Protein synthesis elongation cycle, figure

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.

In non-ruminants, the malonyl CoA is combined with an acyl carrier protein (ACP) which is part of a six-enzyme complex (molecular weight c. 500 kDa) located in the cytoplasm. All subsequent steps in fatty acid synthesis occur attached to this complex through a series of steps and repeated cycles, the fatty acid is elongated by two carbon units per cycle (Figure 3.8, see also Lehninger, Nelson and Cox, 1993). 

Figure 12.2 Initiation reactions and elongation cycle in prokaryotic protein synthesis.

The second phase of protein synthesis is the elongation cycle. This phase begins with the insertion of an aminoacyl-lRKA into the empty A site on the ribosome. The particular species inserted depends on the mRNA codon in the A site. The cognate ami noacyl-tKN A does not simply leave the synthetase and diffuse to the A site. Rather, it is delivered to the A site in association with a 4. Tkd protein called elongation factor Tu (EF-Tu). Elongation factor Tu, another member of the G-protein family, requires GTP to bind aminoacyl-tRNA (Figure 30.23) and to bind the ribosome. The binding of... 

Three key steps in the elongation stage of protein synthesis are required for the addition of each amino acid. Because these steps are repeated for each peptide bond formed, this is sometimes called the elongation cycle. The central theme in elongation is that the fully assembled ribosomal complex functions as a ribonucleoprotein machine which rapidly moves 50 to 30 down the mRNA, much like a ratchet. At the center of this complex are two binding sites which line up over a pair of triplet codons, as shown in Figure 26.11. These two sites are called the P site, for peptidyl (or polypeptide), and the A site, for aminoacyl (or acceptor). A third site, called the E site for tRNA exit site, is also a functional component of the ribosome, but for reasons of clarity, it is not included in the figures. 

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