2. CONTENTS
• Overview of translation
• What is proofreading
• Proofreading at protein level
• Types of proofreading
• Previously asked questions
• References
3. TRANSLATION
• The process of synthesizing an amino acid sequence from messenger rna
which carries information from the genetic material containing genes.
4.
5. Proofreading
• Proofreading in translation can be defined as the mechanism which
works simultananeously along with the synthesis of proteins, which
helps in the correction of any incorrect amino acid which charges the
trna.
8. CHEMICAL PROOFREADING- This type of editing can also be called as double sieve
model , proposed by Fersht and Kaethner
9. CLASS 1 and CLASS 2 (AARS)
• Two classes of AARS (amino acyl trna synthetase) are present which
attaches the amino acids to the trna at different sites. Class 1 attaches
the amino acid at the 2’ end of the trna , whereas the Class 2 AARS
attaches the amino acid at the 3’ end of the trna of the base adenine ,
as CCA is the sequence present in the acceptor arm
• Class 1 recognises amino acids like – Arg, Cys, Glu, Glutamic acid, leu,
iso-leu, try, where as Class 2 recognises amino acids like – Gly, Pro,
Ala, Lys, Phe etc.
10. KINETIC PROOFREADING
• Codon anticodon basepairing
• 16 rRNA forms H bonds with minor groove of codon : anticodon
duplex only when correctly paired.
• Correct base pairing allows EF-TU bound to aa- tRNA to interact with
factor binding center inducing GTP hydrolysis and EF-TU release.
• In case of incorrect base pairing FBC ( factor binding site ) will not be
contacted, and will allow more time for EF-TU GTP release
11. Mechanisms that ensure the correct pairing
between tRNA and mRNA
• To release EF-TU , its GTP must be hydrolysed . Mismatches in the
codon-anticodon pairing alter the position of EF-TU preventing its
interaction with the factor- binding center and reducing its GTPase
activity.
• After release of EF-TU, the tRNA must rotate the aa towards the P-site
in a process called accommodation. Incorrectly paired tRNA will often
dissociate in this process.
• The error rate of translation is approximately one incorrect amino
acids inserted per 2000 residues
12. Quality control mechanisms
• One mechanism of quality control over translation is a type of
proofreading that takes place at the A site.
• In the elongation process the charged trna comes to the A site bound
with molecule of EF-TU GTP.
• The anticodon of the charged trna has to fit tightly into the active
sites to promote the formation of peptide bonds.
• When the codon and anticodon does not match however hydrolysis
of GTP is reduced which allows enough time for the incorrect charged
trna to diffuse away and replaced with the correct ones. This type of
proofreading at A site is called as KINETIC PROOFREADING.
