📚 Enzymes: Structure, Function and Regulation | 酶：结构、功能与调控
Enzymes are biological catalysts that accelerate chemical reactions without being consumed. Understanding their structure, specificity, and the factors that affect their activity is fundamental to biochemistry and many A‑level science specifications.
酶是生物催化剂，能在不被消耗的情况下加速化学反应。理解它们的结构、专一性以及影响其活性的因素，是生物化学以及许多 A‑level 科学课程的基础。
1. What Are Enzymes? | 什么是酶？
Enzymes are globular proteins that act as catalysts in living organisms. They lower the activation energy of reactions, allowing metabolic processes to occur rapidly at body temperature.
酶是球状蛋白质，在生物体内充当催化剂。它们降低反应的活化能，使代谢过程在体温下就能迅速进行。
Most enzymes are highly specific, catalysing only one type of reaction or acting on a single substrate. This specificity arises from the precise three‑dimensional shape of the enzyme, particularly the active site.
大多数酶具有高度专一性，只催化一种类型的反应或只作用于一种底物。这种专一性源于酶精确的三维结构，尤其是活性部位。
The names of enzymes often end in ‘-ase’ and are derived from the substrate they act on, e.g. lipase acts on lipids, protease on proteins.
酶的名称通常以“‑ase”结尾，并来自其作用的底物，例如脂肪酶作用于脂质，蛋白酶作用于蛋白质。
2. Enzyme Structure | 酶的结构
Enzymes possess primary, secondary, tertiary and sometimes quaternary structures. The primary structure is the sequence of amino acids, which folds into α‑helices and β‑pleated sheets (secondary structure).
酶具有一级、二级、三级，有时还有四级结构。一级结构是氨基酸序列，它折叠成 α‑螺旋和 β‑折叠（二级结构）。
The tertiary structure is the overall 3D shape maintained by hydrogen bonds, ionic bonds, disulphide bridges and hydrophobic interactions. This folding creates the active site.
三级结构是由氢键、离子键、二硫键和疏水相互作用维持的整体三维形状。这种折叠形成了活性部位。
Some enzymes consist of more than one polypeptide chain, forming a quaternary structure. For example, lactate dehydrogenase has four subunits.
有些酶由不止一条多肽链组成，形成四级结构。例如，乳酸脱氢酶含有四个亚基。
3. The Active Site | 活性部位
The active site is a small cleft or pocket on the enzyme surface formed by a few key amino acid residues. It is the region where the substrate binds and the chemical reaction takes place.
活性部位是酶表面由少数几个关键氨基酸残基形成的小裂缝或口袋。它正是底物结合并发生化学反应的区域。
The shape and chemical properties of the active site are complementary to the substrate. Charged groups such as R‑groups of amino acids participate in binding and catalysis.
活性部位的形状和化学性质与底物互补。诸如氨基酸 R 基团等带电基团参与结合和催化过程。
Binding of the substrate involves multiple weak interactions, including hydrogen bonds and van der Waals forces, ensuring that the enzyme–substrate complex is both specific and reversible.
底物的结合涉及多种弱相互作用，包括氢键和范德华力，确保酶‑底物复合物既有专一性又可逆。
4. Lock and Key Model | 锁钥模型
The lock and key model proposes that the active site has a rigid, fixed shape that is exactly complementary to the substrate. The substrate fits into the active site like a key into a lock.
锁钥模型提出，活性部位具有刚性的固定形状，与底物精确互补。底物像钥匙插入锁孔一样嵌入活性部位。
This model explains enzyme specificity well, but it does not account for the conformational changes that occur upon substrate binding or the stabilisation of the transition state.
该模型很好地解释了酶的专一性，但不能解释底物结合时发生的构象变化或过渡态的稳定化。
Despite its limitations, the lock and key model remains a useful simplification when introducing the concept of enzyme action.
尽管有其局限性，锁钥模型在引入酶作用概念时仍然是一个有用的简化方式。
5. Induced Fit Model | 诱导契合模型
The induced fit model refines our understanding: the active site is flexible and undergoes a conformational change as the substrate binds. This change strains the substrate and lowers activation energy.
诱导契合模型完善了我们的理解：活性部位是柔性的，在底物结合时发生构象变化。这种变化使底物形变，降低了活化能。
Once the substrate enters, the enzyme moulds itself around it, forming a tighter fit. This places strain on bonds, making the transition state easier to reach.
一旦底物进入，酶会环绕它紧密结合，形成更紧的配合。这使化学键产生张力，更容易达到过渡态。
The model is supported by X‑ray crystallography data showing different conformations of enzymes in the presence and absence of substrate.
该模型得到 X 射线晶体学数据支持，这些数据显示酶在有无底物时呈不同构象。
6. Factors Affecting Enzyme Activity: Temperature | 影响酶活性的因素：温度
As temperature increases, kinetic energy rises, leading to more frequent collisions between enzyme and substrate. The rate of reaction increases up to an optimum temperature, often around 37–40 °C for human enzymes.
随着温度升高，动能增加，酶与底物之间的碰撞更频繁。反应速率逐渐增大，直至最适温度，人体酶的最适温度通常约为 37‑40 °C。
Beyond the optimum, the increased thermal energy disrupts the hydrogen and ionic bonds that maintain the tertiary structure. The active site is denatured and loses its complementary shape, so activity falls sharply.
超过最适温度后，增加的热能破坏了维持三级结构的氢键和离子键。活性部位变性，失去互补形状，活性急剧下降。
Some thermophilic organisms have enzymes with an optimum temperature above 80 °C, stabilised by more disulphide bonds and hydrophobic interactions.
某些嗜热生物的酶最适温度超过 80 °C，它们通过更多的二硫键和疏水相互作用来稳定结构。
7. Factors Affecting Enzyme Activity: pH | 影响酶活性的因素：pH
Each enzyme has an optimum pH at which it is most active. For pepsin this is around pH 2, while for trypsin it is about pH 8.
每种酶都有其最适 pH，在此条件下活性最高。胃蛋白酶的最适 pH 约为 2，而胰蛋白酶约为 pH 8。
Changes in pH alter the ionisation of amino acid R‑groups in the active site. This can disrupt the ionic and hydrogen bonds that maintain the precise shape, leading to denaturation.
pH 的改变影响活性部位氨基酸 R 基团的电离状态。这会破坏维持精确形状的离子键和氢键，导致变性。
Even small deviations from the optimum pH reduce the rate of reaction, as the binding affinity for the substrate decreases.
即使轻微偏离最适 pH，也会降低反应速率，因为酶对底物的结合亲和力下降。
8. Substrate Concentration | 底物浓度
At low substrate concentration, not all active sites are occupied, so the rate increases linearly with substrate concentration. This is a first‑order relationship.
在低底物浓度下，并非所有活性部位都被占据，因此反应速率随底物浓度线性增加，呈一级反应关系。
As substrate concentration rises, more active sites become filled until the enzyme is saturated. The rate reaches a maximum, Vmax, and becomes independent of substrate concentration (zero‑order).
随着底物浓度升高，越来越多的活性部位被填满，直到酶饱和。速率达到最大值 Vmax，不再依赖于底物浓度（零级反应）。
The Michaelis–Menten constant, Km, is the substrate concentration at which the rate is half Vmax. It indicates the affinity of the enzyme for its substrate.
米氏常数 Km 是反应速率为 Vmax 一半时的底物浓度，它反映了酶对底物的亲和力。
9. Enzyme Inhibitors: Competitive | 酶抑制剂：竞争性抑制
Competitive inhibitors have a shape similar to the substrate and bind reversibly to the active site. They compete with the substrate, so increasing substrate concentration can overcome the inhibition.
竞争性抑制剂具有与底物相似的形状，并可逆地与活性部位结合。它们与底物竞争，因此增加底物浓度可以克服抑制。
In the presence of a competitive inhibitor, Vmax remains unchanged because a sufficiently high substrate concentration can out‑compete the inhibitor. However, Km increases, indicating reduced apparent affinity.
在竞争性抑制剂存在下，Vmax 保持不变，因为足够高的底物浓度可以胜出抑制剂；但是 Km 增大，表明表观亲和力下降。
Statins, which inhibit the enzyme HMG‑CoA reductase, act as competitive inhibitors and are used to lower cholesterol.
他汀类药物作为 HMG‑CoA 还原酶的竞争性抑制剂，用于降低胆固醇。
10. Enzyme Inhibitors: Non‑competitive | 酶抑制剂：非竞争性抑制
Non‑competitive inhibitors bind to an allosteric site – a location other than the active site – and change the shape of the enzyme. The substrate can still bind, but the active site is no longer catalytically effective.
非竞争性抑制剂结合于别构部位（活性部位以外的位置），并改变酶的形状。底物仍能结合，但活性部位不再具有催化效力。
Because binding does not occur at the active site, increasing substrate concentration cannot overcome non‑competitive inhibition. Vmax decreases while Km remains unchanged.
由于结合不发生在活性部位，增加底物浓度无法克服非竞争性抑制。Vmax 下降，Km 保持不变。
Heavy metal ions such as Hg²⁺ and Pb²⁺ often act as non‑competitive inhibitors by binding to –SH groups and causing permanent denaturation.
重金属离子如 Hg²⁺ 和 Pb²⁺ 常作为非竞争性抑制剂，与 ‑SH 基团结合，导致永久变性。
11. Cofactors and Coenzymes | 辅因子与辅酶
Cofactors are non‑protein chemical compounds required for enzyme activity. They can be inorganic ions like Zn²⁺ (as in carbonic anhydrase) or Fe²⁺ (in cytochromes).
辅因子是酶活性所需的非蛋白质化合物。它们可以是无机离子，如碳酸酐酶中的 Zn²⁺，或细胞色素中的 Fe²⁺。
Coenzymes are organic cofactors that are often derived from vitamins. NAD⁺, FAD and coenzyme A are key examples, carrying electrons or acyl groups between reactions.
辅酶是有机辅因子，通常来源于维生素。NAD⁺、FAD 和辅酶 A 是重要例子，分别在反应间传递电子或酰基。
Some coenzymes are tightly bound to the enzyme (prosthetic groups), while others associate only transiently during the reaction.
有些辅酶与酶紧密结合（辅基），而另一些仅在反应期间短暂结合。
12. Industrial and Medical Applications | 工业与医学应用
Enzymes are used in biotechnology, for example proteases in biological washing powders digest protein stains, and lactase converts lactose into glucose and galactose for lactose‑free milk.
酶被用于生物技术，例如加酶洗衣粉中的蛋白酶可分解蛋白质污渍，乳糖酶将乳糖转化为葡萄糖和半乳糖以生产无乳糖牛奶。
In medicine, enzymes serve as diagnostic markers: elevated troponin levels indicate a heart attack, and elevated liver enzymes such as ALT suggest liver damage.
在医学上，酶作为诊断标志物：肌钙蛋白升高表明心肌梗死，丙氨酸氨基转移酶等肝酶升高提示肝损伤。
Immobilised enzymes, trapped in alginate beads, allow continuous production in bioreactors and can be reused, reducing costs in the manufacture of high‑fructose corn syrup.
固定化酶被包埋在海藻酸钙珠中，可在生物反应器中连续生产并可重复使用，从而降低高果糖玉米糖浆的生产成本。
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