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| Figure 32.6 Peptide bond formation and translocation. The formation of the peptide bond between each successive amino acid is catalyzed by peptidyl transferase. Once the peptide bond is formed, an elongation factor (EF-2) will move the ribosome down one codon on the mRNA, so that the A site is vacant and ready to receive the next charged tRNA. |
| Figure 32.7 Termination of protein synthesis. Termination of protein synthesis occurs when the A site is placed over a termination codon. A releasing factor (RF) will cause the completed protein to be released and the ribosome, mRNA, and tRNA will dissociate from each other. |
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| Termination of protein synthesis in both eukaryotic and bacterial cells is accomplished when the A site of the ribosome reaches one of the stop-codons of the mRNA. Protein factors called releasing factors recognize these codons, and cause the protein that is attached to the last tRNA molecule in the P site to be released (Fig. 32.7). This process is an energy-dependent reaction catalyzed by the hydrolysis of GTP, which transfers a water molecule to the end of the protein, thus releasing it
from the tRNA. After release of the newly synthesized protein, the ribosomal subunits, tRNA, and mRNA dissociate from each other. An initiation factor, such as eIF-2, binds to the small ribosomal subunit, setting the stage for the translation of another mRNA (see Fig. 32.4).
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| For a newly synthesized protein to become functionally active it must be folded into a unique three-dimensional structure. Newly synthesized proteins achieve their native structures with the help of a class of proteins called chaperones. Chaperones bind to exposed hydrophobic regions of unfolded proteins and prevent their interaction with each other by shielding the interactive surfaces, thereby preventing misfolding of the protein. The chaperones promote the correct folding of newly synthesized proteins by cycles of substrate binding and release regulated by an ATPase activity and by cofactor proteins. The consequences of protein aggregation due to misfolding can be severe. Some forms of Alzheimer's and Huntington's disease are associated with the formation of amyloid, a fibrillar aggregation of mis-folded protein.
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