Elongation cycle

FIGURE 25.13 Double bonds are introduced into the growing fatty acid chain in E. coli by specific dehydrases. Palmitoleoyl-ACP is synthesized by a sequence of reactions involving four rounds of chain elongation, followed by double bond insertion by /3-hydroxydecanoyl thioester dehydrase and three additional elongation steps. Another elongation cycle produces cA-vaccenic acid. 

The binding of the 60S ribosomal subunit to the 48S initiation complex involves hydtolysis of the GTP bound to elF-2 by elF-5. This teaction tesults in telease of the initiation factots bound to the 48S initiation complex (these factots then ate tecycled) and the tapid association of the 40S and 60S subunits to fotm the 80S ribosome. At this point, the met-tRNA is on the P site of the ribosome, ready for the elongation cycle to commence. 

Elongation Cycle Repeats for Each Amino Acid Added... 

The uncharged Val-tRNA then dissociates from the E site. The ribosome is now ready for the next elongation cycle. 

Energy requirements in protein synthesis are high. Four energy-rich phosphoric acid anhydride bonds are hydrolyzed for each amino acid residue. Amino acid activation uses up two energy-rich bonds per amino acid (ATP AMP + PP see p. 248), and two GTPs are consumed per elongation cycle. In addition, initiation and termination each require one GTP per chain. 

Proteins are synthesized by the ribosomal assembly in living organisms, details of which have been the subject of excellent recent reviews. The ribosome is a marvelously complex system of three large RNA molecules and over 50 proteins. This assembly translates the information from mRNA into protein sequence with a fidelity of one mistake per each 10" elongation cycles. ... 

Elongation Step 3 Translocation In the final step of the elongation cycle, translocation, the ribosome moves one codon toward the 3 end of the mRNA (Fig. 27-25a). This movement shifts the anticodon of the dipeptidyl-tRNA, which is still attached to the second codon of the mRNA, from the A site to the P site, and shifts the de-acylated tRNA from the P site to the E site, from where the tRNA is released into the cytosol. The third codon of the mRNA now lies in the A site and the second codon in the P site. Movement of the ribosome along the mRNA requires EF-G (also known as translocase) and the energy provided by hydrolysis of another molecule of GTP. 

The ribosome, with its attached dipeptidyl-tRNA and mRNA, is now ready for the next elongation cycle and attachment of a third amino acid residue. This process occurs in the same way as addition of the second residue (as shown in Figs 27-23, 27-24, and 27-25). For each amino acid residue correctly added to the growing polypeptide, two GTPs are hydrolyzed to GDP and P, as the ribosome moves from codon to codon along the mRNA toward the 3 end. 

The elongation cycle in eukaryotes is quite similar to that in prokaryotes. Three eukaryotic elongation factors (eEFla, eEFljSy, and eEF2) have functions analogous to those of the bacterial elongation factors (EF-Tu, EF-Ts, and EF-G, respectively). Eukaryotic ribosomes do not have an E site uncharged tRNAs are expelled directly from the P site. 

After many such elongation cycles, synthesis of the polypeptide is terminated with the aid of release factors. At least four high-energy phosphate equivalents (from ATP and GTP) are required to generate each peptide bond, an energy investment required to guarantee fidelity of translation. 

The elongation cycle for E. coli is shown in Fig. 29-12. That for eukaryotic ribosomes is similar except that 40S and 60S subunits are involved in formation of the complete 80S ribosome. 

Figure 29-12 (A) Classic version of the polypeptide elongation cycle. The green color traces the pathway of an incoming tRNA carrying a new aminoa-cyl group. The decoding center is at the lower end of the A and P sites, while the peptidyltransferase is at the upper edge. The drawing is schematic, and the orientations of the tRNAs in the three sites are not pictured correctly. Here Tu=EF-Tu and G=EF-G. (B) Path of transfer RNA through ribosome.

At the start of the first round of elongation (Fig. 5), the initiation codon (AUG) is positioned in the P site with fMet-tRNAfMet bound to it via codon-anticodon base-pairing. The next codon in the mRNA is positioned in the A site. Elongation of the polypeptide chain occurs in three steps called the elongation cycle, namely aminoacyl-tRNA binding, peptide bond formation and translocation ... 

The pathway The first committed step in fatty acid biosynthesis is the carboxylation of acetyl CoA to form malonyl CoA which is catalyzed by the biotin-containing enzyme acetyl CoA carboxylase. Acetyl CoA and malonyl CoA are then converted into their ACP derivatives. The elongation cycle in fatty acid synthesis involves four reactions condensation of acetyl-ACP and malonyl-ACP to form acetoacetyl-ACP releasing free ACP and C02, then reduction by NADPH to form D-3-hydroxybutyryl-ACP, followed by dehydration to crotonyl-ACP, and finally reduction by NADPH to form butyryl-ACP. Further rounds of elongation add more two-carbon units from malonyl-ACP on to the growing hydrocarbon chain, until the C16 palmitate is formed. Further elongation of fatty acids takes place on the cytosolic surface of the smooth endoplasmic reticulum (SER). 

The elongation cycle of fatty acid synthesis has four stages for each round of synthesis (Fig. 3). For the first round of synthesis these are ... 

In prokaryotes, each of the reactions of fatty acid synthesis is catalyzed by a separate enzyme. However, in eukaryotes, the enzymes of the fatty acid synthesis elongation cycle are present in a single polypeptide chain, multifunctional enzyme complex, called fatty acid synthase. The fatty acid synthase complex exists as a dimer, with the ACP moiety shuttling the fatty acyl chain between successive catalytic sites, and from one subunit of the dimer to the other. It is, in effect, a highly efficient production line for fatty acid biosynthesis. 

Details of elongation cycle (original method above, alternative method below)... 

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