Improving Translational Accuracy

Elongation factors EF-Tu and EF-G in bacteria (or EF1 and EF2 in eukaryotes) associate with GTP and aminoacyl-tRNA to deliver the correct tRNA to the ribosome's A site. They facilitate codon-anticodon duplex formation and enable the two-checkpoint proofreading system that ensures translational fidelity throughout protein synthesis.

The first checkpoint involves complementary base pairing between the mRNA codon and tRNA anticodon; incorrect tRNAs dissociate due to weaker binding. The second checkpoint uses 16S rRNA contacts that form a network of ribosomal interactions specific to each codon position. Mismatched tRNAs fail to make these contacts and dissociate, while correct tRNAs induce conformational changes in the ribosome's catalytic center.

Induced fit occurs when a correct tRNA changes the conformation of the ribosome's catalytic center, triggering EF-Tu to hydrolyze GTP and dissociate. This conformational change ensures only correctly paired tRNAs proceed to peptide bond formation, preventing incorrect aminoacyl-tRNAs from being incorporated into the growing polypeptide chain.

Kinetic proofreading introduces a time delay after EF-Tu·GDP dissociation, during which incorrect codon-anticodon pairs are more likely to dissociate than correct pairs. Since incorrect tRNAs form weaker base pairs with the codon, they cannot withstand the extended delay before entering the peptidyl transferase center active site for catalysis.

The free energy difference between correct and nearly correct base pairs is only 3 kcal/mol, making base complementarity an unreliable sole mechanism. Without additional proofreading steps, error frequency would reach one wrong amino acid per 100 incorporated. Two additional checkpoints—induced fit and kinetic proofreading—reduce actual error rates to far lower levels observed in organisms.

If an aminoacyl-tRNA with a codon-anticodon mismatch enters the P site, it increases error rates in subsequent A site selections. Successive rounds of misincorporation produce a faulty polypeptide chain that is prematurely terminated by release factors and subsequently degraded through regulated protein degradation pathways.

The peptidyl transferase center functions as an enzyme with an active site that discriminates substrates based on molecular structure. Only correct tRNAs induce the conformational changes necessary for the PTC to catalyze covalent bond formation between amino acids. Incorrect tRNAs cannot trigger these conformational changes and fail to participate in peptide bond formation.