Question

The chemical mechanisms used to avoid errors in protein synthesis are different from those used during DNA replication. DNA polymerases use a 3 ′ to 5 ′ exonuclease proofreading activity to remove mispaired nucleotides incorrectly inserted into a growing DNA strand. There is no analogous proofreading function on ribosomes. Instead, proofreading for protein synthesis is carried out by the aminoacyl‑tRNA synthetases. ldentify the mechanisms aminoacyl-tRNA synthetases use to ensure that the correct amino acid is attached to the appropriate tRNA.(A) To distinguish between two amino acids with similar structures, some aminoacyl-tRNA synthetases use a proofreading site that recognizes and hydrolyzes the incorrect amino acid.(B) The active site in each aminoacyl-tRNA synthetase has a high specificity for the correct amino acid substrate. After binding the correct amino acid, an aminoacyl-tRNA synthetase recruits the appropriate tRNA to the active site by recognizing a number of sequences and structural features in a specific tRNA.(C) Aminoacyl-tRNA synthetases have an additional active site that binds to non-cognate tRNAs. The tRNAs that bind to this second active are hydrolyzed and released from the enzyme.(D) First, the aminoacyl-tRNA synthetase binds to its cognate tRNA by recognizing a number of specific sequences and structural features in the tRNA. Next, the synthetase-tRNA complex recruits the correct amino acid to the enzyme active site.(E) Aminoacyl-tRNA synthetases can rapidly hydrolyze the ester linkage between incorrectly paired RNAs and amino acids

Answer 1

Answer:

(C) Aminoacyl-tRNA synthetases have an additional active site that binds to non-cognate tRNAs. The tRNAs that bind to this second active are hydrolyzed and released from the enzyme.

Explanation:

In case of translation, proof reading is done by aminoacyl-tRNA synthetases only. Aminoacyl-tRNA synthetases have two mechanisms to avoid error during translation which are mentioned as under:

(1) Chemical proof reading: Incorrect amino acids rather than being hydrolyzed in catalytic pocket get hydrolyzed in editing pocket and thus they hardly get attached to tRNA.  

For example: For distinguishing similar amino acids like isoleucine and valine, isoleucyl-tRNA synthetase uses a second active site which is meant for only valine not for isoleucine. In this particular site, valine which had entered the enzyme is cleaved away with the help of editing reaction after which the enzyme is well prepared to process isoleucine which is the correct amino acid for this enzyme.

(2) Kinetic proof reading: Even if an incorrect amino acid has entered a particular aminoacyl-tRNA synthetase, it does not cause appropriate conformational change in the enzyme because of which the incorrect amino acid loosens from the enzyme and does not get incorporated.

Note: In this example, only chemical proof reading is mentioned not kinetic proof reading.