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Here is the question: How many GTP are necessary to synthesize 2 linear peptides of 11 amino acids each using an in vitro prokaryotic system? Assume the tRNAs provided are already loaded with their proper amino acids. All 3 steps of the protein synthesis reaction must be completed.

My answer was 46. I thought that translation initiation uses 2 GTPs (the GTPs on eIF2 and eIF5B are hydrolyzed). Elongation involves 2 GTPs for each amino acid after the first methionine (the GTPs on EF1𝛂 and EF2 are hydrolyzed) - there are 10 amino acids left, so 2x10=20 GTPs are used. Termination involves 1 GTP (the GTP on eRF3 is hydrolyzed). So for one linear chain of 11 amino acids, 2+20+1=23 GTPs are used, and for 2 chains, 46 GTPs are used.

The correct answer is 44. I'm not sure where in my reasoning I went wrong. In what steps were there less GTPs used, and which proteins instead did not actually have GTP hydrolysis? Thank you.

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    $\begingroup$ Welcome to SE Biology. Please finish reading the Tour to learn how this site works and what sort of questions are suitable. The question specifies prokaryotic translation but all the factors you mention are eukaryotic (that's what 'e' stands for). Despite that, you are correct. It looks as if whoever set the question forgot about termination. However I am voting to close this question rather than answering it because the usage of GTP in protein sythesis is standard text-book material, and this site exists to answer points about biology, not to discuss poor or sloopy teaching. $\endgroup$
    – David
    Dec 1, 2023 at 14:22

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