Aminoacyl-tRNA binding factor

The elongation reactions begin with the binding of the aminoacyl-tRNA specified by the codon immediately adjacent to the initiator codon. The binding of this aminoacyl-tRNA is catalyzed by an aminoacyl-tRNA binding factor,... 

The final step in elongation is known as translocation (fig. 29.17). This reaction, like aminoacyl-tRNA binding, is catalyzed by a factor (the translocation factor, known as EF-G in prokaryotic systems and EF-2 in eukaryotic systems) that cycles on and off the ribosome and hydrolyzes GTP in the process. The overall purpose of translocation is to move the ribosome physically along the mRNA to expose the next codon for translation. 

Figure 29.24. Mechanism of Protein Synthesis. The cycle begins with peptidyl-tRNA in the P site. An aminoacyl-tRNA binds in the A site. With both sites occupied, a new peptide bond is formed. The tRNAs and the mRNA are translocated through the action of elongation factor G, which moves the deacylated tRNA to the E site. Once there, it is free to dissociate to complete the cycle.

How is EF-Tu in the GDP form reset to bind another aminoacyl-tRNA Elongation Factor Ts, a second elongation factor, joins the EF-Tu complex and induces the dissociation of GDP. Finally, GTP binds to EF-Tu, and EF-Ts is concomitantly released. It is noteworthy that EF-Tu does not interact with fMet-tRNA Hence, this initiator tRNA is not... 

Eukaryotic peptide elongation follows similar processes as prokaryotes via aminoacyl-tRNA binding, transpeptidation and translocation, involving the A, P and E sites of the ribosome. Two elongation factors, EFl and EF2 mediate the elongation steps. EFl consists of two components, EFl A equivalent of EF-Tu and EFIB equivalent of EF-Ts. EF2 is the translocation factor that binds GTP and catalyzes hydrolysis of GTP that accompanies translocation. 

I have deliberately emphasized in this article that EF-Tu-promoted aminoacyl-tRNA binding and EF-G-promoted translocation precede the hydrolysis of GTP. In order to avoid any misinterpretation of the above statement, I would like to call again the readers attention to the fact that we are using a substrate amount of factors for these experiments. 

After dissociation of the three initiation factors, the 70S initiation complex is ready to bind a ternary complex consisting of aminoacyl-tRNA, elongation factor EF-Xu and GXP. This complex binds to the ribosomal acceptor-site, the A-site, in such a way that the anticodon of the tRNA is in close contact with the complementary codon on the... 

K.-W. Hwang, F. Jumak, D.L. Miller A mutation that hinders the GTP induced aminoacyl-tRNA binding of elongation factor Tu, in L. Bosch, B. Kraal, A. Parmeggiani (eds.) Guanine-nucleotide binding proteins. Plenum Press, New York,, p. 77 (1989)... 

Elongation is a cycUc process on the ribosome in which one amino acid at a time is added to the nascent peptide chain. The peptide sequence is determined by the order of the codons in the mRNA. Elongation involves several steps catalyzed by proteins called elongation factors (EFs). These steps are (1) binding of aminoacyl-tRNA to the A site, (2) peptide bond formation, and (3) translocation. 

Figure 38-9. Diagrammatic representation of the termination process of protein synthesis. The peptidyl-tRNAand aminoacyl-tRNA sites are indicated as P site and A site, respectively. The termination (stop) codon is indicated by the three vertical bars. Releasing factor RF1 binds to the stop codon. Releasing factor RF3, with bound GTP, binds to RFl. Flydrolysisofthe peptidyl-tRNA complex is shown by the entry of HjO. N and C indicate the amino and carboxyl terminal amino acids, respectively, and illustrate the polarity of protein synthesis.

Stage 2 Initiation The mRNA bearing the code for the polypeptide to be made binds to the smaller of two ri-bosomal subunits and to the initiating aminoacyl-tRNA. The large ribosomal subunit then binds to form an initiation complex. The initiating aminoacyl-tRNA base-pairs with the mRNA codon AUG that signals the beginning of the polypeptide. This process, which requires GTP, is promoted by cytosolic proteins called initiation factors. 

Codon-specific binding of an aminoacyl-tRNA (decoding). The binding of an aminoacyl-tRNA to the A site of the 70S or 80S initiation complex depends upon a protein called elongation factor Tu (EF-Tu... 

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