Final answer:
Aminoacyl-tRNA synthetases discriminate chemically similar amino acids through selective binding and an editing function that corrects misselection by hydrolysis. While aaRSs are highly specific during tRNA aminoacylation, they show relaxed specificity with CoA-SH aminoacylation, hinting at an evolutionary legacy from non-ribosomal peptide synthesis systems.
Step-by-step explanation:
Chemically similar amino acids are discriminated by aminoacyl-tRNA synthetases (aaRS) through their high selectivity for the correct amino acid and an editing function that corrects errors in amino acid selection. The tRNA aminoacylation reaction has a high degree of specificity, with error rates for amino acid selection being between 10-4 and 10-5. AaRSs achieve this specificity by preferentially binding to cognate amino acids and utilizing a quality control mechanism to selectively edit non-cognate amino acids through hydrolysis at either a catalytic domain or a separate editing domain. Despite their high selectivity during tRNA aminoacylation, AARSs exhibit relaxed amino acid selectivity in the CoA-SH aminoacylation reaction.
The ability of aaRSs to discriminate between similar amino acids in tRNA aminoacylation but demonstrate broader selectivity in CoA-SH aminoacylation reflects the evolutionary history of these enzymes, suggesting they evolved from primordial non-ribosomal peptide synthesis systems. Understanding the elaborate mechanisms of selectivity and quality control employed by aaRSs can provide insights into the origin and evolution of the Genetic Code.