Translation in Protein Synthesis | 蛋白质合成中的翻译

📚 Translation in Protein Synthesis | 蛋白质合成中的翻译

Translation is the second major stage of protein synthesis, in which the sequence of codons on messenger RNA (mRNA) is decoded by ribosomes to assemble a specific polypeptide chain. It depends on the precise interaction between mRNA, transfer RNA (tRNA), ribosomes, and various protein factors, converting the nucleic acid language into the amino acid language of proteins.

翻译是蛋白质合成的第二个主要阶段，在此过程中，信使 RNA (mRNA) 上的密码子序列由核糖体解码，以组装特定的多肽链。它依赖 mRNA、转运 RNA (tRNA)、核糖体和多种蛋白质因子之间的精确相互作用，将核酸语言转化为蛋白质的氨基酸语言。

1. Overview of Translation | 翻译概述

Translation occurs in the cytoplasm on ribosomes, using the mRNA transcript produced by transcription as a template. The sequence of bases in mRNA is read in groups of three, called codons, each specifying one amino acid. The ribosome catalyses the formation of peptide bonds between amino acids brought by tRNA molecules, building a polypeptide chain from the N-terminus to the C-terminus.

翻译发生在细胞质中的核糖体上，使用转录产生的 mRNA 转录本作为模板。mRNA 的碱基序列以三个为一组读取，称为密码子，每个密码子指定一种氨基酸。核糖体催化由 tRNA 分子带来的氨基酸之间形成肽键，从 N 端到 C 端构建多肽链。

2. The Genetic Code | 遗传密码

The genetic code is a set of rules by which 64 possible mRNA codons specify the 20 amino acids used in proteins, plus three stop signals. It is degenerate, meaning that most amino acids are encoded by more than one codon; for example, leucine is specified by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). The code is unambiguous – each codon codes for only one amino acid – and it is nearly universal across all organisms.

遗传密码是一套规则，通过它 64 个可能的 mRNA 密码子指定蛋白质中使用的 20 种氨基酸以及三个终止信号。它是简并的，意味着大多数氨基酸由多个密码子编码；例如，亮氨酸由六个不同的密码子指定 (UUA, UUG, CUU, CUC, CUA, CUG)。该密码是明确的——每个密码子只编码一种氨基酸——并且在所有生物中几乎通用。

The start codon AUG (coding for methionine in eukaryotes or a modified methionine in prokaryotes) signals the beginning of translation and sets the reading frame. Stop codons UAA, UAG, and UGA do not code for any amino acid; they signal the termination of polypeptide synthesis.

起始密码子 AUG（在真核生物中编码甲硫氨酸，在原核生物中编码修饰的甲硫氨酸）指示翻译的起始并设定阅读框。终止密码子 UAA、UAG 和 UGA 不编码任何氨基酸；它们发出多肽合成终止的信号。

3. Ribosomes: The Site of Translation | 核糖体：翻译的场所

Ribosomes are complex macromolecular machines composed of ribosomal RNA (rRNA) and proteins. Each ribosome consists of a large subunit and a small subunit. In eukaryotes, the subunits are 60S and 40S, forming an 80S ribosome; in prokaryotes, they are 50S and 30S, forming a 70S ribosome. The small subunit binds to mRNA and reads the codons, while the large subunit catalyses peptide bond formation and houses the growing polypeptide chain.

核糖体是由核糖体 RNA (rRNA) 和蛋白质组成的复杂大分子机器。每个核糖体由一个大亚基和一个小亚基组成。在真核生物中，亚基为 60S 和 40S，形成 80S 核糖体；在原核生物中，为 50S 和 30S，形成 70S 核糖体。小亚基与 mRNA 结合并读取密码子，大亚基则催化肽键形成并容纳生长中的多肽链。

The ribosome has three key sites for tRNA binding: the A site (aminoacyl site) where incoming aminoacyl–tRNA enters; the P site (peptidyl site) where the tRNA carrying the growing peptide is held; and the E site (exit site) from which deacylated tRNA leaves the ribosome.

核糖体有三个关键的 tRNA 结合位点：A 位点（氨酰基位点），进入的氨酰 tRNA 在此处；P 位点（肽基位点），携带延伸中多肽的 tRNA 位于此；E 位点（出口位点），去酰化的 tRNA 从此离开核糖体。

4. Transfer RNA (tRNA) and Anticodons | 转运 RNA 与反密码子

tRNA molecules are adapters that link the genetic code to amino acids. Each tRNA has a cloverleaf secondary structure and an L-shaped tertiary structure. At one end is the 3′ CCA tail where the specific amino acid is attached by an aminoacyl–tRNA synthetase. At the opposite loop is the anticodon – a triplet of bases complementary to a specific mRNA codon.

tRNA 分子是将遗传密码与氨基酸连接起来的适配器。每个 tRNA 具有三叶草形的二级结构和 L 形的三级结构。一端是 3′ CCA 尾部，特定的氨基酸由氨酰 tRNA 合成酶连接于此。在相对的环上是反密码子——与特定 mRNA 密码子互补的三碱基序列。

The pairing between codon and anticodon follows standard base-pairing rules (A–U, G–C) but allows wobble at the third position, where a single tRNA can recognise more than one codon for the same amino acid thanks to modified bases like inosine. This wobble contributes to the efficiency and degeneracy of the code.

密码子与反密码子的配对遵循标准碱基配对规则 (A–U, G–C)，但在第三位允许摆动，此时一个 tRNA 可以识别同一氨基酸的多个密码子，这得益于像次黄苷这样的修饰碱基。这种摆动提高了密码的效率和简并性。

5. Initiation of Translation | 翻译的起始

In eukaryotes, translation initiation involves the assembly of the small ribosomal subunit, an initiator tRNA carrying methionine (Met–tRNAᵢ), and mRNA. The small subunit binds to the 5′ cap of mRNA and scans along until it encounters the start codon AUG in a favourable context (Kozak sequence). The large subunit then joins, forming a complete ribosome with the initiator tRNA in the P site.

在真核生物中，翻译起始涉及小核糖体亚基、携带甲硫氨酸的起始 tRNA (Met–tRNAᵢ) 和 mRNA 的组装。小亚基与 mRNA 的 5′ 帽结合并沿其扫描，直到在适宜的环境中（Kozak 序列）遇到起始密码子 AUG。然后大亚基加入，形成完整的核糖体，起始 tRNA 位于 P 位点。

In prokaryotes, initiation is mediated by the Shine–Dalgarno sequence on mRNA, which base-pairs with the 16S rRNA of the small subunit, positioning the start codon at the P site. The initiator tRNA carries N-formylmethionine (fMet). Initiation factors (IFs) assist in the process and are released before elongation begins.

在原核生物中，起始由 mRNA 上的 Shine–Dalgarno 序列介导，该序列与小亚基的 16S rRNA 碱基配对，将起始密码子定位在 P 位点。起始 tRNA 携带 N-甲酰甲硫氨酸 (fMet)。起始因子 (IFs) 协助该过程，并在延伸开始前被释放。

6. Elongation: Peptide Bond Formation | 延伸：肽键的形成

Elongation is the cyclic addition of amino acids to the growing polypeptide chain. An aminoacyl–tRNA whose anticodon is complementary to the codon in the A site enters the ribosome, guided by elongation factors (EF-Tu in prokaryotes, eEF1 in eukaryotes) and GTP hydrolysis. The peptidyl transferase centre of the large subunit (composed of rRNA) catalyses the formation of a peptide bond between the amino acid on the P-site tRNA and the amino acid on the A-site tRNA.

延伸是将氨基酸循环添加到生长中多肽链的过程。由延伸因子（原核生物中的 EF-Tu，真核生物中的 eEF1）和 GTP 水解引导，反密码子与 A 位点密码子互补的氨酰 tRNA 进入核糖体。大亚基的肽基转移酶中心（由 rRNA 组成）催化 P 位点 tRNA 上的氨基酸与 A 位点 tRNA 上的氨基酸之间形成肽键。

After peptide bond formation, the ribosome translocates: it moves by one codon along the mRNA with the help of elongation factor EF‑G (eEF2 in eukaryotes) and GTP. The tRNA that was in the P site moves to the E site and exits; the tRNA now carrying the growing peptide moves from A site to P site, leaving the A site free for the next aminoacyl–tRNA. This elongation cycle repeats until a stop codon is encountered.

肽键形成后，核糖体进行移位：在延伸因子 EF-G（真核生物中为 eEF2）和 GTP 的帮助下，核糖体沿着 mRNA 移动一个密码子。原本在 P 位点的 tRNA 移到 E 位点并离开；现在携带生长中多肽的 tRNA 从 A 位点移到 P 位点，使 A 位点空出以接纳下一个氨酰 tRNA。这个延伸循环不断重复，直到遇到终止密码子。

7. Termination: Release of the Polypeptide | 终止：多肽的释放

When a stop codon (UAA, UAG, or UGA) enters the A site, no tRNA can bind to it. Instead, release factors (RFs) recognise the stop codon. In prokaryotes, RF1 or RF2 binds and stimulates the ribosome to hydrolyse the bond between the completed polypeptide and the tRNA in the P site. In eukaryotes, a single release factor eRF1 recognises all three stop codons.

当终止密码子 (UAA, UAG 或 UGA) 进入 A 位点时，没有 tRNA 能与之结合。取而代之的是释放因子 (RFs) 识别终止密码子。在原核生物中，RF1 或 RF2 结合并刺激核糖体水解已完成的肽链与 P 位点 tRNA 之间的键。在真核生物中，单一的释放因子 eRF1 识别所有三种终止密码子。

The newly synthesized polypeptide is released into the cytoplasm, and the ribosomal subunits dissociate, ready to be recycled for another round of translation.

新合成的多肽被释放到细胞质中，核糖体亚基解离，准备回收以进行下一轮翻译。

8. Post-Translational Modifications | 翻译后修饰

After translation, the polypeptide often undergoes folding and chemical modifications to become a functional protein. Molecular chaperones assist in correct folding. Modifications include cleavage of signal peptides, formation of disulfide bridges, addition of carbohydrate groups (glycosylation), phosphorylation, acetylation, and ubiquitination. These modifications can affect protein activity, stability, localisation, and interactions.

翻译后，多肽通常需要经过折叠和化学修饰才能成为有功能的蛋白质。分子伴侣协助正确折叠。修饰包括信号肽的切除、二硫键的形成、添加碳水化合物基团（糖基化）、磷酸化、乙酰化和泛素化。这些修饰可以影响蛋白质的活性、稳定性、定位和相互作用。

In many proteins, the initial methionine is removed, and the protein may be targeted to specific organelles such as the endoplasmic reticulum, mitochondria, or chloroplasts via signal sequences.

在许多蛋白质中，最初的甲硫氨酸被切除，蛋白质可能通过信号序列被靶向到特定的细胞器，如内质网、线粒体或叶绿体。

9. Comparison with Transcription | 与转录的比较

Aspect | 方面	Transcription | 转录	Translation | 翻译
Location | 位置	Nucleus (eukaryotes), cytoplasm (prokaryotes) | 细胞核（真核），细胞质（原核）	Cytoplasm on ribosomes | 细胞质中的核糖体
Template | 模板	DNA (template strand) | DNA（模板链）	mRNA | mRNA
Product | 产物	RNA (mRNA, tRNA, rRNA) | RNA（mRNA, tRNA, rRNA）	Polypeptide (protein) | 多肽（蛋白质）
Key enzyme/complex | 关键酶/复合物	RNA polymerase | RNA 聚合酶	Ribosome (peptidyl transferase) | 核糖体（肽基转移酶）
Direction of synthesis | 合成方向	5′ → 3′ | 5′ → 3′	N-terminus → C-terminus | N 端 → C 端
Start/stop signals | 起始/终止信号	Promoter and terminator sequences | 启动子和终止子序列	Start codon (AUG), stop codons (UAA, UAG, UGA) | 起始密码子 (AUG)，终止密码子 (UAA, UAG, UGA)

Although both processes read nucleic acid templates in a 5’→3′ direction, transcription produces an RNA copy of a gene, whereas translation decodes that RNA into a sequence of amino acids. Coupled transcription–translation can occur in prokaryotes because there is no nuclear membrane.

虽然两个过程都以 5’→3′ 方向读取核酸模板，但转录产生基因的 RNA 副本，而翻译则将该 RNA 解码为氨基酸序列。原核生物中转录与翻译可以偶联进行，因为没有核膜。

10. Regulation of Translation | 翻译的调控

Translation is tightly regulated at multiple levels. Control can occur through the stability of mRNA, the availability of translation initiation factors, and the presence of regulatory RNA molecules such as microRNAs (miRNAs) that bind to complementary sequences on mRNA and block translation or promote degradation. In response to cellular conditions, phosphorylation of initiation factors (e.g., eIF2) can inhibit global translation.

翻译在多个水平上受到严格调控。控制可以通过 mRNA 的稳定性、翻译起始因子的可用性以及调节性 RNA 分子（如 microRNA）的存在来实现，这些分子与 mRNA 上的互补序列结合，阻断翻译或促进降解。响应细胞条件，起始因子（如 eIF2）的磷酸化可以抑制全局翻译。

In prokaryotes, operons allow coordinated regulation of translation of multiple proteins from one mRNA, as seen in the lac operon. Riboswitches – structured RNA elements in the 5′ UTR of mRNA – can bind small molecules and alter translation efficiency.

在原核生物中，操纵子允许从一个 mRNA 上协调调控多个蛋白质的翻译，如乳糖操纵子所示。核糖开关——mRNA 5′ UTR 中的结构化 RNA 元件——可以结合小分子并改变翻译效率。

11. Clinical Relevance: Antibiotics Targeting Translation | 临床相关：靶向翻译的抗生素

Many antibiotics selectively inhibit prokaryotic translation without harming eukaryotic cells, exploiting differences in ribosomal structure. For example, tetracyclines block the binding of aminoacyl–tRNA to the A site; chloramphenicol inhibits peptidyl transferase activity of the 50S subunit; erythromycin binds to the 50S subunit and blocks translocation; and streptomycin causes misreading of the genetic code by binding to the 30S subunit.

许多抗生素选择性抑制原核生物的翻译而不伤害真核细胞，利用了核糖体结构的差异。例如，四环素类阻断氨酰 tRNA 与 A 位点的结合；氯霉素抑制 50S 亚基的肽基转移酶活性；红霉素与 50S 亚基结合并阻断移位；链霉素通过与 30S 亚基结合导致遗传密码的误读。

Understanding the molecular details of translation helps in designing new antimicrobial drugs and understanding resistance mechanisms, such as mutations in ribosomal RNA or enzymatic modification of the antibiotic.

理解翻译的分子细节有助于设计新的抗菌药物并理解耐药机制，例如核糖体 RNA 的突变或抗生素的酶促修饰。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

• Use precise terminology: always refer to ‘codon’ for mRNA triplets and ‘anticodon’ for tRNA triplets. Do not confuse the two. | 使用准确的术语：始终将 mRNA 的三联体称为“密码子”，将 tRNA 的三联体称为“反密码子”。不要混淆两者。

• Do not say that amino acids bind to codons; amino acids are attached to tRNA molecules via specific enzymes. | 不要说氨基酸与密码子结合；氨基酸是通过特定的酶连接到 tRNA 分子上的。

• The ribosome moves along the mRNA, not the other way round. Translocation is an active process driven by GTP hydrolysis. | 核糖体沿着 mRNA 移动，而不是反过来。移位是一个由 GTP 水解驱动的主动过程。

• Be clear about the direction of polypeptide synthesis: from amino (N) terminus to carboxyl (C) terminus. The mRNA is read 5′ → 3′. | 清楚地说明多肽合成的方向：从氨基 (N) 端到羧基 (C) 端。mRNA 按 5′ → 3′ 方向读取。

• Avoid stating that the mRNA ‘becomes’ a protein; it acts as a template and is not reused after translation (it may be recycled for multiple rounds). | 不要说 mRNA “变成”了蛋白质；它作为模板，翻译后不会被消耗（它可以循环进行多轮翻译）。

• When comparing prokaryotic and eukaryotic translation, note differences in ribosome size, initiation mechanisms, and coupling with transcription. | 当比较原核和真核翻译时，注意核糖体大小、起始机制以及与转录偶联的差异。

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