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Before translation begins, amino acids are covalently attached to their tRNAs, which recognize codons in the mRNA that designate that amino acid. The process of attaching an amino acid to its tRNA is known as amino acylation. The amino acid is covalently linked to the tRNA’s acceptor arm, which always ends in 5′ CCA 3′.
Aminoacylation of tRNA
The amino acid’s carboxyl group connects to the 3′-hydroxyl of the acceptor arm’s terminal adenine. Aminoacyl tRNA synthesizes enzymes that catalyze the charging reaction, which requires ATP hydrolysis. Each amino acid has its own enzyme that can charge all of the amino acid’s iso-acceptor tRNAs.
The aminoacyl tRNA synthetase recognizes both the appropriate amino acid and the corresponding tRNA.
When the correct amino acid is connected to the tRNA, it identifies the amino acid’s codon in the mRNA, allowing it to insert the amino acid in the correct location as determined by the mRNA sequence. This ensures that the amino acid sequence encoded by the mRNA is correctly translated.
Codon recognition is accomplished through the tRNA’s anticodon loop, specifically three nucleotides in the anticodon loop that bind to the codon via complementary base-pairing.
A 3-pin plug with a socketed base is recognized in the same way as a full codon – anticodon fitting. Both the pin and the socket are highly specific. DNA’s four nucleotides can be combined to create 64 codons. Three codons indicate the completion of translation, while the remaining 61 code for the 20 amino acids found in proteins. As a result, most amino acids have multiple codons.
The cytoplasm contains inactive amino acids. When they obtain energy from ATP, they become active. When an amino acid binds to ATR, the reaction is triggered. This step is carried out by aminoacyl RNA synthetase, a specific activating enzyme.
A high-energy acyl bond is formed between the a-phosphate of ATP and the carboxyl group of an amino acid during the production of aminoacyl adenylate. ATP phosphates are converted into inorganic pyrophosphate.
The active amino acid is transported to the corresponding t-RNA. A high-energy ester bond is formed by the carboxyl group of the amino acid and the 3′-hydroxyl group of the terminal adenosine of tRNA. When the aminoacyl AMP-enzyme complex combines with the appropriate tRNA, the aminoacyl-tRNA complex is formed.
Aminoacylation is a two-step process that is catalyzed by a group of enzymes known as aminoacyltRNA synthetases. There are twenty aminoacyltRNA synthetases in each cell, one for each amino acid in the genetic code. In the first step of aminoacyltRNA production, ATP and the appropriate amino acid produce an aminoacyl adenylate intermediate. The enzyme inorganic pyrophosphatase catalyzes the breakdown of inorganic pyrophosphate to free phosphate. The aminoacyl adenylate intermediate is “high-energy,” and amino acid transfer to the acceptor end of tRNA occurs without the need for additional ATP in the second step.
The aminoacyltRNA synthetase edits tRNA to prevent mis-acylated tRNA from being used in protein synthesis. Because the ribosome must treat all aminoacyltRNAs the same in order to form the peptide bond, any tRNA containing the wrong amino acid would be used for protein synthesis, potentially resulting in the production of a dangerous protein. A second active site on the aminoacyltRNA synthetase edits aminoacyltRNAs for accuracy.
Enzymes in Acylation
The enzyme’s function is to cleave the incorrect aminoacyl tRNA and release free amino acids and tRNA. This process is similar to the editing that occurs during DNA synthesis due to the 3′5′ exonucleolytic activity of DNA polymerases. Like that procedure, aminoacyltRNA editing results in a “futile cycle,” in which the enzyme uses energy to build a link and then breaks it down. Both scenarios preserve information processing fidelity at the expense of energy “wastage.” Because mistakes are so harmful to the cell, the cost is reasonable.
Conclusion
The active amino acid is transported to the corresponding t-RNA. A high-energy ester bond is formed by the carboxyl group of the amino acid and the 3′-hydroxyl group of the terminal adenosine of tRNA. When the aminoacyl AMP-enzyme complex combines with the appropriate tRNA, the aminoacyl-tRNA complex is formed.
