Question

Puromycin is a powerful inhibitor of protein synthesis. It is an analog of the 3’ end of aminoacyl tRNA. When puromycin is added to a cell-free system containing all the necessary machinery for protein synthesis, incomplete polypeptide chains are released from ribosomes. Each such chain has py y uromycin covalently attached to one end.

a) Explain these results.

b) To which end of the polypeptide chains would you expect the puromycin to be bound? Explain fully.

c) Would you expect puromycin to bind to the A or P site on the ribosome, or to both? Explain fully.

Answer 1

a- Puromycin resembles its structure with the nucleoside monophosphate and its structure is somewhat same to the 3' end of tRNA. Due to its structure similarity, it can easily bind to the A site of the ribosome. As a result of binding ribosome attaches puromycin to the synthesizing peptide. Upon binding to the peptide it moves to the P site. Since ribosome can cleaves only the ester linkage which is found between the Amino-acid and tRNA and not the amide linkage which is found between the puromycin and the peptide, the peptide prematurely released from the ribosome.

B- As we have seen that the puromycin is the last compound that is bound to the peptide chain. So, it binds to the C-terminal of the peptide.

B- Puromycin binds to the A- site of the ribosome as it mimics the 3' end of the tRNA. once the ribosome attaches it to the peptide it moves to the P-site of the ribosome. Upon reaching the P-site, ribosome releases the peptide since it cannot cleave the amide bond which is formed between the peptide and the puromycin.