate metabolic pathways.
酶抑制剂在医学中很重要。例如，阿司匹林不可逆地抑制环氧合酶，减轻炎症和疼痛。许多药物作为酶抑制剂来调节代谢途径。
8. Cofactors and Coenzymes | 辅因子与辅酶
Some enzymes require additional non-protein components to function. Cofactors are inorganic ions such as Zn²⁺, Mg²⁺, or Fe²⁺ that bind to the enzyme and help stabilise the enzyme-substrate complex.
一些酶需要额外的非蛋白质成分才能发挥作用。辅因子是无机离子，如 Zn²⁺、Mg²⁺ 或 Fe²⁺，它们与酶结合并帮助稳定酶-底物复合物。
Coenzymes are organic molecules, often derived from vitamins, that assist enzymes by transferring chemical groups, electrons, or atoms during the reaction. Examples include NAD⁺, FAD, and coenzyme A.
辅酶是有机分子，通常来源于维生素，通过在反应过程中转移化学基团、电子或原子来协助酶。例如 NAD⁺、FAD 和辅酶 A。
Apoenzyme is the protein part of an enzyme without its cofactor or coenzyme. The complete, active enzyme with its cofactor is called the holoenzyme.
脱辅基酶是没有辅因子或辅酶的酶蛋白部分。带有辅因子的完整活性酶称为全酶。
9. Immobilised Enzymes | 固定化酶
Immobilised enzymes are enzymes attached to an inert, insoluble material such as alginate beads, silica, or cellulose. This allows the enzyme to be reused and easily separated from the reaction mixture.
固定化酶是附着在惰性不溶性材料（如海藻酸盐珠、二氧化硅或纤维素）上的酶。这使得酶可以重复使用，并易于从反应混合物中分离。
Methods of immobilisation include adsorption, covalent bonding, entrapment in a gel matrix, and encapsulation. Entrapment in alginate beads is commonly used in school experiments with lactase.
固定化方法包括吸附、共价键合、包埋于凝胶基质和封装。在海藻酸盐珠中包埋是学校实验中乳糖酶常用的方法。
Immobilised enzymes have industrial advantages: higher stability, continuous production, lower contamination, and reduced cost because the enzyme is not lost with the product.
固定化酶具有工业优势：稳定性更高、可连续生产、污染更低、成本更低，因为酶不会随产物一起损失。
10. Industrial and Medical Applications of Enzymes | 酶的工业与医学应用
Enzymes are widely used in industry. Proteases in biological washing powders break down protein stains such as blood and egg. Lipases remove grease stains, and amylases break down starch.
酶在工业中应用广泛。生物洗衣粉中的蛋白酶分解血液和蛋清等蛋白质污渍。脂肪酶去除油脂污渍，淀粉酶分解淀粉。
In food production, pectinase clarifies fruit juices by breaking down pectin, while lactase converts lactose into glucose and galactose to produce lactose-free milk for lactose-intolerant individuals.
在食品生产中，果胶酶通过分解果胶来澄清果汁，而乳糖酶将乳糖转化为葡萄糖和半乳糖，为乳糖不耐受者生产无乳糖牛奶。
In medicine, enzymes are used in diagnostic tests. For example, glucose oxidase is used in biosensors to measure blood glucose levels in diabetic patients.
在医学中，酶用于诊断测试。例如，葡萄糖氧化酶用于生物传感器中测量糖尿病患者的血糖水平。
11. Measuring Enzyme Activity and Experimental Design | 酶活性的测量与实验设计
The rate of an enzyme-catalysed reaction can be measured by monitoring the appearance of product or the disappearance of substrate over time. Common methods include measuring gas volume, colour change, or mass loss.
酶催化反应的速率可以通过监测产物出现或底物消失随时间的变化来测量。常用方法包括测量气体体积、颜色变化或质量损失。
A typical experiment investigates the effect of temperature on catalase activity using hydrogen peroxide. The volume of oxygen produced per unit time is recorded at different temperatures, keeping all other variables constant.
一个典型的实验是利用过氧化氢研究温度对过氧化氢酶活性的影响。在不同温度下记录单位时间内产生的氧气体积，同时保持所有其他变量不变。
Controlled variables include enzyme concentration, substrate concentration, pH, and buffer volume. A water bath is used to maintain temperature, and the reaction is timed precisely.
控制变量包括酶浓度、底物浓度、pH 和缓冲液体积。使用水浴维持温度，并精确计时反应。
Results are plotted as rate against temperature, producing a curve that rises to an optimum then falls due to denaturation. Repeat measurements improve reliability.
结果绘制为速率对温度的曲线，呈现先上升至最适值后因变性而下降的曲线。重复测量可提高可靠性。
12. Summary and Key Points | 总结与要点
Enzymes are protein catalysts that lower activation energy and are specific to their substrates. The induced fit model best explains their action.
酶是降低活化能并对底物具有特异性的蛋白质催化剂。诱导契合模型最好地解释了它们的作用。
Temperature and pH affect enzyme activity by altering molecular motion and disrupting bonds. Extreme conditions cause denaturation.
温度和 pH 通过改变分子运动和破坏键来影响酶活性。极端条件会导致变性。
Enzyme concentration and substrate concentration determine reaction rate, with the latter reaching a maximum velocity Vmax. Inhibitors can reduce activity, and cofactors or coenzymes are often required for full function.
酶浓度和底物浓度决定反应速率，后者达到最大速率 Vmax。抑制剂可以降低活性，而辅因子或辅酶通常是发挥完整功能所必需的。
Immobilised enzymes and industrial applications demonstrate the practical importance of understanding enzyme kinetics and stability.
固定化酶和工业应用展示了理解酶动力学和稳定性的实际重要性。
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