ID #2129

A homework problem asked: "Which one of the following steps in elongation is directly dependent on ATP or GTP hydrolysis?" I picked polypeptide bond formation but the correct answer is "attachment of charged tRNA to A site." I am confused, doesn't attaching tRNA's just involve hydrogen bonding? Why is peptide bond formation not the correct answer?

The tRNA is bound to an elongation factor (EF-Tu) which is bound to GTP. If the anticodon matches the codon, then the ribosome deems it a "good fit", the tRNA leans backwards and the ribosome with GTPase activity hydrolyzes GTP and the tRNA changes conformation, dissociates from EF-Tu and EF-Tu leaves. Peptide bond formation does not require a direct input of energy because the bond between the amino acid and the tRNA in the P site is broken which provides energy for the formation of a new bond. There is energy "built in" since ATP was used to charge the tRNA and that energy is then used when the bond between the tRNA and the amino acid is broken.

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