Enzymes for IB & OCR Biology | IB OCR 生物：酶考点精讲

📚 Enzymes for IB & OCR Biology | IB OCR 生物：酶考点精讲

Enzymes are biological catalysts that accelerate almost all metabolic reactions in living organisms. They are globular proteins with highly specific active sites, and their function is central to understanding biochemistry at the molecular level. This article covers key concepts required for IB and OCR Biology, from enzyme structure and mechanism to kinetics, inhibition, and practical applications.

酶是加速生物体内几乎所有代谢反应的生物催化剂。它们是具有高度特异性活性位点的球状蛋白，其功能是理解分子水平生物化学的核心。本文涵盖IB和OCR生物学所需的关键概念，从酶的结构与作用机制到动力学、抑制作用以及实际应用。

1. What Are Enzymes? | 什么是酶？

Enzymes are mostly proteins (with the exception of ribozymes, which are RNA molecules with catalytic activity). They lower the activation energy of biochemical reactions without being consumed or permanently altered in the process. Each enzyme typically catalyses only one specific reaction or a group of closely related reactions.

酶绝大多数是蛋白质（核酶除外，它们是具有催化活性的RNA分子）。酶能降低生化反应的活化能，而自身在反应过程中不被消耗或永久改变。每种酶通常只催化一种特定反应或一组密切相关的反应。

The substance on which an enzyme acts is called the substrate. The enzyme binds to the substrate to form an enzyme-substrate complex, converting the substrate into product(s), which are then released, leaving the enzyme free to catalyse another reaction cycle.

酶作用的底物称为底物。酶与底物结合形成酶-底物复合物，将底物转化为产物，产物释放后，酶恢复自由状态，可继续催化下一个反应循环。

2. Enzyme Structure and the Active Site | 酶的结构与活性位点

Enzymes are globular proteins with a specific three-dimensional conformation. This conformation is maintained by hydrogen bonds, ionic interactions, hydrophobic interactions, and disulfide bridges. The active site is a cleft or pocket on the enzyme surface, formed by just a few amino acid residues. The shape and chemical properties of the active site are complementary to the substrate.

酶是具有特定三维构象的球状蛋白。这种构象由氢键、离子相互作用、疏水相互作用和二硫键维持。活性位点是酶表面的一个裂隙或凹陷，仅由少数几个氨基酸残基组成。活性位点的形状和化学性质与底物互补。

Specificity arises from the precise arrangement of amino acids in the active site. This is explained by the lock-and-key hypothesis (rigid complementarity) and, more accurately, by the induced-fit model, where the active site undergoes a conformational change upon substrate binding, moulding around the substrate for a tighter fit.

特异性来源于活性位点氨基酸的精确排列。这可用锁钥假说（刚性互补）解释，但更准确的是诱导契合模型，即底物结合后活性位点发生构象变化，紧紧包裹底物以实现更紧密的结合。

3. Mechanism of Enzyme Catalysis | 酶催化的机制

Enzymes function by lowering the activation energy (Ea) of a reaction. They do this by providing an alternative reaction pathway with a lower energy transition state. This is achieved through several mechanisms: bringing substrates into close proximity and correct orientation, stressing or distorting bonds in the substrate (strain), providing a microenvironment with favourable pH or charge, and participating in acid-base or covalent catalysis.

酶通过降低反应的活化能（Ea）来发挥作用。它们提供一条具有较低能量过渡态的替代反应途径。这通过以下几种机制实现：使底物彼此靠近并正确取向、对底物施加拉力使化学键扭曲（应变）、提供有利的pH或电荷微环境，以及参与酸碱催化或共价催化。

For example, lysozyme catalyses the hydrolysis of polysaccharides in bacterial cell walls by inducing strain on the glycosidic bond and using active-site residues as acid-base catalysts. In the IB and OCR specifications, you need to be able to interpret energy profile graphs showing the reduction in Ea by an enzyme.

例如，溶菌酶通过诱导糖苷键的张力并利用活性位点残基作为酸碱催化剂，催化细菌细胞壁多糖的水解。在IB和OCR考纲中，你需要能够解释酶降低活化能的能谱图。

4. Enzyme Kinetics: Michaelis-Menten Model | 酶促反应动力学：Michaelis-Menten模型

Enzyme kinetics describes the rate of an enzyme-catalysed reaction as a function of substrate concentration. At low substrate concentrations, the rate increases almost linearly with [S]. As [S] rises further, more active sites become occupied, and the rate begins to level off, eventually reaching a maximum velocity (Vmax) when all active sites are saturated.

酶促反应动力学描述了酶催化反应速率随底物浓度变化的关系。在低底物浓度下，反应速率几乎随[S]线性增加。随着[S]进一步升高，更多活性位点被占据，速率开始趋于平稳，当所有活性位点饱和时达到最大反应速率（Vmax）。

The Michaelis constant, Km, is defined as the substrate concentration at which the reaction rate is half of Vmax. Km is a measure of the affinity of the enzyme for its substrate: a low Km indicates high affinity, and a high Km indicates low affinity. The Michaelis-Menten equation is v = (Vmax [S]) / (Km + [S]).

米氏常数Km定义为反应速率达到Vmax一半时的底物浓度。Km是衡量酶与底物亲和力的指标：低Km意味着高亲和力，高Km意味着低亲和力。米氏方程为 v = (Vmax [S]) / (Km + [S])。

5. Factors Affecting Enzyme Activity | 影响酶活性的因素

Factor	Effect on Enzyme Activity
Temperature	Increasing temperature initially raises kinetic energy and collision frequency, increasing rate. Beyond an optimum temperature, bonds maintaining tertiary structure break, leading to denaturation and rapid loss of activity.
pH	Enzymes have an optimum pH. Deviations alter the ionisation of active-site residues, disrupting substrate binding and catalysis. Extreme pH can denature the enzyme.
Substrate concentration	Rate increases until saturation (Vmax).
Enzyme concentration	For a given saturated substrate concentration, rate is directly proportional to enzyme concentration.

因素	对酶活性的影响
温度	温度升高最初增加动能和碰撞频率，速率上升。超过最适温度后，维持三级结构的键断裂，导致变性，活性迅速丧失。
pH	酶有其最适pH。偏离会改变活性位点残基的电离状态，破坏底物结合与催化。极端pH可使酶变性。
底物浓度	速率增加直至饱和 (Vmax)。
酶浓度	在底物饱和条件下，速率与酶浓度成正比。

6. Enzyme Inhibition | 酶的抑制

Inhibitors are molecules that reduce the rate of enzyme-catalysed reactions. Competitive inhibitors resemble the substrate in shape and compete for binding at the active site. They can be overcome by increasing substrate concentration, meaning Vmax remains unchanged but Km increases (apparent affinity decreases).

抑制剂是降低酶催化反应速率的分子。竞争性抑制剂形状与底物相似，竞争结合活性位点。它们可通过增加底物浓度来克服，因此Vmax不变，但Km增大（表观亲和力下降）。

Non-competitive inhibitors bind to an allosteric site (a site other than the active site), altering the enzyme’s conformation so that the active site is no longer catalytically functional. This reduces the effective concentration of functional enzyme molecules, lowering Vmax without changing Km. Uncompetitive inhibition (binding only to the enzyme-substrate complex) is also tested in some specifications.

非竞争性抑制剂结合于别构位点（活性位点以外的位点），改变酶构象，使活性位点丧失催化功能。这降低了功能性酶分子的有效浓度，从而降低Vmax，而Km不变。某些考纲还涉及反竞争性抑制（仅与酶-底物复合物结合）。

End-product inhibition is a common regulatory mechanism in metabolic pathways, where the final product of a sequence of reactions acts as a non-competitive inhibitor of the first enzyme, controlling flux through the pathway.

终产物抑制是代谢途径中常见的调节机制，一系列反应的最终产物作为第一个酶的别构抑制剂，控制整个途径的通量。

7. Regulation of Enzyme Activity | 酶活性的调节

Allosteric enzymes have regulatory sites separate from their active sites. Binding of an activator or inhibitor to these sites changes the shape of the active site. Many allosteric enzymes display cooperative binding, where the binding of one substrate molecule makes it easier for subsequent substrate molecules to bind (positive cooperativity), producing a sigmoidal velocity-substrate curve.

别构酶拥有与活性位点分离的调节位点。激活剂或抑制剂与这些位点结合会改变活性位点的形状。许多别构酶表现出协同结合，即一个底物分子的结合使后续底物分子更容易结合（正协同性），产生S形速率-底物曲线。

Covalent modification, such as phosphorylation/dephosphorylation catalysed by kinases and phosphatases, can rapidly switch enzymes on or off. Proteolytic cleavage (e.g., conversion of pepsinogen to pepsin) irreversibly activates digestive enzymes. Environmental factors like pH and temperature also provide coarse regulation.

共价修饰，如由激酶和磷酸酶催化的磷酸化/去磷酸化，可快速开启或关闭酶。蛋白水解切割（例如胃蛋白酶原转化为胃蛋白酶）不可逆地激活消化酶。pH和温度等环境因素也提供粗放调节。

8. Cofactors and Coenzymes | 辅因子与辅酶

Many enzymes require non-protein components called cofactors for activity. Inorganic cofactors include metal ions such as Zn²⁺, Mg²⁺, and Fe²⁺, which may help orient substrates or participate directly in catalysis. Organic cofactors are called coenzymes; they often act as carriers of chemical groups, electrons, or atoms between reactions.

许多酶需要非蛋白质组分即辅因子才能发挥作用。无机辅因子包括Zn²⁺、Mg²⁺、Fe²⁺等金属离子，它们可能帮助定向底物或直接参与催化。有机辅因子称为辅酶，它们通常作为反应间化学基团、电子或原子的载体。

Examples include NAD⁺ and NADP⁺ (hydrogen/electron carriers), coenzyme A (acetyl group carrier), and ATP (energy currency). Vitamins frequently serve as precursors for coenzymes; for instance, niacin is a precursor of NAD⁺. The hol enzyme is the active enzyme-cofactor complex, while the apoenzyme is the protein component alone, which is inactive.

实例包括 NAD⁺ 和 NADP⁺（氢/电子载体）、辅酶A（乙酰基载体）和 ATP（能量货币）。维生素常作为辅酶的前体，例如烟酸是 NAD⁺ 的前体。全酶是活性酶-辅因子复合物，而脱辅基酶是单独的蛋白质组分，无活性。

9. Classification and Naming of Enzymes | 酶的分类与命名

Enzymes are classified into six main classes by the type of reaction they catalyse: oxidoreductases (redox reactions), transferases (transfer of functional groups), hydrolases (hydrolysis), lyases (addition/removal of groups to form double bonds), isomerases (rearrangement of isomers), and ligases (joining of two molecules using ATP).

酶根据催化的反应类型分为六大类：氧化还原酶（氧化还原反应）、转移酶（官能团转移）、水解酶（水解反应）、裂合酶（加成/去除基团形成双键）、异构酶（异构体重排）和连接酶（利用ATP连接两个分子）。

Enzymes are often named by adding the suffix ‘-ase’ to the substrate or reaction name (e.g., protease, lipase, DNA polymerase). The systematic name is longer and precisely describes the reaction. IB and OCR exams may require recognition of examples like catalase, amylase, and protease, as well as linking them to their reactions.

酶的命名通常在底物或反应名称后加后缀“-ase”（如蛋白酶、脂肪酶、DNA聚合酶）。系统名称较长，精确描述反应。IB和OCR考试可能要求识别如过氧化氢酶、淀粉酶、蛋白酶等实例，并将其与各自的反应联系起来。

10. Applications of Enzymes in Biotechnology | 酶在生物技术中的应用

Enzymes are widely used in industrial processes and medicine. In the food industry, pectinase clarifies fruit juice, lactase produces lactose-free milk, and proteases tenderise meat. In molecular biology, restriction enzymes cut DNA at specific sequences, and ligases join DNA fragments.

酶广泛应用于工业过程和医学领域。在食品工业中，果胶酶用于澄清水果汁，乳糖酶生产无乳糖牛奶，蛋白酶用于嫩化肉类。在分子生物学中，限制酶在特定序列切割DNA，连接酶连接DNA片段。

Immobilised enzymes, attached to inert materials like alginate beads, allow continuous use and easier product separation. Lactase immobilised in columns is used for treating milk on an industrial scale. This technique is a required practical in several specifications: students investigate the effect of different factors on immobilised enzyme activity.

固定化酶附着于藻酸盐珠粒等惰性材料上，可实现持续使用并便于产品分离。固定化乳糖酶柱用于工业规模处理牛奶。此项技术是多个考纲的必修实验：学生探究不同因素对固定化酶活性的影响。

In medicine, enzymes are used diagnostically (e.g., glucose oxidase in biosensors for blood glucose) and therapeutically (e.g., proteolytic enzymes to remove dead tissue). Enzyme replacement therapy is used for certain genetic deficiencies.

在医学上，酶被用于诊断（如生物传感器中的葡萄糖氧化酶检测血糖）和治疗（如蛋白水解酶去除坏死组织）。酶替代疗法用于某些遗传缺陷疾病。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Enzymes for IB & OCR Biology | IB OCR 生物：酶 考点精讲