Elongation Also Is a Multistep Process Figure 388

Elongation is a cyclic 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.

(met)

A. Binding of Aminoacyl-tRNA to the A Site

In the complete 80S ribosome formed during the process of initiation, the A site (aminoacyl or acceptor site) is free. The binding of the proper aminoacyl-tRNA in the A site requires proper codon recognition. Elongation factor EF1A forms a ternary complex with GTP and the entering aminoacyl-tRNA (Figure 38-8). This complex then allows the aminoacyl-tRNA to enter the A site with the release of EFIA^GDP and phosphate. GTP hydrolysis is catalyzed by an active site on the ribosome. As shown in Figure 38-8, EF1A-GDP then recycles to EF1A-GTP with the aid of other soluble protein factors and GTP.

B. Peptide Bond Formation_

The a-amino group of the new aminoacyl-tRNA in the A site carries out a nucleophilic attack on the esterified carboxyl group of the peptidyl-tRNA occupying the P site (peptidyl or polypeptide site). At initiation, this site is occupied by aminoacyl-tRNA met1. This reaction is catalyzed by a peptidyltransferase, a component of the 28S RNA of the 60S ribosomal subunit. This is another example of ribozyme activity and indicates an impor-tant—and previously unsuspected—direct role for RNA in protein synthesis (Table 38-3). Because the amino acid on the aminoacyl-tRNA is already "activated," no further energy source is required for this reaction. The reaction results in attachment of the growing peptide chain to the tRNA in the A site.

C. Translocation_

The now deacylated tRNA is attached by its anticodon to the P site at one end and by the open CCA tail to an exit (E) site on the large ribosomal subunit (Figure 38-8). At this point, elongation factor 2 (EF2) binds to and displaces the peptidyl tRNA from the A site to the P site. In turn, the deacylated tRNA is on the E site, from which it leaves the ribosome. The EF2-GTP complex is hydrolyzed to EF2-GDP, effectively moving the mRNA forward by one codon and leaving the A site open for occupancy by another ternary complex of amino acid tRNA-EF1A-GTP and another cycle of elongation.

Figure 38-8. Diagrammatic representation of the peptide elongation process of protein synthesis. The small circles labeled n - 1, n, n + 1, etc, represent the amino acid residues of the newly formed protein molecule. EFIA and EF2 represent elongation factors 1 and 2, respectively. The peptidyl-tRNA and aminoacyl-tRNA sites on the ribosome are represented by P site and A site, respectively.

Termination

codon

Releasing factor (RF1)

GTP Releasing factor (RF3)

Peptide

Termination

codon

Peptide

Releasing factor (RF1)

GTP Releasing factor (RF3)

Figure 38-9. Diagrammatic representation of the termination process of protein synthesis. The peptidyl-tRNA and 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 RF1. Hydrolysis of the peptidyl-tRNA complex is shown by the entry of H2O. N and C indicate the amino and carboxyl terminal amino acids, respectively, and illustrate the polarity of protein synthesis.

Figure 38-9. Diagrammatic representation of the termination process of protein synthesis. The peptidyl-tRNA and 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 RF1. Hydrolysis of the peptidyl-tRNA complex is shown by the entry of H2O. N and C indicate the amino and carboxyl terminal amino acids, respectively, and illustrate the polarity of protein synthesis.

Table 38-3. Evidence that rRNA is peptidyltransferase.

• Ribosomes can make peptide bonds even when proteins are removed or inactivated.

• Certain parts of the rRNA sequence are highly conserved in all species.

• These conserved regions are on the surface of the RNA molecule.

• Mutations that result in antibiotic resistance at the level of protein synthesis are more often found in rRNA than in the protein components of the ribosome.

The charging of the tRNA molecule with the aminoacyl moiety requires the hydrolysis of an ATP to an AMP, equivalent to the hydrolysis of two ATPs to two ADPs and phosphates. The entry of the aminoacyl-tRNA into the A site results in the hydrolysis of one GTP to GDP. Translocation of the newly formed pep-tidyl-tRNA in the A site into the P site by EF2 similarly results in hydrolysis of GTP to GDP and phosphate. Thus, the energy requirements for the formation of one peptide bond include the equivalent of the hydrolysis of two ATP molecules to ADP and of two GTP molecules to GDP, or the hydrolysis of four high-energy phosphate bonds. A eukaryotic ribosome can incorporate as many as six amino acids per second; prokaryotic ribo-somes incorporate as many as 18 per second. Thus, the process of peptide synthesis occurs with great speed and accuracy until a termination codon is reached.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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