Model Answer
0 min readIntroduction
Protein synthesis, or translation, is the fundamental biological process by which cells create proteins using messenger RNA (mRNA) as a template. In prokaryotes, this process occurs in the cytoplasm and is highly efficient, allowing for rapid adaptation to environmental changes. It involves three main stages: initiation, elongation, and termination. Elongation is the core phase where amino acids are sequentially added to a growing polypeptide chain, dictated by the codons on the mRNA. This intricate process ensures the accurate and timely production of functional proteins essential for cellular activities.
The Process of Chain Elongation in Prokaryotes
Chain elongation in prokaryotic protein synthesis is a cyclical, highly coordinated process involving the ribosome, mRNA, transfer RNA (tRNA), and various elongation factors. It proceeds in three distinct steps, each powered by the hydrolysis of Guanosine Triphosphate (GTP).
1. Aminoacyl-tRNA Binding (Decoding)
- Initial State: At the start of elongation, the ribosome has the initiator tRNA (carrying formylmethionine, fMet-tRNAfMet) positioned in the P (peptidyl) site, base-paired with the start codon (AUG) on the mRNA. The A (aminoacyl) site is open and ready to receive the next charged tRNA.
- Formation of Ternary Complex: The incoming aminoacyl-tRNA (charged with the appropriate amino acid corresponding to the mRNA codon in the A site) associates with Elongation Factor Tu (EF-Tu) and a molecule of GTP, forming a ternary complex (aminoacyl-tRNA/EF-Tu/GTP).
- A-site Entry and Codon Recognition: This ternary complex enters the A site of the ribosome. Correct codon-anticodon base pairing between the mRNA codon in the A site and the anticodon of the incoming tRNA leads to a conformational change in the ribosome and EF-Tu.
- GTP Hydrolysis and EF-Tu Release: Upon successful codon recognition, EF-Tu hydrolyzes its bound GTP to GDP and inorganic phosphate (Pi). This hydrolysis is crucial for proofreading and ensures the accuracy of amino acid selection. The EF-Tu-GDP complex then dissociates from the ribosome, leaving the new aminoacyl-tRNA stably bound in the A site.
- Regeneration of EF-Tu: The EF-Tu-GDP complex is reactivated by Elongation Factor Ts (EF-Ts), which facilitates the exchange of GDP for a new GTP molecule, making EF-Tu ready for another cycle.
2. Peptide Bond Formation
- Peptidyl Transferase Activity: Once the aminoacyl-tRNA is correctly seated in the A site, the ribosome catalyzes the formation of a peptide bond. This reaction is carried out by the peptidyl transferase activity, which is an intrinsic ribosomal RNA (rRNA) function (specifically of the 23S rRNA in the 50S large ribosomal subunit) acting as a ribozyme.
- Transfer of Polypeptide Chain: The carboxyl group of the amino acid (or the nascent polypeptide chain) attached to the tRNA in the P site is unlinked from its tRNA and forms a peptide bond with the free amino group of the amino acid on the tRNA in the A site. This effectively transfers the growing polypeptide chain from the P-site tRNA to the A-site tRNA.
- Resulting State: After peptide bond formation, the tRNA in the P site is now deacylated (uncharged), and the tRNA in the A site holds the growing polypeptide chain (peptidyl-tRNA).
3. Translocation
- Role of EF-G: The final step in an elongation cycle is translocation, which is catalyzed by Elongation Factor G (EF-G), also known as translocase. EF-G binds to the ribosome in its GTP-bound form.
- mRNA and tRNA Movement: EF-G-GTP binds to the A site and, upon GTP hydrolysis to GDP and Pi, undergoes a conformational change that drives the simultaneous movement of the mRNA and the tRNAs. Specifically, the deacylated tRNA from the P site moves to the E (exit) site, and the peptidyl-tRNA from the A site moves to the P site. The mRNA moves exactly one codon (three nucleotides) relative to the ribosome, exposing a new codon in the now-empty A site.
- Dissociation of EF-G: The EF-G-GDP complex then dissociates from the ribosome, leaving the A site open for the next aminoacyl-tRNA.
- Exit of Deacylated tRNA: The deacylated tRNA in the E site is subsequently released from the ribosome.
This three-step cycle (aminoacyl-tRNA binding, peptide bond formation, and translocation) repeats for each amino acid to be added, progressively extending the polypeptide chain until a stop codon is encountered, signaling termination.
The efficiency of prokaryotic translation is remarkable; for instance, in E. coli, approximately 15-20 amino acids can be added per second, enabling rapid protein synthesis vital for bacterial survival and growth.
| Elongation Factor | Function | Energy Source (GTP) |
|---|---|---|
| EF-Tu | Delivers aminoacyl-tRNA to the A site; ensures accurate codon-anticodon pairing. | Yes (hydrolysis to GDP) |
| EF-Ts | Regenerates active EF-Tu by exchanging GDP for GTP. | No (acts as a nucleotide exchange factor) |
| EF-G | Catalyzes translocation of mRNA and tRNAs on the ribosome. | Yes (hydrolysis to GDP) |
Conclusion
The chain elongation process in prokaryotes is a finely orchestrated series of events critical for the accurate and efficient synthesis of proteins. Driven by ribosomal machinery and specific elongation factors, each cycle involves the precise delivery of amino acids, the formation of robust peptide bonds, and the directional movement of the mRNA template. The energetic input from GTP hydrolysis at key stages ensures fidelity and speed, enabling prokaryotic cells to rapidly produce the diverse array of proteins necessary for their metabolism, growth, and response to environmental stimuli.
Answer Length
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