protein synthesis

Biosynthesis of proteins from genetic information on DNA transcribed mRNA

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protein Synthesis refers to the process by which a living organism follows Deoxyribonucleic acid Transcribed Messenger ribonucleic acid (mRNA) genetic information Synthetic protein Process. Protein Biosynthesis It is also called translation, that is, putting the mRNA into Base The sequence is changed into protein or Polypeptide chain In sequence of amino acid Process.

This is gene expression The second step of Gene product The final stage of protein. different Histiocyte Have different physiological function , because they express different genes, produce proteins with special functions, and participate in Protein Biosynthesis There are at least 200 kinds of components of tRNA 、 Riboribosome And related enzymes and protein factors.

Chinese name: protein synthesis
Foreign name: Protein Synthesis
Synthesis process: The base sequence is changed into protein

Translation template: Molecular size of different mRNA sequences
Genetic code table: In the open reading frame region of mRNA
Main body: mRNA、tRNA

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Synthesis process

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protein synthesis

prokaryote And Eukaryote There are many differences in the protein synthesis process of prokaryotes. Eukaryotes have a more complex process. The following focuses on the protein synthesis process of prokaryotes, and points out the differences between eukaryotes and prokaryotes. Protein Biosynthesis It can be divided into five stages: amino acid activation Polypeptide chain Synthesis initiation, peptide chain extension Peptide chain Termination and release of protein, and processing and modification after protein synthesis.

Direct template

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Translation template

protein biosynthesis

Different mRNA sequences Molecular size and Base The arrangement order is different, but all have 5 ˊ - end Non translation area , open reading frame area, and 3 ˊ - end non translation area; Eukaryote The 5-terminal of mRNA of Hat structure 3 ˊˊˊˊˊˊˊˊˊˊˊˊˊˊˊˊ Polyadenylate (polyA) tail. The hat structure can be combined with the hat and participate in the mRNA ribosome Positioning combination on, start protein Biological synthesis; The cap structure and ployA tail also play a stabilizing role RNA ； The open reading frame region corresponds to the gene sequence encoding the protein.

Genetic code table

In the open reading frame region of mRNA nucleotide Is a group, representing an amino acid or Other information This triad situation is called codon. The general open reading frame area contains more than 500 Codon 。

Characteristics of genetic code

one directional : codon and its components Base The arrangement in the mRNA sequence is directional, and the reading direction during translation can only be 5 ˊ → 3 ˊ.

two Continuity : each on the mRNA sequence Codon The bases of codon and codon are arranged in a continuous manner. There is no space between the bases of codon and codon. Each base is read only once, without overlapping reading.

three Degeneracy An amino acid can have two or more codons for its coding. Genetic code The table shows that each amino acid has 2, 3, 4 or 6 codons for its coding (except methionine But each codon only corresponds to one amino acid, or to termination information.

IV. Universality: Biosphere Almost all creatures of

V. Oscillation: tRNA The last bit of the is incompletely corresponding to the mRNA, resulting in Degeneracy

Synthesis site

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ribosome Like a small mobile factory, peptide chains are rapidly synthesized along the mRNA template. Aminoacyl tRNA enters the ribosome at a very high rate amino acid go to Peptide chain The ribosome is discharged from another position, Elongation factor It also constantly combines with and dissociates from ribosomes. Ribosomes and Additional factor One provides the active region for each step of protein synthesis.

Related Introduction

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Amino acid activation

protein synthesis

Before the synthesis of polypeptide chain, it must be activated first, and then combined with its specific tRNA to bring it to the corresponding position of mRNA. This process relies on tRNA synthesis Enzyme catalysis This enzyme catalyzes the combination of specific amino acids with specific tRNA to generate various aminoacyl tRNAs synthetase Catalyzed by ATP Energy supply, in amino acid carboxyl To form aminoacyl AMP, and then combine with aminoacyl tRNA synthetase to form a triplet complex, which then reacts with specific tRNA to transfer aminoacyl to tRNA Amino acid arm (i.e. 3 '- end CCA- OH ）On.

brief introduction

Prokaryotic cell After the activation of the starting amino acid in the middle, it will be formylated to form Formyl methionine tRNA， By N10 Formyl Tetrahydrofolate Provider A acyl 。 and Eukaryotic cell There is no such process. As mentioned earlier, a group of different tRNAs carrying the same amino acid is called Isomorphism tRNA。 A group of isofunctional tRNAs is catalyzed by the same aminoacyl tRNA synthetase. Aminoacyl tRNA synthetase is specific to both tRNA and amino acids. It has high specificity for amino acid recognition, but low specificity for tRNA recognition. How does aminoacyl tRNA synthetase choose the right amino acid and tRNA? According to the general principle, the correct combination of enzyme and substrate is the combination of both geometry It is determined that only suitable amino acids and suitable tRNAs enter the corresponding sites of synthetase, can correct aminoacyl tRNAs be synthesized. Synthases are known to bind to the medial side of L-shaped tRNA, and the binding points include the proximity arm, DHU arm and Anticodon arm 。 Aminoacyl tRNA synthetase and tRNA It can be seen from the interaction of amino acid acceptor. At first glance, Anticodon It seems that it should be related to the correct load of amino acids, which is true for some tRNAs. However, this is not the case for most tRNAs. It has long been known that when the anti codon on some tRNAs mutates, the amino acids they carry remain unchanged. Hou Zhiminghe in 1988 Schimmel Experimental proof of alanine G3: U70 on the amino acid arm of tRNA acid molecule Base The mutation affects the correct recognition of alanyl tRNA synthetase, indicating that G3: U70 is the main factor determining the essence of alanine tRNA molecule. The region on the tRNA molecule that determines the amino acid it carries is called Paracodon 。 An aminoacyl tRNA synthase can recognize a group of isofunctional tRNAs, which indicates that they have common characteristics. For example, three alanine tRNAs (tRNAAmm/CUA, tRNAIm/GGC, tRNAIn/UGC all have G3: U70 codon.) However, there is no sufficient evidence that other aminoacyl tRNA synthetases also recognize the same paracodon in the homologous tRNA group. In addition, there is no fixed position of the paracodon, or there may be more than one Base pair 。

Polypeptide chain

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Ribosome size Subunit ，mRNA Initial tRNA and Starting factor Joint participation Peptide chain Start of synthesis.

1、 Escherichia coli Cells Translation initiation complex Formation process:

⑴ Ribosome 30S Small subunit Attach to the mRNA starting signal site: prokaryote Each mRNA in Ribosome binding site It is a short segment located at 8-13 nucleotides upstream of AUG called SD sequence 。 This sequence is just complementary to a part of the 16S rRNA3 'end sequence in the 30S small subunit, so the SD sequence is also called Ribosome binding sequence This complementarity means ribosome It can select the correct position of AUG on mRNA to start the synthesis of peptide chain, and the binding reaction is Starting factor 3 (IF-3). In addition, IF-1 promotes the combination of IF-3 and small subunits, so IF3-30S subunit mRNA is formed first Ternary complex 。

(2) Before 30s Starting complex Formation of, Formyl Methionyl initiation tRNA And MRNA molecule AUG in, namely Codon And Anticodon At the same time, IF3 falls off from the ternary complex to form the initial complex before 30S, that is, IF2-3S subunit mRNA-fMet-tRNAfmet complex. This step requires GTP And Mg2+.

protein synthesis

⑶70S Starting complex Formation of: the 30S pre initiation complex mentioned above of the 50S subunit is combined, and IF2 falls off to form the 70S initiation complex, that is, the 30S subunit-mRNA-50S subunit-mRNA-fMet-tRNAfmet complex. At this time, fMet tRNAfmet occupies the peptidyl site of 50S subunit. The A position is empty, waiting for the corresponding aminoacyl tRNA of the second code in the corresponding mRNA to enter, thus entering the extension phase.

2、 Eukaryotic cell The initiation of protein synthesis The formation of eukaryotic protein synthesis initiation complex requires more initiation factors, so the initiation process is more complex.

(1) Special Initial tRNA That is, - tRNAfmet, and does not need N-end Formylation. Discovered Eukaryotic initiation factor Nearly 10 species (eukaryote Initiation factor, eIF)

⑵ Starting complex Formed upstream of AUG at mRNA5 'end Hat structure , (except for some viral mRNA)

⑶ ATP hydrolysis It supplies ADP with the energy required for mRNA binding.

Eukaryotic cell The formation process of the initial complex is:

Translation Start It is also combined with eIF-3 in 40S Small subunit Upward promotion 80S ribosome 60 s dissociation Large subunit At the beginning, eIF-2 binds to Met-tRNAfmet and GTP under the action of eIF-2, and then binds to 40S small subunit and then to mRNA through the action of eIF-3 and eIF-4C. When mRNA binds to the 40S small subunit, eIF-1, eIF-4A and eIF-4B are required in addition to eIF-3, and are hydrolyzed by ATP to ADP and Pi Energy supply And transferred to small subunits through the cap binding factor and the cap binding of mRNA. However, no S-D sequence that can pair with small subunit 18SRNA was found at the end of mRNA5 '. After cap binding, the mRNA moves upstream and downstream on the small subunit for scanning, which can fix the starting code AUG on the mRNA at the anti code position of Met tRNAfmet Translation Start 。

Peptide chain steps

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The extension of polypeptide chain Polypeptide chain Each addition of an amino acid requires three steps: carry, transpeptide and translocation The codon specific amino acid tRNA binds to Ribosome The A bit of is called carry. Aminoacyl tRNA needs three types before carry Elongation factor The function of, that is, the heat unstable E (Unstable temperature, EF) EF-Tu ， Thermal stability EF (stable temperature EF, EF-Ts ）And relying on GTP Transposition factor 。 EF Tu first combines with GTP, and then combines with aminoacyl tRNA to form a ternary complex, such as Ternary complex To enter position A. At this time, GTP is hydrolyzed into gross domestic product , EF Tu and GDP were separated from aminoacyl tRNA bound at the A position.

Peptide Chain Details

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Processing and finishing

⑴ N-end Formyl methionine or methionine Removal of: N-terminal methionine is Polypeptide chain Start of synthesis amino acid , must be folded into a certain space structure Previously removed. The process is: ① Deformylation; ② Egg free acyl group.

⑵ Amino acid modification: Specificity Of Enzyme catalysis Decorate, including Glycosylation 、 Hydroxylation Phosphorylation, formylation, etc.

⑶ Disulfide bond Formation of: catalyzed by specific oxidase- SH Oxidation to - S-S.

⑷ Peptide segment resection: from specific protease Catalysis, partial peptide segments are removed.

Formation of advanced structure

(1) Formation of conformation: Intramolecular chaperone , auxiliary enzyme and Molecular chaperone With the help of, form a specific spatial conformation.

⑵ Subunit Polymerization. ⑶ Cofactor Connection of.

Targeted transport

protein After synthesis, it is directionally transported to performing function The site of is called target delivery. In most cases, the transported protein molecules need to pass through the membrane structure to reach a specific location. Therefore, in the amino End, generally with a hydrophobic peptide segment, called signal peptide 。 Secretory type The directional transport of protein depends on signal peptide And Cytoplasm In Signal peptide recognition particle （ SRP ）Recognize and specifically bind to the docking protein on the membrane through SRP（ DP ）After recognition and binding, the protein carried is sent out of the cell.

Signal peptide hypothesis : Signal peptide is located at Neosynthesis Of Secretory protein N end. Protein secreting Targeted transport Play a decisive role. ① Intracellular signal peptide recognition particle (SRP) recognizes signal peptide, temporarily stops peptide chain synthesis, and SRP guides Ribosome Joint roughness Endoplasmic reticulum Membrane; ② SRP recognizes and binds to the docking protein on the endoplasmic reticulum membrane, hydrolyzes GTP to separate SRP, and the polypeptide chain continues to extend; ③ After the signal peptide guided lengthening polypeptide enters the endoplasmic reticulum cavity Signal peptidase Excision. Secreted protein Golgi apparatus Packed into secretory granules Exocytosis 。

Biological regulation

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Regulation of protein synthesis

The speed of protein synthesis in the organism is mainly at the transcription level, followed by regulation and control in the process of translation. It is affected by gender, hormone cell cycle 、 Growth and development , health status, living environment and many other factors, as well as many biochemical substances involved in protein synthesis. because prokaryote The translation and transcription of coupling Because of the short life span of their mRNA, the speed of protein synthesis is mainly determined by the speed of transcription. Weakening effect It is a way to regulate the speed of translation by first affecting transcription through the excess and deficiency of translation products. The structure and properties of mRNA can also regulate the rate of protein synthesis.

HCR two states

Eukaryote Transcription and translation are not coupled, usually protein synthesis Speed ratio prokaryote Slow. Eukaryotes, except mainly through transcription and Post transcriptional processing And the structure and properties of mRNA (such as Hat structure And poly A tail, etc.) (see messenger RNA) Globin biosynthesis The research shows that the eukaryotic initiation factor eIF-2 is the translation speed Limiting factor Therefore, the factors affecting eIF-2 can adjust the speed of translation. use mammal Reticulocyte In vitro studies on cell-free preparations of hemoglobin , not necessary Synthetic protein 。 Experimental proof hemoglobin The regulation of Repressor (HCR). HCR has active and inactive states.

Effect of heme

Heme regulates proteins by influencing eIF-2. When heme exists, it inhibits the synthesis of cellular proteins, and can also promote the synthesis of proteins in cells that usually do not synthesize hemoglobin, such as liver cancer Cells Hella cell and ascites Tumor cell Protein synthesis without cellular agents.

Protein inhibitor

Inhibitors of protein biosynthesis Many proteins biosynthesis Inhibitors are highly specific, which is important for studying the synthesis mechanism. Many clinically effective antibiotics are specifically inhibited prokaryote They inhibit bacterial growth without damaging Human cells 。 Using the difference in the synthesis of two kinds of biological proteins, we can find out the drugs for treating diseases caused by bacterial infection. Some important protein biosynthesis are listed in the table inhibitor And Action site And specificity.

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