Elongation

<p>The mechanism of <b>elongation in prokaryotic and eukaryotic cells is very similar</b> (Figure 7.11). The ribosome has three sites for tRNA binding, designated the P (peptidyl), A (aminoacyl), and E (exit) sites. The initiator methionyl tRNA is bound at the P site. The first step in elongation is the binding of the next aminoacyl tRNA to the A site by pairing with the second codon of the mRNA. The aminoacyl tRNA is escorted to the ribosome by an elongation factor (EF-Tu in prokaryotes, eEF-1α in eukaryotes), which is complexed to GTP. The GTP is hydrolyzed to GDP as the correct aminoacyl tRNA is inserted into the A site of the ribosome and the elongation factor bound to GDP is released. The requirement for hydrolysis of GTP before EF-Tu or eEF-1α is released from the ribosome is the rate-limiting step in elongation and provides a time interval during which an incorrect aminoacyl tRNA, which would bind less strongly to the mRNA codon, can dissociate from the ribosome rather than being used for protein synthesis. Thus, the expenditure of a high-energyGTP at this step is an important contribution to accurate protein synthesis; it allows time for proofreading of the codon-anticodon pairing before the peptide bond forms.

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<b>Elongation

Elongation</b> is the stepwise addition of amino acids to the growing protein chain. The order of amino acids is specified by the sequence of codons in the mRNA,

<i>

Protein factors, eEFs (eukaryotic elongation factors)</i>, are necessary to speed up the elongation cycle. The elongation process proceeds in the following order:

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1. Amino acid-containing tRNA molecules (aminoacyl-tRNAs, aa-tRNA) are picked up by elongation factor eEF-1 in the presence of GTP.
   
2. The formed complex enters the empty A-site on a ribosome carrying an initiator Met-tRNAi or a peptidyl-tRNA.
   
3. On the ribosome, the anticodon of the incoming aminoacyl-tRNA is matched against the mRNA codon positioned in the A-site. As the three bases in the codon can be arranged in 64 different combinations, the translational machinery must be able to select the aminoacyl-tRNA carrying the matching anticodon. During this proof-reading, aminoacyl-tRNAs with non-cognate anticodons are thrown out of the ribosome and replaced by new aa-tRNAs that are to be checked.
   
4. When the right aminoacyl-tRNA enters the A-site the growing polypeptide in the P-site is almost immediately linked to the new amino acid in the A-site via a peptide bond. Formation of the peptide bond is catalysed by the ribosome itself. The reaction leaves an empty tRNA in the ribosomal P-site and the new peptidyl-tRNA in the A-site.
   
5. In the next step the ribosome moves one codon forward on the mRNA. Simultaneously, the empty tRNA is displaced from the P-site to the E-site as the peptidyl tRNA is translocated from the A-site to the P-site. The process is facilitated by elongation factor eEF-2 and GTP.
   
6. After the translocation, the peptidyl-tRNA is positioned in the P-site and the next codon on the mRNA is made available for interaction with a new aminaoacyl-tRNA in the A-site.
   
7. These reaction steps are repeated until the ribosome encounters an in-frame stop-codon. At this point the translation is terminated.</p>