| The fundamental role of aminoacylation reaction catalyzed by aaRSs implies that representatives of the family are thought to be among the earliest proteins to appear . Organization of amino acid biosynthetic pathways and clustering of aaRSs into different classes are intimately related to one another. A plausible explanation for such a relationship is dictated by early link between aaRSs and amino acids biosynthetic proteins. The aaRSs catalytic cores are highly relevant to the ancient metabolic reactions, namely, amino acids and cofactors biosynthesis . |  |