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The below mentioned article provides a short note on the aminoacylation of tRNA.
Role of Transfer RNA:
Transfer RNA molecules play a key role in the process by delivering amino acids to the ribosome in an order specified by the mRNA sequence; this ensures that the amino acids are joined together in the correct order. Cells usually contain many species of tRNA, each of which binds specifically to one of the 20 amino acids.
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Consequently, there may be more than one tRNA for each amino acid. Transfer RNAs that bind the same amino acid are called iso-acceptors.
Before translation begins, amino acids become covalently linked to their tRNAs which then recognize codons in the mRNA specifying that amino acid. The attachment of an amino acid to its tRNA is called amino acylation or charging. The amino acid is covalently attached to the end of the acceptor arm of the tRNA which always ends with the base sequence 5′ CCA 3′.
A bond forms between the carboxyl group of the amino acid and the 3′-hydroxyl of the terminal adenine of the acceptor arm. Charging is catalyzed by enzymes called aminoacyi tRNA synthetizes in a reaction requiring the hydrolysis of ATP. A separate enzyme exists for each amino acid and each enzyme can charge all the iso-acceptors tRNAs for that amino acid.
The aminoacyi tRNA synthetase recognizes both the appropriate amino acid and the corresponding tRNA.
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When the correct amino acid has been attached to the tRNA, it recognizes the codon for that amino acid in the mRNA allowing it to place the amino acid in the correct position, as specified by the sequence of the mRNA. This ensures that the amino acid sequence encoded by the mRNA is translated faithfully.
Codon recognition takes place via the anticodon loop of the tRNA and specifically by three nucleotides in the loop known as the anticodon which binds to the codon by complementary base-paring.
The entire codon – anticodon fitting is comparable to recognition of a 3-pin plug with the socketed base. Both the pin and the socket are highly specific. The four bases present in DNA can combine as 64 codons. Three codons act as signals for translation to stop and the remaining 61 encode the 20 amino acids present in proteins. Consequently, most amino acids are represented by more than one codon.
Activation of Amino Acid and Attachment with tRNA:
Amino acid in cytoplasm occurs in inactive state. They are activated by gaining energy which comes from ATP. The reaction is brought about by the binding of amino acid with ATR. The step is mediated by specific activating enzyme known as aminoacyi RNA synthetase.
A high energy acyl bond is formed between the a-phosphate of ATP and the carboxyl group of amino acid with the formation of aminoacyi adenylate. The β and γ phosphates of ATP break away as inorganic pyrophosphate.
The activated amino acid is transferred to its specific t-RNA. A high energy ester bond is formed between the carboxyl group of the amino acid and the 3′-hydroxyl group of the terminal adenosine of tRNA. The aminoacyi AMP-enzyme complex reacts with the specific tRNA to form an aminoacyl-tRNA complex.
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(AMP –AA1)Enz, + tRNA → AA1 – tRNA, + AMP + Enz1
