📚 Proteins: Structure and Function | 蛋白质：结构与功能
Proteins are the workhorses of the living world. These large, intricate macromolecules are polymers made from amino acid monomers linked by peptide bonds. Every protein has a unique three-dimensional shape that is directly related to its biological role, whether it is acting as an enzyme, transporting oxygen, providing mechanical support, or defending the body against pathogens. In the Cambridge A-Level Biology syllabus, an understanding of protein structure—from the sequence of amino acids to the complex folding patterns—is fundamental to grasping how cells function.
蛋白质是生命世界的主力军。这些巨大而复杂的大分子是由氨基酸单体通过肽键连接而成的聚合物。每种蛋白质都具有独特的三维形状，这与其生物学功能直接相关，无论是作为酶、运输氧气、提供机械支撑还是保护机体抵御病原体。在剑桥A-Level生物学课程中，理解蛋白质结构——从氨基酸序列到复杂的折叠模式——是掌握细胞如何运作的基础。
1. Introduction to Proteins | 蛋白质简介
Proteins account for more than half the dry mass of most cells and are involved in practically every cellular process. They are synthesised on ribosomes according to the genetic code and can be composed of one or more polypeptide chains. The diversity of proteins arises from the huge variety of possible sequences of the 20 standard amino acids and from the subsequent folding and modifications that occur after translation.
蛋白质占大多数细胞干重的一半以上，几乎参与所有细胞过程。它们根据遗传密码在核糖体上合成，可以由一条或多条多肽链组成。蛋白质的多样性源于20种标准氨基酸可能排列顺序的巨大变化，以及翻译后发生的折叠和修饰。
2. Amino Acids: The Building Blocks | 氨基酸：构成蛋白质的单体
All amino acids share a common structure: a central (alpha) carbon atom bonded to a hydrogen atom, an amino group (—NH₂), a carboxyl group (—COOH), and a variable R group (side chain). This can be represented as H₂N—CHR—COOH. The R group differs among the 20 amino acids and determines the chemical properties of each. For example, glycine has the simplest R group (—H), while cysteine contains a sulfhydryl group (—SH) capable of forming disulfide bonds.
所有氨基酸都有一个共同的结构：中心（α）碳原子连接一个氢原子、一个氨基（—NH₂）、一个羧基（—COOH）以及一个可变的R基团（侧链）。这可以表示为H₂N—CHR—COOH。R基团在20种氨基酸中各不相同，决定了每种氨基酸的化学性质。例如，甘氨酸具有最简单的R基团（—H），而半胱氨酸含有能够形成二硫键的巯基（—SH）。
In aqueous solutions at physiological pH, the amino group is protonated (—NH₃⁺) and the carboxyl group is deprotonated (—COO⁻), giving amino acids a zwitterionic character. This dual charge influences protein folding and interactions. The carbon atom in the centre is chiral in all amino acids except glycine, giving rise to optical isomers (L- and D- forms), but only L-amino acids are used in protein synthesis.
在生理pH值的水溶液中，氨基被质子化（—NH₃⁺），而羧基去质子化（—COO⁻），使氨基酸具有两性离子特性。这种双重电荷影响了蛋白质的折叠和相互作用。除甘氨酸外，所有氨基酸的中心碳原子都是手性的，产生光学异构体（L型和D型），但只有L-氨基酸被用于蛋白质合成。
3. Formation of a Dipeptide | 二肽的形成
Proteins are built via condensation reactions between amino acids. The carboxyl group of one amino acid reacts with the amino group of another, releasing a molecule of water and forming a covalent peptide bond (—CO—NH—). A dipeptide is the simplest product of this reaction. For example, glycine and alanine can condense to form glycylalanine, with the peptide bond linking the carbon of the carboxyl group to the nitrogen of the amino group. The reaction can be summarised as:
蛋白质是通过氨基酸之间的缩合反应构建的。一个氨基酸的羧基与另一个氨基酸的氨基发生反应，释放一分子水并形成共价肽键（—CO—NH—）。二肽是该反应最简单的产物。例如，甘氨酸和丙氨酸可以缩合形成甘氨酰丙氨酸，肽键将羧基的碳与氨基的氮连接起来。该反应可概括为：
Amino acid₁ + Amino acid₂ → Dipeptide + H₂O
氨基酸₁ + 氨基酸₂ → 二肽 + H₂O
This reaction is anabolic, requiring energy input in the cell, and is catalysed by peptidyl transferase in the ribosome. Repeated condensation reactions lead to the formation of a polypeptide chain, with a free amino group at one end (N-terminus) and a free carboxyl group at the other (C-terminus).
这是一个合成代谢反应，在细胞中需要输入能量，由核糖体中的肽基转移酶催化。重复的缩合反应导致多肽链的形成，一端为游离的氨基（N端），另一端为游离的羧基（C端）。
4. Primary Structure of Proteins | 蛋白质的一级结构
The primary structure is the unique linear sequence of amino acids in a polypeptide chain, determined by the base sequence of the corresponding gene. This sequence is held together exclusively by covalent peptide bonds and dictates all higher levels of protein organisation. Even a single amino acid substitution can profoundly affect a protein’s function, as demonstrated by the mutation causing sickle cell anaemia, where valine replaces glutamic acid at position 6 of the β-haemoglobin chain.
一级结构是多肽链中氨基酸的独特线性序列，由相应基因的碱基序列决定。该序列仅由共价肽键维系，并决定了蛋白质所有更高级的组织形式。哪怕单个氨基酸的替换也可能深刻影响蛋白质的功能，如导致镰状细胞贫血的突变所示，该突变使得β-血红蛋白链第6位的谷氨酸被缬氨酸取代。
The primary structure is synthesised from the N-terminus to the C-terminus. Because the peptide bond has partial double-bond character due to resonance, rotation around the C—N bond is restricted, influencing the possible conformations the chain can adopt. The side chains (R groups) protrude from the backbone and later participate in folding.
一级结构从N端向C端合成。由于共振作用，肽键具有部分双键特性，限制C—N键的旋转，从而影响多肽链可能采取的构象。侧链（R基团）从主链伸出，稍后参与折叠。
5. Secondary Structure: α-Helix and β-Pleated Sheet | 二级结构：α-螺旋与β-折叠片
Secondary structure refers to local, regular folding patterns within a polypeptide chain, stabilised primarily by hydrogen bonds between the carbonyl oxygen (C=O) of one peptide bond and the amide hydrogen (N—H) of another. The two most common types are the α-helix and the β-pleated sheet.
二级结构是指多肽链内局部的、有规律的折叠模式，主要通过一个肽键的羰基氧（C=O）与另一个肽键的酰胺氢（N—H）之间的氢键来稳定。最常见的两种类型是α-螺旋和β-折叠片。
In an α-helix, the polypeptide backbone coils into a right-handed spiral, with 3.6 amino acid residues per turn. The hydrogen bonds run almost parallel to the helix axis, forming between the C=O of residue n and the N—H of residue n+4. The R groups extend outwards, preventing steric hindrance. α-keratin, found in hair and nails, is rich in α-helices.
在α-螺旋中，多肽主链盘绕成右手螺旋，每圈包含3.6个氨基酸残基。氢键几乎与螺旋轴平行，在残基n的C=O与残基n+4的N—H之间形成。R基团向外伸展，避免了空间位阻。存在于头发和指甲中的α-角蛋白富含α-螺旋。
In a β-pleated sheet, the polypeptide chain folds back and forth to form a sheet-like structure. Adjacent strands can run in the same direction (parallel) or in opposite directions (antiparallel). Hydrogen bonds form between C=O and N—H groups on adjacent strands, holding the sheet together. Fibroin, the silk protein, predominantly shows β-pleated sheets, giving it flexibility and strength.
在β-折叠片中，多肽链来回折叠形成片状结构。相邻的链可以同向排列（平行），也可以反向排列（反平行）。氢键在相邻链的C=O和N—H基团之间形成，将折叠片维系在一起。丝蛋白（丝心蛋白）主要呈β-折叠片结构，这赋予其柔韧性和强度。
6. Tertiary Structure | 三级结构
The tertiary structure is the overall three-dimensional folding of a single polypeptide chain, determined by interactions among the side chains (R groups). These interactions include hydrogen bonds, ionic bonds (between charged R groups such as —NH₃⁺ and —COO⁻), hydrophobic interactions (clustering of non-polar side chains in the protein interior), and covalent disulfide bonds (—S—S—) formed between the sulfhydryl groups of two cysteine residues. Disulfide bonds are the strongest of these forces and play a key role in stabilising secreted proteins like insulin.
三级结构是单条多肽链的整体三维折叠，由侧链（R基团）之间的相互作用决定。这些相互作用包括氢键、离子键（在带相反电荷的R基团之间，如—NH₃⁺和—COO⁻）、疏水相互作用（非极性侧链在蛋白质内部聚集）以及两个半胱氨酸残基的巯基之间形成的共价二硫键（—S—S—）。二硫键是这些作用力中最强的，在稳定胰岛素等分泌蛋白中起关键作用。
For water-soluble globular proteins, tertiary folding typically places hydrophobic residues in the core, shielded from water, while hydrophilic residues are exposed on the surface. This arrangement makes the protein soluble and often essential for its biological activity. The precise shape creates active sites in enzymes or binding pockets in transport proteins.
对于水溶性球状蛋白，三级折叠通常将疏水残基置于核心，隔离水分子，而亲水残基暴露在表面。这种排列使蛋白质可溶，且通常对其生物活性至关重要。精确的形状创造出酶的活性位点或运输蛋白的结合口袋。
7. Quaternary Structure: Haemoglobin as an Example | 四级结构：以血红蛋白为例
Quaternary structure exists in proteins composed of two or more polypeptide chains (subunits) that are held together by the same types of interactions found in tertiary structure, though disulfide bonds are less common between subunits. The subunits may be identical or different, and the resulting complex functions as a single entity.
四级结构存在于由两个或多个多肽链（亚基）组成的蛋白质中，这些亚基通过类似于三级结构中的相互作用组装在一起，但亚基间二硫键较少见。亚基可以相同或不同，形成的复合体作为一个整体发挥功能。
A classic example is haemoglobin, the oxygen-carrying protein in red blood cells. Adult haemoglobin consists of two α-globin subunits (141 amino acids each) and two β-globin subunits (146 amino acids each), giving a formula α₂β₂. Each subunit contains a haem prosthetic group with an iron (Fe²⁺) atom that reversibly binds one O₂ molecule. The binding of one oxygen molecule induces a conformational change in the haemoglobin molecule, increasing its affinity for subsequent oxygen molecules—a phenomenon called cooperative binding, best described by the sigmoid oxygen dissociation curve.
一个经典例子是血红蛋白，即红细胞中运载氧的蛋白质。成人血红蛋白由两个α-珠蛋白亚基（各141个氨基酸）和两个β-珠蛋白亚基（各146个氨基酸）组成，分子式为α₂β₂。每个亚基含有一个血红素辅基，其中的铁（Fe²⁺）原子可逆地结合一个O₂分子。一个氧分子的结合会诱导血红蛋白分子构象变化，提高其对后续氧分子的亲和力——这种现象称为协同结合，最好用S形氧解离曲线来描述。
8. Globular and Fibrous Proteins | 球状蛋白与纤维状蛋白
Proteins can be broadly classified into globular and fibrous types based on their overall shape and solubility. Globular proteins, such as enzymes, haemoglobin, myoglobin, and insulin, have compact, roughly spherical structures. They are usually soluble in water because their hydrophilic side chains are exposed on the surface while hydrophobic residues are buried inside. Their precise tertiary and quaternary structures are crucial for their specific functions, which involve molecular recognition, catalysis, and transport.
根据整体形状和溶解性，蛋白质可大致分为球状和纤维状两类。酶、血红蛋白、肌红蛋白和胰岛素等球状蛋白具有紧凑的、大致呈球形的结构。它们通常可溶于水，因为亲水侧链暴露在表面，而疏水残基包埋在内部。它们精确的三级和四级结构对其特定功能至关重要，这些功能包括分子识别、催化和运输。
Fibrous proteins have long, thread-like shapes and are generally insoluble in water. They often play structural or mechanical roles. Collagen, the most abundant protein in mammals, consists of three intertwined left-handed helices forming a right-handed triple helix. This quaternary arrangement provides immense tensile strength for tendons, ligaments, and skin. Keratin, found in hair and nails, contains a high proportion of cysteine residues that cross-link polypeptide chains through numerous disulfide bonds, giving it durability. Fibrous proteins are often composed of repetitive amino acid sequences that facilitate regular secondary structures extended over great lengths.
纤维状蛋白呈长丝状，通常不溶于水。它们通常起结构或机械支持作用。胶原蛋白是哺乳动物体内最丰富的蛋白质，由三条左手螺旋相互缠绕形成右手三股螺旋。这种四级结构为肌腱、韧带和皮肤提供了巨大的抗张强度。角蛋白存在于头发和指甲中，含有高比例的半胱氨酸残基，通过大量二硫键使多肽链交联，从而赋予其耐久性。纤维状蛋白常由重复的氨基酸序列构成，有利于在长距离上延伸规则的二级结构。
9. Diverse Functions of Proteins | 蛋白质的多种功能
Proteins perform an extraordinary array of functions in living organisms. Enzymes, which are almost all globular proteins, act as biological catalysts that lower activation energy, speeding up metabolic reactions with high specificity. Transport proteins such as haemoglobin carry oxygen, while channel and carrier proteins in cell membranes regulate the passage of ions and molecules. Structural proteins like collagen and keratin provide mechanical support. Hormones, including insulin and glucagon, are protein-based signalling molecules that coordinate physiological processes.
蛋白质在生物体中发挥着极其多样的功能。酶几乎都是球状蛋白，作为生物催化剂通过降低活化能来加速代谢反应，且具有高度特异性。运输蛋白如血红蛋白携带氧气，而细胞膜上的通道蛋白和载体蛋白调控离子和分子的通过。结构蛋白如胶原蛋白和角蛋白提供机械支撑。包括胰岛素和胰高血糖素在内的激素是基于蛋白质的信号分子，协调生理过程。
Other functions include immune defence (antibodies, also called immunoglobulins, recognise and neutralise foreign antigens), contractile movement (actin and myosin in muscles), and storage of nutrients (casein in milk, ferritin that stores iron). Receptors embedded in cell membranes are proteins that receive extracellular signals and initiate intracellular responses. The diversity of protein function illustrates the central role of these macromolecules in sustaining life.
其他功能包括免疫防御（抗体，又称免疫球蛋白，识别并中和外来抗原）、收缩运动（肌肉中的肌动蛋白和肌球蛋白）以及养分储存（乳汁中的酪蛋白、储存铁的铁蛋白）。镶嵌在细胞膜上的受体是蛋白质，接收细胞外信号并启动细胞内反应。蛋白质功能的多样性说明了这些大分子在维持生命中的核心作用。
10. Enzyme Function and Denaturation | 酶功能与变性
As proteins, enzymes owe their catalytic power to their unique three-dimensional conformations. The active site of an enzyme is a groove or pocket formed by the folding of a polypeptide chain, precisely shaped to accommodate a specific substrate. The binding of the substrate often induces a conformational change known as the induced-fit model, which stresses the substrate’s bonds and lowers the activation energy for the reaction. The rate of an enzyme-catalysed reaction depends on temperature, pH, enzyme concentration, and substrate concentration.
作为蛋白质，酶的催化能力归功于其独特的三维构象。酶的活性位点是由多肽链折叠形成的凹槽或口袋，其形状精确，可容纳特定的底物。底物的结合通常诱导构象变化，这被称为诱导契合模型，该模型对底物的化学键施加应力，降低反应的活化能。酶促反应的速率取决于温度、pH值、酶浓度和底物浓度。
Denaturation is the irreversible loss of a protein’s native conformation, leading to loss of biological activity. It is caused by factors that disrupt the non-covalent interactions (hydrogen, ionic, hydrophobic) and sometimes disulfide bonds maintaining the secondary, tertiary, and quaternary structures. High temperatures increase kinetic energy, breaking hydrogen and ionic bonds. Extremes of pH alter ionic charges on side chains, disrupting electrostatic interactions. Heavy metals and organic solvents can also denature proteins. Importantly, the primary structure (peptide bonds) usually remains intact during denaturation. A denatured enzyme can no longer bind its substrate because the shape of the active site is destroyed.
变性是蛋白质天然构象不可逆的丧失，导致生物活性丧失。它由破坏维持二级、三级和四级结构的非共价相互作用（氢键、离子键、疏水作用）以及有时二硫键的因素引起。高温增加动能，破坏氢键和离子键。极端的pH值改变侧链上的离子电荷，破坏静电相互作用。重金属和有机溶剂也可使蛋白质变性。重要的是，在变性过程中一级结构（肽键）通常保持完整。变性的酶无法再与其底物结合，因为活性位点的形状已被破坏。
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