Translation

Translation is the process of translating the sequence of a mRNA molecule to a sequence of amino acids during protein synthesis. The ribosome reads the sequence of mRNA in groups of three bases to assemble the protein. 40S ribosome binds mRNA and aminoacyl t RNA and locates AUG start codon on mRNA. 60 S ribosome has peptidyl transferase activity. Translation has three stages as follows:

1. Initiation: There is an area near the 5’ end of mRNA that is known as the untranslated region (UTR) or leader sequence. This portion of mRNA is located between the first nucleotide that is transcribed and the start codon (AUG) of the coding region. The leader sequence is important because it contains a ribosome-binding site.

   First, three initiation factor proteins (known as IF1, IF2, and IF3) bind to the small subunit of the ribosome. This preinitiation complex and a methionine-carrying tRNA then bind to the mRNA, near the AUG start codon, forming the initiation complex on the mRNA. The small subunit of the ribosome has three binding sites: an amino acid site (A), a polypeptide site (P), and an exit site (E). The initiator tRNA molecule carrying the amino acid methionine binds to the AUG start codon of the mRNA transcript at the ribosome’s P site where it will become the first amino acid incorporated into the growing polypeptide chain. Once the initiation complex is formed on the mRNA, the large ribosomal subunit binds to this complex, which causes the release of IFs (initiation factors).

The large subunit of the ribosome has three sites at which tRNA molecules can bind. he A (amino acid) site is the location at which the aminoacyl-tRNA anticodon base pairs up with the mRNA codon, ensuring that correct amino acid is added to the growing polypeptide chain. The P (polypeptide) site is the location at which the amino acid is transferred from its tRNA to the growing polypeptide chain. Finally, the E (exit) site is the location at which the “empty” tRNA sits before being released back into the cytoplasm to bind another amino acid and repeat the process. The initiator methionine tRNA is the only aminoacyl-tRNA that can bind in the P site of the ribosome, and the A site is aligned with the second mRNA codon. The ribosome is thus ready to bind the second aminoacyl-tRNA at the A site, which will be joined to the initiator methionine by the first peptide bond.

2. Elongation: First, the ribosome moves along the mRNA in the 5’ to 3’direction, which requires the elongation factor G, in a process called translocation. This is followed by binding of the tRNA-amino acid complex in the A site and peptide bond formation between first and second amino acids by peptidyl transferase activity of the 60S ribosome. After the peptide bond is formed, the ribosome shifts, or translocates again, thus causing the tRNA to occupy the E site. The tRNA is then released to the cytoplasm to pick up another amino acid. In addition, the A site is now empty and ready to receive the tRNA for the next codon. This process is repeated until all the codons in the mRNA have been read.
3. Termination: When any of the stop codons (UUA,UGA or UAG) is reached, translation is terminated as no tRNAs can recognize the stop codons. Release factors bind and facilitate release of the mRNA from the ribosome and subsequent dissociation of the ribosome.

Post-translational modifications: Proteins and polypeptides are modified after translation to be fully functional. Following modifications are seen :

• Prosthetic groups are added by covalent bonds
• Glycosylation involves addition of sugar groups to amino acids like serine, threonine, asparagine to form blood group antigens, membrane proteins etc.
• Proteolytic cleavage and activation of enzymes like trypsinogen to trypsin, clotting factors and hormones like proinsulin.
• Hydroxylation of lysine and proline in collagen chains
• Gamma carboxylation of glutamine residues in Vit K dependent clotting factors
• Phosphorylation of rate limiting enzymes of metabolism like glycogen synthase.