Question

You have a front-row seat to translation occurring for a 250 amino acid protein. You notice that amino acid number 40, bound to the P site is an alanine. The next amino acid to be added to the growing protein chain will be a glycine. Since the tRNA carrying glycine has yet to reach the ribosome you know that:

A) glycine has a free n-terminus and alanine has a free n-terminus
B) glycine has a free c-terminus and alanine has a free c-terminus
C) glycine has both c and n-terminals free, where alanine has only a free c-teminus.
D) glycine has a free n-terminus, where alanine has both c and n-terminals free
E) both glycine and alanine have free c and n-terminals

Answer 1

The correct answer is D) glycine has a free n-terminus, where alanine has both c and n-terminals free, meaning the glycine is not yet part of the growing chain and has both terminals free, whereas alanine is part of the chain and only has a free C-terminus.

Step-by-step explanation:

If you are observing a 250 amino acid protein being translated and see that amino acid number 40, which is an alanine, is bound to the P site, and the next amino acid to be added is a glycine, it is important to understand the structure of the growing polypeptide chain. At this stage, the alanine at the P site is part of the growing polypeptide chain and, therefore, has its N-terminus linked to the previous amino acids in the chain. The glycine, represented by the incoming aminoacyl-tRNA to the A site, has both its N-terminus and C-terminus free since it has not yet formed a peptide bond with the alanine or any other amino acid. In peptide bond formation, the carboxyl group of the P-site amino acid (alanine) will bond with the amine group of the A-site amino acid (glycine).

The correct answer to the question is therefore: D) glycine has a free n-terminus, where alanine has both c and n-terminals free.