Mode of Action of Enzymes | 酶的作用方式

📚 Mode of Action of Enzymes | 酶的作用方式

All living cells depend on thousands of enzyme-catalysed reactions. Understanding how enzymes work is therefore central to A-Level Biology, linking protein structure to metabolic control.

所有活细胞都依赖成千上万种酶催化的反应。因此，理解酶的作用方式是 A-Level 生物学的核心内容，将蛋白质结构与代谢调控联系起来。

1. Enzymes as Biological Catalysts | 酶作为生物催化剂

Enzymes are mostly globular proteins with a specific three-dimensional conformation. They accelerate reactions by factors of up to 10¹⁰ or more, and they remain chemically unchanged at the end of the reaction.

酶大多数是球状蛋白质，具有特定的三维构象。它们可将反应速率提高高达 10¹⁰ 倍或更多，并在反应结束时化学性质保持不变。

Because they are not used up, a single enzyme molecule can catalyse many substrate molecules in succession, which allows cells to use enzymes in very small quantities.

由于酶不会被耗尽，一个酶分子可以连续催化许多底物分子，因此细胞只需极少量的酶即可完成反应。

Enzymes are selective: each enzyme normally catalyses only one type of reaction or acts on a very limited range of substrates.

酶具有选择性：每种酶通常只催化一种类型的反应，或仅作用于范围非常有限的底物。

2. Activation Energy and Reaction Pathways | 活化能与反应途径

Every chemical reaction requires an input of energy to break existing bonds before new bonds can form. This energy barrier is called the activation energy, Eₐ.

每个化学反应都需要先输入能量来断裂旧键，然后才能形成新键。这个能量屏障称为活化能 Eₐ。

Enzymes do not change the overall free-energy change ΔG; instead, they lower the activation energy by providing an alternative reaction pathway.

酶不会改变总自由能变化 ΔG；相反，它们通过提供替代反应途径来降低活化能。

With a lower activation energy, a much larger proportion of substrate molecules possess enough energy to react at a given temperature, so the rate of reaction increases.

活化能降低后，在给定温度下具有足够能量发生反应的底物分子比例大大增加，因此反应速率提高。

E + S ⇌ ES → E + P

3. The Active Site and Substrate Specificity | 活性位点与底物特异性

The active site is a small cleft or pocket on the enzyme surface formed by a specific arrangement of amino acid R groups. It contains binding residues and catalytic residues.

活性位点是酶表面由特定氨基酸 R 基团排列形成的小裂隙或口袋。它包含结合残基和催化残基。

The shape, charge, hydrophobicity and hydrogen-bonding ability of the active site make it complementary to the substrate, which explains enzyme specificity.

活性位点的形状、电荷、疏水性和形成氢键的能力使其与底物互补，这解释了酶的专一性。

Specificity can be broad, such as lipase acting on several lipids, or absolute, such as urease acting only on urea.

酶的专一性有宽有窄，例如脂肪酶可作用于多种脂类，而脲酶只能作用于尿素。

4. The Lock-and-Key Model | 锁钥模型

The lock-and-key model, proposed by Emil Fischer in 1894, suggests that the shape of the active site is rigid and exactly complementary to the substrate, like a key fitting a lock.

锁钥模型由 Emil Fischer 于 1894 年提出，认为活性位点的形状是刚性的，并与底物完全互补，就像钥匙插入锁中一样。

This model explains specificity well, but it does not explain how enzymes can stabilise the transition state or how some enzymes can catalyse reversible reactions.

该模型可以很好地解释专一性，但无法解释酶如何稳定过渡态，也无法解释某些酶如何催化可逆反应。

Modern evidence shows that many active sites are flexible rather than rigid, leading to the induced-fit model.

现代证据表明，许多活性位点是柔性的而非刚性的，因此提出了诱导契合模型。

5. The Induced-Fit Model | 诱导契合模型

The induced-fit model, proposed by Daniel Koshland in 1958, states that the active site is not perfectly complementary to the substrate until binding occurs.

诱导契合模型由 Daniel Koshland 于 1958 年提出，认为在底物结合之前，活性位点并不与底物完全互补。

When the substrate binds, it induces a conformational change in the enzyme. The active site moulds around the substrate, bringing catalytic side chains into the correct positions.

当底物结合时，会诱导酶发生构象变化。活性位点围绕底物发生形变，使催化侧链进入正确位置。

Induced fit also places strain on the substrate, weakening particular bonds and therefore lowering the activation energy more effectively than a simple rigid pocket would.

诱导契合还会对底物施加张力，削弱特定化学键，因此比简单的刚性口袋更有效地降低活化能。

6. Formation of Enzyme-Substrate Complexes | 酶-底物复合物的形成

The first step in catalysis is the formation of an enzyme-substrate complex (ES). The substrate binds through non-covalent interactions such as hydrogen bonds, ionic bonds and hydrophobic interactions.

催化的第一步是形成酶-底物复合物（ES）。底物通过氢键、离子键和疏水相互作用等非共价键结合。

Once the ES complex forms, the enzyme provides catalytic groups that donate or accept protons, form temporary covalent bonds, or stabilise the transition state.

一旦 ES 复合物形成，酶会提供催化基团，这些基团可以提供或接受质子、形成临时共价键或稳定过渡态。

The reaction can be represented by the equation:

该反应可用方程式表示：

E + S ⇌ ES → E + P

The products have a lower affinity for the active site and are released, leaving the enzyme free for another catalytic cycle.

产物对活性位点的亲和力较低，会被释放出来，使酶可以自由进行下一轮催化循环。

7. Catalytic Mechanisms: Lowering Activation Energy | 催化机制：降低活化能

Enzymes lower activation energy through several mechanisms. Proximity and orientation effects hold substrates close together in the correct alignment for reaction.

酶通过多种机制降低活化能。邻近效应和定向效应使底物以正确的方向紧密靠在一起，便于反应。

Acid-base catalysis involves the transfer of protons from acidic or basic R groups in the active site, while covalent catalysis forms a temporary intermediate between enzyme and substrate.

酸碱催化涉及活性位点中酸性或碱性 R 基团的质子转移，而共价催化则在酶与底物之间形成临时中间体。

Metal ions such as Zn²⁺ and Mg²⁺ can act as cofactors, stabilising negative charges or polarising bonds. Strain and distortion can also weaken bonds in the substrate.

Zn²⁺ 和 Mg²⁺ 等金属离子可作为辅因子，稳定负电荷或使化学键极化。张力和扭曲作用也会削弱底物中的化学键。

All these effects combine to stabilise the high-energy transition state, which is the key requirement for lowering activation energy.

所有这些效应共同作用，稳定高能过渡态，这是降低活化能的关键要求。

8. Enzyme Cofactors and Coenzymes | 酶辅因子与辅酶

Many enzymes require additional non-protein components to function. These may be inorganic cofactors such as Zn²⁺, Fe²⁺ or Mg²⁺, or organic coenzymes such as NAD⁺, FAD and coenzyme A.

许多酶需要额外的非蛋白质成分才能发挥作用。这些成分可以是 Zn²⁺、Fe²⁺ 或 Mg²⁺ 等无机辅因子，也可以是 NAD⁺、FAD 和辅酶 A 等有机辅酶。

Coenzymes often act as carriers of electrons, protons or chemical groups. For example, NAD⁺ accepts electrons and a proton to become reduced NADH during respiration.

辅酶通常作为电子、质子或化学基团的载体。例如，在呼吸作用中，NAD⁺ 接受电子和质子变成还原型 NADH。

A complete active enzyme, including its cofactor or coenzyme, is called a holoenzyme; the protein part alone is an apoenzyme, which is usually inactive.

包含辅因子或辅酶的完整活性酶称为全酶；单独的蛋白质部分称为脱辅基酶，通常无活性。

9. Measuring Enzyme Action: Initial Rates | 测量酶作用：初速率

The rate of an enzyme-catalysed reaction is usually measured as the initial rate, before significant substrate depletion or product accumulation.

酶催化反应的速率通常以初速率来测量，即在底物大量消耗或产物明显积累之前进行测量。

Initial rates allow valid comparison because the substrate concentration is known and the reaction is still linear with time under most practical conditions.

初速率能够进行有效比较，因为在大多数实际条件下底物浓度已知，而且反应随时间仍呈线性关系。

Common methods include measuring the rate of product formation, such as oxygen gas from catalase, or the disappearance of substrate, such as starch with amylase.

常用方法包括测量产物生成的速率，例如过氧化氢酶产生氧气的速率，或底物消失的速率，例如淀粉酶分解淀粉的速率。

10. Factors Affecting Enzyme Action | 影响酶作用的因素

Temperature and pH affect enzyme activity because they change the kinetic energy of molecules and the ionisation of amino acid side chains in the active site.

温度和 pH 会影响酶活性，因为它们会改变分子的动能以及活性位点中氨基酸侧链的电离状态。

As temperature rises, collisions increase and the rate rises up to an optimum; beyond this, thermal energy breaks hydrogen bonds and hydrophobic interactions, causing denaturation.

随着温度升高，碰撞增多，反应速率上升直至最适温度；超过此温度，热能会破坏氢键和疏水相互作用，导致酶变性。

Each enzyme has an optimum pH. For example, pepsin works best around pH 2, whereas trypsin works best around pH 8.

每种酶都有最适 pH。例如，胃蛋白酶的最适 pH 约为 2，而胰蛋白酶的最适 pH 约为 8。

Substrate concentration affects rate: at low substrate concentration the active sites are not saturated, and the rate increases; at high concentration all active sites are occupied and the rate reaches Vmax.

底物浓度会影响速率：底物浓度低时活性位点未被饱和，速率随之增加；底物浓度高时所有活性位点被占据，速率达到 Vmax。

11. Enzyme Inhibition | 酶抑制

Competitive inhibitors have a shape similar to the substrate and bind reversibly to the active site, blocking substrate access. Their effect can be overcome by increasing substrate concentration.

竞争性抑制剂具有与底物相似的形状，可逆地与活性位点结合，阻断底物进入。增加底物浓度可以克服其抑制作用。

Non-competitive inhibitors bind to an allosteric site, changing the shape of the active site. Increasing substrate concentration does not overcome their effect.

非竞争性抑制剂结合别构位点，改变活性位点的形状。增加底物浓度不能克服其抑制作用。

Inhibition is an important regulatory mechanism in cells, controlling metabolic pathways through feedback inhibition and drug action.

抑制作用是细胞中重要的调控机制，通过反馈抑制和药物作用来控制代谢途径。

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Mode of Action of Enzymes | 酶的作用方式