Final answer:
The two classes of Aminoacyl-tRNA synthetases are classified based on structural and functional attributes, with Class I binding larger amino acids and Class II binding smaller amino acids. Evolutionary analysis suggests these classes originated from ancestral forms associated with non-coded peptide synthesis, with origins preceding the Last Universal Common Ancestor (LUCA).
Step-by-step explanation:
The two classes of Aminoacyl-tRNA synthetases (aaRS) are distinguished based on their structural and functional characteristics. Class I aaRS enzymes typically bind to larger amino acids and are associated with 2' acylation on the tRNA, featuring a Rossmann-fold catalytic domain. They bind ATP in an extended conformation and bind tRNA via the minor groove side of its amino acid acceptor stem helix. Conversely, Class II aaRS enzymes are generally characterized by their binding to smaller amino acids and 3' acylation on the tRNA, with a biotin synthase-like domain. Class II aaRS bind ATP in a bent conformation with the y-phosphate folding back over the adenine ring, and they bind tRNA via the major groove side. Some aaRS enzymes, like LysRS and PheRS, exhibit atypical behaviors, switching between the two classes or presenting unique functional properties, as seen in their ability to catalyze the formation of other important biomolecules like the signaling molecule diadenosine 5',5"-p1,p4-tetraphosphate (AppppA).
The evolutionary implications of the division of aaRS into two classes suggest they originated from ancestral forms involved in non-coded thioester-dependent peptide synthesis, similar to non-ribosomal peptide synthetases. Phylogenetic analysis indicates that these two classes of aaRS have diversified from common ancestors and are linked with the pre-LUCA phase of evolution. The specificity interaction between tRNAs and amino acids in the early translation system likely involved primitive RNA molecules relative to those observed in modern translation systems.