Protein synthesis is a fundamental process in all living organisms, translating genetic information into functional proteins. This process follows the central dogma of molecular biology, where information flows from DNA to RNA and then to protein. Translation, the final stage, involves decoding messenger RNA (mRNA) to build a polypeptide chain.
Translation elongation is the cyclical phase where amino acids are progressively added to a growing protein chain. This process ensures the accurate and efficient construction of proteins, which perform nearly all cellular functions.
Key Components of the Elongation Machinery
Translation elongation relies on several molecular players. The ribosome, a large complex composed of ribosomal RNA (rRNA) and proteins, serves as the cellular workbench for protein synthesis. It provides the structural framework and catalytic activity for polypeptide formation.
The ribosome has three distinct binding sites for transfer RNA (tRNA) molecules: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site. The A site is the entry point for incoming tRNAs carrying new amino acids, while the P site holds the tRNA attached to the growing polypeptide chain. The E site is where uncharged tRNAs, having delivered their amino acid, prepare to exit the ribosome.
Messenger RNA (mRNA) carries the coded instructions from DNA in the form of codons. Each codon, a sequence of three nucleotides, specifies a particular amino acid. The mRNA strand threads through the ribosome, dictating the sequence of amino acids.
Transfer RNA (tRNA) molecules act as adapter units, bridging the genetic code on mRNA with specific amino acids. One end carries a particular amino acid, while the other has a three-nucleotide sequence called an anticodon. This anticodon precisely matches a complementary mRNA codon, ensuring the correct amino acid is incorporated.
Elongation factors (EFs) are helper proteins that facilitate the elongation process by utilizing energy from guanosine triphosphate (GTP) hydrolysis. These factors facilitate the precise movements and interactions required for efficient and accurate protein synthesis. They are transiently associated with the ribosome during the cycle.
The Elongation Cycle Step-by-Step
The elongation of a polypeptide chain proceeds through a repetitive cycle involving three main steps. This assembly-line-like process ensures that each amino acid is added in the correct order, as dictated by the mRNA template.
The cycle begins with codon recognition, where an aminoacyl-tRNA, carrying its specific amino acid, enters the ribosome’s A site. This entry is guided by an elongation factor and GTP. The anticodon of the incoming tRNA must accurately pair with the mRNA codon positioned in the A site.
Upon correct codon-anticodon pairing, the GTP is hydrolyzed, and the elongation factor dissociates. This hydrolysis acts as a proofreading step, allowing time for incorrect tRNAs to exit. The aminoacyl-tRNA is then properly seated in the A site, ready for peptide bond formation.
Following successful codon recognition, the ribosome catalyzes the formation of a peptide bond, connecting the new amino acid to the growing polypeptide chain. This reaction is carried out by the peptidyl transferase center. The polypeptide chain, previously attached to the tRNA in the P site, is transferred to the amino acid on the tRNA in the A site.
This transfer results in the growing polypeptide now being linked to the tRNA in the A site, while the tRNA in the P site becomes uncharged, having released its amino acid.
The third step is translocation, a coordinated movement of the ribosome along the mRNA. An elongation factor powers this movement, shifting the ribosome precisely one codon down the mRNA strand.
This translocation event causes the tRNA with the newly elongated polypeptide to move from the A site to the P site. Simultaneously, the uncharged tRNA from the P site moves to the E site, from where it is subsequently released from the ribosome. This makes the A site available for the next incoming aminoacyl-tRNA, beginning a new round of elongation.
Ensuring Fidelity and Speed
The cellular machinery responsible for translation elongation operates with accuracy and speed to produce functional proteins. Maintaining accuracy during this process is achieved through several layers of quality control. Beyond the initial codon-anticodon pairing, a kinetic proofreading mechanism enhances accuracy.
After the initial binding of an aminoacyl-tRNA to the A site and the hydrolysis of GTP, a brief delay occurs before peptide bond formation. During this window, an incorrectly paired tRNA is more likely to dissociate from the ribosome than a correctly paired one. This two-step verification process significantly reduces the rate of amino acid misincorporation.
The speed of translation elongation allows cells to rapidly respond to changing protein demands. Ribosomes can add amino acids to a polypeptide chain quickly, allowing for the production of thousands of protein molecules needed for cellular function and growth.
Inhibition of Elongation
Understanding the process of translation elongation has been important in developing therapeutic agents, particularly antibiotics. Many antibiotics specifically target bacterial ribosomes to inhibit protein synthesis, stopping bacterial growth. These drugs exploit structural differences between bacterial and eukaryotic ribosomes, allowing for selective toxicity.
Tetracyclines interfere with the codon recognition step. These drugs bind to the 30S ribosomal subunit, blocking the A site. This prevents the aminoacyl-tRNA from binding to its complementary codon on the mRNA, halting the addition of new amino acids.
Macrolides bind to the 50S ribosomal subunit. They obstruct the polypeptide exit tunnel, a channel through which the growing protein chain emerges from the ribosome. This interferes with the translocation step, stalling the ribosome’s movement and preventing further polypeptide elongation.
Chloramphenicol targets the bacterial 50S ribosomal subunit. It inhibits the peptidyl transferase activity of the ribosome. By binding to the peptidyl transferase center, chloramphenicol prevents peptide bond formation between amino acids, blocking protein chain elongation.