tRNA | tRNA Charging | tRNA Function | BIOCHEM I PART-7 | Protein Synthesis| Basic Science Series
Channel: Basic Science Series
Date: 2020-05-25

00:00 Introduction
00:18 Subscription request
00:33 Recap of the previous chapter (RNA editing)
00:52 Any question?
01:02 What is a tRNA molecule
01:46 Structure of tRNA molecule
02:55 tRNA Charging 
04:03 Coupling of amino acid to tRNA
04:20 Why specificity of coupling is important? 
04:50 Thank you note

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Transfer ribonucleic acid (tRNA) is a type of RNA molecule that helps decode a messenger RNA sequence into a protein.  tRNA is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length. IT serves as the physical link between the mRNA and the amino acid sequence of proteins. tRNA does this by carrying an amino acid to the protein synthetic machinery of a cell. 

Previous lecture on RNA Editing: https://youtu.be/1hjJGfRFh0o

It has a 5'-terminal phosphate group.
The acceptor stem of tRNA is a 7- to 9-base pair (bp) stem made by the base pairing of the 5'-terminal nucleotide with the 3'-terminal nucleotide (which contains the CCA 3'-terminal group used to attach the amino acid). 
Next is the D arm is a 4- to 6-bp stem ending in a loop that often contains dihydrouridine.
The anticodon arm of tRNA is a 5-bp stem and its loop contains the anticodon. 
And finally, the T arm, which is a 4- to 5- bp stem containing the sequence TΨC where Ψ is pseudouridine, modified uridine. 

Amino acid activation (also known as aminoacylation or tRNA charging) refers to the attachment of an amino acid to its Transfer RNA (tRNA).
In the first step, Aminoacyl transferase binds Adenosine triphosphate (ATP) to amino acid, and a pyrophosphate molecule is released. 
In the second step, Aminoacyl TRNA synthetase binds AMP-amino acid to tRNA. The AMP is used in this step. 
During amino acid activation the amino acids (aa) are attached to their corresponding tRNA.[1] The coupling reactions are catalyzed by a group of enzymes called aminoacyl-tRNA synthetases 

aa + ATP ⟶ aa-AMP + PP, (pyrophosphate)
aa-AMP + tRNA ⟶ aa-tRNA + AMP

The amino acid is coupled at the 3’-end of the tRNA via an ester bond. The formation of the ester bond conserves a considerable part of the energy from the activation reaction. This stored energy provides the majority of the energy needed for peptide bond formation during translation.

The specificity of the amino acid activation is as critical for translational accuracy as the correct matching of the codon with the anticodon. 
The reason is that the ribosome only sees the anticodon of the tRNA during translation. Thus, the ribosome will not be able to discriminate between tRNAs with the same anticodon but linked to different amino acids.  

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