Aminoacyl-tRNA synthetases (aaRS) ensure the fidelity of translation by covalently attaching the correct amino acid to its cognate tRNA. The 'editing/proofreading' mechanism of class I aaRS involves:

• A A Zn2+-containing active site that directly discriminates amino acid side chain size
• B A separate editing domain (CP1 domain) that cleaves incorrectly charged aminoacyl-tRNA via a distinct catalytic site geometrically too small for the correct amino acid ✓
• C The anticodon loop of the tRNA forming a proofreading loop that verifies amino acid structure
• D EF-Tu GTPase hydrolysis activity rejecting non-cognate aminoacyl-tRNAs on the ribosome A-site

Explanation

Many aminoacyl-tRNA synthetases, particularly those handling small hydrophobic amino acids (e.g., IleRS, ValRS, LeuRS), possess a separate editing active site (for class I aaRS, located in the connective peptide 1 or CP1 domain) that acts as a 'sieve' to hydrolyse incorrectly charged aminoacyl-tRNAs. The editing site is sterically smaller than the synthetic site—it can accommodate the non-cognate amino acid (which is structurally similar but smaller) to cleave it but excludes the correct cognate amino acid (larger) from hydrolysis. This double-sieve model ensures error rates below 1/10,000. EF-Tu does provide kinetic proofreading on the ribosome A-site, but the aaRS editing domain acts prior to ribosomal use. Zinc motifs are structural in some aaRS but not the proofreading mechanism.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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