📚 Proteins: Structure and Function | 蛋白质：结构与功能
Proteins are large, complex macromolecules made of amino acids. They carry out nearly all biological tasks, from catalysis to structural support. Understanding protein structure explains how enzymes, antibodies, haemoglobin and collagen work.
蛋白质是由氨基酸组成的大型复杂大分子。它们执行几乎所有的生物学任务，从催化到结构支持。理解蛋白质结构有助于解释酶、抗体、血红蛋白和胶原蛋白如何发挥作用。
1. Amino Acids: The Building Blocks | 氨基酸：基本构件
All proteins are polymers of amino acids. Each amino acid has a central carbon atom (the α-carbon) bonded to a hydrogen atom, an amino group (NH₂), a carboxyl group (COOH), and a distinctive R group. The R group differs among the 20 amino acids and determines their properties.
所有蛋白质都是氨基酸的聚合物。每个氨基酸都有一个中心碳原子（α-碳），连接一个氢原子、一个氨基（NH₂）、一个羧基（COOH）和一个独特的 R 基团。R 基团在 20 种氨基酸中各不相同，并决定了它们的性质。
Some R groups are non-polar and hydrophobic, while others are polar or charged and hydrophilic. This variety allows proteins to fold into specific shapes.
有些 R 基团是非极性和疏水的，而另一些是极性或带电且亲水的。这种多样性使蛋白质能够折叠成特定的形状。
2. Peptide Bonds and Polypeptides | 肽键与多肽
Amino acids join by condensation reactions. The carboxyl group of one amino acid reacts with the amino group of the next, removing a water molecule and forming a covalent peptide bond (-CO-NH-).
氨基酸通过缩合反应连接。一个氨基酸的羧基与下一个氨基酸的氨基反应，脱去一分子水，形成共价肽键（-CO-NH-）。
Chains of amino acids are called polypeptides. A protein may consist of one or more polypeptide chains folded into a functional shape.
氨基酸链称为多肽。蛋白质可能由一个或多个多肽链折叠成功能性形状组成。
3. Primary Structure: The Amino Acid Sequence | 一级结构：氨基酸序列
The primary structure is the linear sequence of amino acids in a polypeptide. This sequence is coded by DNA and determines all higher levels of structure.
一级结构是多肽中氨基酸的线性序列。该序列由 DNA 编码，并决定所有更高级的结构。
Even a single amino acid substitution can alter function. For example, in sickle cell disease, one amino acid change in haemoglobin reduces its solubility.
即使单个氨基酸被替换也能改变功能。例如，在镰状细胞病中，血红蛋白中一个氨基酸的改变降低了其溶解度。
4. Secondary Structure: Alpha Helix and Beta Sheet | 二级结构：α 螺旋与 β 折叠
Secondary structure is the local folding of the polypeptide backbone. The two main types are the α-helix and the β-pleated sheet.
二级结构是多肽主链的局部折叠。两种主要类型是 α 螺旋和 β 折叠片。
Hydrogen bonds form between the C=O group of one peptide bond and the N-H group of another. In an α-helix, the backbone winds into a right-handed coil. In a β-pleated sheet, two or more sections of the chain lie side by side, held by hydrogen bonds.
氢键形成于一个肽键的 C=O 基团与另一个肽键的 N-H 基团之间。在 α 螺旋中，主链盘绕成右手螺旋。在 β 折叠片中，两条或多条链段并排排列，由氢键维持。
The R groups are not directly involved in these hydrogen bonds, so secondary structure can form in many different amino acid sequences.
R 基团不直接参与这些氢键，因此二级结构可以在许多不同的氨基酸序列中形成。
5. Tertiary Structure: Overall 3D Folding | 三级结构：整体三维折叠
Tertiary structure is the overall three-dimensional shape of a single polypeptide. It is stabilised by interactions between R groups, including hydrogen bonds, ionic bonds between charged R groups, hydrophobic interactions, and covalent disulfide bridges between cysteine residues.
三级结构是单条多肽的整体三维形状。它通过 R 基团之间的相互作用稳定，包括氢键、带电 R 基团之间的离子键、疏水相互作用以及半胱氨酸残基之间的共价二硫键。
Disulfide bridges are strong and form when two cysteine R groups are oxidised, creating an -S-S- link. Hydrophobic residues usually cluster in the interior, away from water.
二硫键很强，当两个半胱氨酸 R 基团被氧化时形成 -S-S- 连接。疏水残基通常聚集在内部，远离水。
6. Quaternary Structure: Multiple Subunits | 四级结构：多个亚基
Quaternary structure exists only in proteins with more than one polypeptide chain or subunit. The subunits associate through the same types of bonds as in tertiary structure.
四级结构只存在于含有多条多肽链或亚基的蛋白质中。亚基通过与三级结构相同类型的键结合在一起。
Haemoglobin is a classic example. It has two α-chains and two β-chains, plus four haem groups that contain iron and bind oxygen.
血红蛋白是一个典型例子。它含有两条 α 链和两条 β 链，以及四个含铁并能结合氧的血红素基团。
7. Globular and Fibrous Proteins | 球状蛋白与纤维状蛋白
Proteins can be classified by shape. Globular proteins are compact and roughly spherical, usually soluble in water, and often have metabolic roles. Fibrous proteins are long, narrow, insoluble, and mainly structural.
蛋白质可按形状分类。球状蛋白结构紧凑、大致呈球形，通常可溶于水，常具有代谢功能。纤维状蛋白细长、不溶，主要起结构作用。
Haemoglobin and enzymes are globular, while collagen and keratin are fibrous.
血红蛋白和酶是球状蛋白，而胶原蛋白和角蛋白是纤维状蛋白。
| Feature | Globular proteins | Fibrous proteins |
|---|---|---|
| Shape | Compact, spherical | Long, narrow |
| Solubility | Usually soluble in water | Usually insoluble in water |
| Main role | Metabolic | Structural |
| Examples | Haemoglobin, enzymes | Collagen, keratin |
8. Haemoglobin: A Globular Protein | 血红蛋白：一种球状蛋白
Haemoglobin is a water-soluble globular protein found in red blood cells. Its quaternary structure allows cooperative oxygen binding: when one haem group binds O₂, the protein changes shape and makes it easier for the remaining haem groups to bind oxygen.
血红蛋白是红细胞中的一种水溶性球状蛋白。其四级结构允许协同结合氧：当一个血红素基团结合 O₂ 时，蛋白质改变形状，使其余血红素基团更容易结合氧。
The hydrophobic R groups face the interior, while hydrophilic R groups face the surface. This makes haemoglobin soluble in the cytoplasm of red blood cells.
疏水 R 基团朝向内部，而亲水 R 基团朝向表面。这使得血红蛋白可溶于红细胞的细胞质中。
Each haem group contains an iron ion (Fe²⁺) that can reversibly bind one O₂ molecule. This allows haemoglobin to pick up oxygen in the lungs and release it in tissues.
每个血红素基团都含有一个铁离子（Fe²⁺），可以可逆地结合一个 O₂ 分子。这使得血红蛋白能够在肺部结合氧，并在组织中释放氧。
9. Collagen: A Fibrous Protein | 胶原蛋白：一种纤维状蛋白
Collagen is a fibrous protein found in skin, tendons, ligaments, and bone. It consists of three polypeptide chains wound into a triple helix.
胶原蛋白是一种纤维状蛋白，存在于皮肤、肌腱、韧带和骨骼中。它由三条多肽链缠绕成三股螺旋组成。
Every third amino acid is glycine, the smallest amino acid. This allows the three chains to pack tightly. Covalent cross-links between chains give collagen high tensile strength.
每第三个氨基酸是甘氨酸，这是最小的氨基酸。这使三条链能够紧密排列。链之间的共价交联赋予胶原蛋白很高的抗张强度。
Collagen is insoluble in water and provides mechanical support, making it ideal for tissues that must resist stretching.
胶原蛋白不溶于水，并提供机械支撑，非常适合需要抵抗拉伸的组织。
10. Functions of Proteins | 蛋白质的功能
Proteins perform an enormous range of functions because their shapes and R groups vary widely. The table below summarises some major roles.
蛋白质执行极其广泛的功能，因为它们的形状和 R 基团变化很大。下表总结了一些主要作用。
| Function | Examples |
|---|---|
| Enzymes | Amylase, catalase |
| Transport | Haemoglobin (O₂), albumin (fatty acids) |
| Structural support | Collagen, keratin |
| Hormones | Insulin, glucagon |
| Defence | Antibodies |
| Movement | Actin, myosin |
| Storage | Ferritin (iron), casein |
11. The Biuret Test for Proteins | 蛋白质的双缩脲检测
The biuret test detects peptide bonds. Add a few drops of dilute copper(II) sulfate solution to a solution of the sample, after adding sodium hydroxide. If protein is present, the mixture turns purple or violet.
双缩脲检测用于检测肽键。向样品溶液中先加入氢氧化钠，然后加入几滴稀硫酸铜（II）溶液。如果存在蛋白质，混合物会变成紫色或紫罗兰色。
The violet colour arises from coordination complexes between Cu²⁺ ions and peptide bonds. A negative result remains blue. The test gives a positive result for any substance with peptide bonds, so short peptides also respond.
紫色来自 Cu²⁺ 离子与肽键形成的配位复合物。阴性结果保持蓝色。该检测对任何含有肽键的物质都会产生阳性结果，因此短肽也会反应。
12. Denaturation of Proteins | 蛋白质的变性
Denaturation is the loss of the specific three-dimensional shape of a protein without breaking its primary structure. Heat, pH changes, or chemicals can disrupt hydrogen bonds, ionic bonds, and hydrophobic interactions.
变性是蛋白质失去特定三维形状但不破坏其一级结构的过程。高温、pH 变化或化学物质可以破坏氢键、离子键和疏水相互作用。
Because the shape determines function, a denatured protein usually loses its biological activity. If conditions return to normal, some proteins can renature, but many cannot.
由于形状决定功能，变性蛋白质通常会失去其生物活性。如果条件恢复正常，有些蛋白质可以复性，但许多不能。
Denaturation does not break peptide bonds, so the primary structure remains intact. This explains why the biuret test can still be positive after a protein has been denatured.
变性不会破坏肽键，因此一级结构保持完整。这解释了为什么蛋白质变性后双缩脲检测仍然可以呈阳性。
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