tRNA charging, or amino acid activation, is essential to initiate protein synthesis and translation. Since synthesising peptide bonds is a thermodynamically unfavourable endergonic process, amino acid activation is necessary by covalent bonding to tRNA molecules. Peptide bond production is fueled by the energy contained in the aminoacyl-tRNA bond. Thus, the amino acid becomes more reactive due to activation, which promotes the formation of peptide bonds.
Aminoacyl-tRNA is a type of tRNA (also known as charged tRNA or aa-tRNA) with a charged chemical bonding with its cognate amino acid. The amino acid is brought to the ribosome by the aa-tRNA and is then incorporated into the polypeptide chain that is created during translation by certain elongation factors.
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tRNA Meaning
Transfer RNA, often known as tRNA, is a form of RNA that helps in protein synthesis from the mRNA. It was previously referred to as soluble RNA or sRNA. During translation, tRNA performs the role of an adaptor molecule. It serves as an adaptor by combining amino acids and nucleic acids. It carries the amino acid to be included in the peptide chain and identifies the corresponding codon in the mRNA molecule.
The three hairpin loops that constitute the distinct folded shape of the tRNA molecule give it the appearance of a three-leafed clover. A section of these hairpin loops known as the anticodon can identify and decode any mRNA codon. Each tRNA is ended with the appropriate amino acid attached.
The tRNA adopts an “L” shape structure in 3D, with the anticodon on one end and the acceptor end on the other.
tRNA Charging Meaning
The process of attaching an amino acid to its corresponding transfer RNA (tRNA) is called amino acid activation, also known as tRNA charging or aminoacylation.
The aminoacyl tRNA synthetase enzyme catalyses the bond of adenosine triphosphate (ATP) to its respective amino acid, creating a reactive intermediate of aminoacyl adenylate (AMP-amino acid) and producing inorganic pyrophosphate (PPi).
The AMP-amino acid is then attached to a tRNA molecule by aminoacyl tRNA synthetase, which also releases AMP. The resultant aminoacyl-tRNA is considered charged.
Mechanism of tRNA Charging
Steps
Aminoacyl-tRNA synthetases are a class of enzymes that catalyse the coupling process. Each amino acid (aa) is linked to its appropriate tRNA molecule during amino acid activation. There are two steps in the coupling reaction:
The carboxyl group of the amino acid forms a covalent bond to the ATP molecule’s α-phosphate, generating inorganic pyrophosphate (PPi) and producing a 5’ aminoacyl adenylate intermediate (aa-AMP).
aa + ATP ⟶ aa-AMP + PPi
Next, a nucleophilic attack on the intermediate of aminoacyl adenylate results in the attachment of an aminoacyl group to the 3′-OH of the tRNA and the release of an AMP molecule. Thus, the tRNA molecule produced is now “charged” and prepared for use.
aa-AMP + tRNA ⟶ aa-tRNA + AMP
Class I and class II are the two categories of aminoacyl t-RNA synthetases. The resultant aminoacyl-tRNA molecule remains the same, irrespective of the type of enzyme used.
The overall reaction is:
aa + ATP + tRNA ⟶ aa-tRNA + AMP + PPi
Aminoacyl-tRNA Synthetase Enzymes
The structure of the enzymes varies greatly, but they always carry out a similar activity by binding ATP, an amino acid, and its associated tRNA. The specific aminoacyl-tRNA synthetase recognises the 20 amino acids. The protein subunits that constitute synthetases range from one to four.
Transfer RNA (tRNA)
According to Francis Crick’s adaptor hypothesis, the tRNA serves as an adaptor during the activation process. In other words, the tRNA binds to the desired amino acid at one end and the mRNA codon sequence at the other. The tRNA molecule is a link between the two to translate the genetic code into an amino acid sequence.
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