BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 6. AMINO ACID METABOLISM AND FUNCTIONS. PROTEIN BIOSYNTHESIS

6.10. Protein biosynthesis

6.10.7. Translation termination

The elongation phase on the ribosome continues until stop codons are reached in the mRNA molecule: UAA, UGA, UAG. These do not code for Amino Acids and, therefore, cannot interact complementarily with corresponding aminoacyl-tRNAs, thereby promoting the termination of Protein Synthesis. Protein termination factors, known as releasing factors (RF), are involved in this process. Prokaryotes have three such factors: RF-1 (molecular mass 47·10³), RF-2 (molecular mass 35·10³-48·10³), and RF-3 (molecular mass 46·10³). They participate in activating the peptidyl esterase function of the ribosome, resulting in the release of the free polypeptide and deacylated tRNA, as well as the dissociation of the 70S ribosome into two subunits.

The RF-1 factor can recognize the UAG and UAA codons, while RF-2 recognizes UGA and UAA. Thus, once the stop codons enter the A-site of the ribosome, the corresponding protein termination factor binds to it (likely due to the molecular mimicry of certain domains of these factors with the corresponding structures of tRNA molecules). This induces a shift in the Specificity of peptidyltransferase activity, catalyzing The transfer of the peptide chain to a Water molecule, which is followed by the Hydrolysis of the ester bond between the peptidyl-tRNA and the synthesized peptide. A GTP molecule participates in termination, the hydrolysis of which is catalyzed by the RF-3 release factor. As a result, the synthesized polypeptide, along with the termination factors, is released from the ribosome, which in turn dissociates into two subunits (Fig. 6.46).

In eukaryotes, the termination process involves factors such as eRF-1 (molecular mass 48.5·10³), an analog of RF-1 and RF-2, and eRF-3 (molecular mass 70·10³). eRF-1 recognizes all stop codons in the ribosomal A-site, whereas eRF-3 catalyzes GTP hydrolysis. The termination process in eukaryotes proceeds similarly to that in prokaryotes, though with certain distinct features. A connection has been discovered between eRF-3 and the poly(A)-binding protein (PABP), which is involved in Translation initiation. This finding suggests the possibility of translation reinitiation on pseudo-closed (circular) eukaryotic mRNA molecules.

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Fig. 6.46. Translation termination in prokaryotes:

protein termination factors RF-1, RF-2, RF-3

It should be noted that translation factors exerting their effects through GTP hydrolysis are members of the G-protein family, which also includes Proteins involved in signal Transduction. Upon binding GTP, these proteins are functionally active and participate in various metabolic processes, whereas the hydrolysis of GTP to GDP in their active center converts them into inactive forms.

Thus, the template process of translation is the sequential delivery of aminoacyl-tRNAs to the ribosome for protein synthesis, which is strictly determined by the mRNA—meaning the order of codons in the mRNA chain dictates The Structure of the synthesized protein. The ribosome scans the mRNA chain in triplets and sequentially selects the appropriate aminoacyl-tRNAs, releasing deacylated tRNAs during elongation. The large and small subunits perform distinct Functions during translation: the small subunit binds mRNA and decodes the information, while the large subunit is responsible for peptide bond formation.