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How aminoacyl-tRNA synthetases
translate the genetic code.
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Protein synthesis requires a continual
supply of aminoacylated tRNAs.
These are used by the ribosome to
synthesise the polypeptide chain
corresponding to the sequence of
nucleotide triplets on the messenger RNA.
This process actually involves a two-step
decoding of the genetic code, and
both steps have to be highly accurate
in order to guarantee the fidelity of
protein synthesis.
First of all enzymes known as
aminoacyl-tRNA synthetases
ensure the correct
identification of an amino
acid with its corresponding
tRNA anticodon triplet.
Synthetases do this by covalently
attaching the amino acid to the 3'
end of its cognate tRNA,
a process known as aminoacylation.
The second step is ensuring that
the tRNA anticodon is correctly paired
with the codon on the mRNA,
this is the job of
the ribosome using a mechanism described
in another talk in this series.
The ribosome does not check the identity
of the amino acid attached to a tRNA,
so if a synthetase charges
a tRNA with a wrong amino acid,
this can immediately lead to
an error in protein sequence.
The overall error rate in protein
synthesis is about 1:10,000 amino acids.
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The synthetases aminoacylate tRNA
in a two-step chemical reaction,
both steps of which occur in
the same active site on the enzyme.
Firstly, the amino acid is
activated using ATP to form
an intermediate called
the aminoacyl-adenylate,
this remains tightly bound to the enzyme
while pyrophosphate is released.
In the second step, the ribose of
the terminal adenosine at the 3'
end of the tRNA is charged with
the amino acid, at either the 2' or
3'-hydroxyl, with the concomitant
formation of AMP.
The aminoacylated tRNA can then leave
the enzyme and is subsequently escorted
to the ribosome by elongation factor
Ef-Tu, to be used in protein synthesis.
