(64)
14. Beskrive termineringsfasen (stopkodon i A-sitet + releasefaktorer)
Devlin, s.251
Stryer, s. 835-6, fig. 29.31
 

The final phase of translation is termination, when a stop codon of mRNA occupies the A-site.

Aminoacyl tRNA does not normally bind to the A site of a ribosome if the codon Is UUA, UAG or UGA, because normal cells do not contain tRNAs with anticodons complementary to these trop signals. Instead, the stop codons are recognized by release factors.
 

The release factors are G-proteins that mimic the tRNA structure. They use ATP to transfer a water molecule to the growing polypeptide chain (the reaction of transfering a normal amino acid to the growing polypeptide does not require ATP since the energy has already been stored when the amino acid is acitvated).

The realease factors contain a special tRNA fold that contains a water molecule that can be brought in the peptidyltransferase center and hydrolyse the ester bond between the carboxy-group of the amino acid (peptide chain) and the OH-group of the adenylate ribose. The water molecule is basically transferred to the growing polypeptide chain and forms its –COOH end.

The polypeptide chain is then freed. GTP of the release factor is hydrolyzed GDP and Pi, presumably altering the confirmation of the release factor. The entire complex then disassociates.

So, the release factors free the polypeptide chain and disassemble the ribosome.

tilbage til molekylær biologi