Protein Synthesis: From mRNA to Polypeptide Chain
Ribosome Binding Sites
In the ribosome, there are three different attachment sites for tRNA: the P site, where the polypeptide chain is formed; the A site, into which the amino acid (AA) is going to join the protein chain; and the E site, where the tRNA is located before leaving the ribosome.
Transfer RNA (tRNA)
Transfer RNA molecules are responsible for transporting amino acids to the ribosome and incorporating them into the growing protein chain according to the sequence indicated by the messenger RNA (mRNA). There are over 20 different tRNAs. Each tRNA has two important regions:
- The anticodon: Consisting of three nitrogenous bases that are complementary to those that form an mRNA codon. Thus, each type of tRNA recognizes a specific codon in the mRNA.
- The 3′ end: The site to which the amino acid corresponding to the codon recognized by this tRNA binds.
Amino Acid Activation
Activation is the union of an amino acid to its corresponding tRNA. This process is catalyzed by the enzyme aminoacyl-tRNA synthetase, resulting in a complex called aminoacyl-tRNA. The reaction requires energy, which is provided by an ATP molecule. The binding of the amino acid and tRNA occurs between the carboxyl group of the amino acid and the -OH group of the tRNA’s 3′ end.
Protein Synthesis Process
Initiation of the Polypeptide Chain
Synthesis begins when the small subunit of the ribosome and mRNA join at a point near the AUG codon, which is the initiator codon and marks the beginning of the protein. Then, a first aminoacyl-tRNA, whose anticodon consists of three bases complementary to the initiator codon, enters the ribosome’s P site. The first tRNA carries the amino acid N-formylmethionine in prokaryotes, and methionine in eukaryotes. The small subunit of the ribosome, mRNA, and the first aminoacyl-tRNA form the initiation complex, to which the large subunit of the ribosome then binds.
Elongation of the Polypeptide Chain
Elongation consists of lengthening the protein chain. A second aminoacyl-tRNA “enters” the ribosome and occupies the free A site. The next step is the formation of a peptide bond between the amino acid occupying the P site and the new amino acid occupying the A site. The reaction forming the new peptide bond is catalyzed by the enzyme peptidyl transferase, which resides in the catalytic RNA that is part of the large subunit. Following the formation of this peptide bond, the second tRNA is joined at one end to the newly formed dipeptide and, at the other end, to the complementary codon. This is followed by the translocation of the ribosome. Translocation involves the ribosome moving along the mRNA in the 5′-3′ direction. Since the displacement is exactly three bases, the first tRNA leaves the ribosome through the E site, and the dipeptide-tRNA moves to occupy the P site, leaving the A site free. Under these conditions, another aminoacyl-tRNA can be incorporated into the A site, allowing the process of protein chain elongation to continue.
Termination
Termination occurs when the ribosome reaches an mRNA sequence containing a termination codon. This codon is not recognized by any tRNA but by release factors that bind to the A site and cause the peptidyl transferase to detach the polypeptide chain from the tRNA. Once translation is complete, the newly formed protein, mRNA, and tRNA leave the ribosome, which dissociates into its two subunits until a new synthesis is initiated. In prokaryotes, due to the absence of a division between the nucleus and cytoplasm, translation and transcription are simultaneous: the mRNA begins to be translated before the end of transcription.
Components Required for Translation
To carry out the process of translation or protein synthesis, the following are needed:
- Ribosomes, where protein synthesis takes place.
- Messenger RNA (mRNA), carrying the information to synthesize each protein.
- Amino acids, which are the building blocks of proteins.
- Transfer RNA (tRNA), which delivers the amino acids in the precise order.
In the small subunit, mRNA binds, while in the large subunit, amino acids are joined to form the polypeptide chain. Both subunits come together when proteins need to be synthesized.