Minor groove extended conformation

Both classes of enzymes catalyze the common aminoacylation reaction but via different mechanisms (1). Class I and Class II aaRSs bind ATP in an extended and bent conformation, respectively (Fig. 2). In addition, class I enzymes bind the tRNA acceptor stem from the minor groove side, which orients the 2 -hydroxyl group of the A76 ribose for attachment of the amino acid (Fig. 3). In contrast. Class II aaRSs aminoacylate the 3 -hydroxyl of the terminal adenosine, because the enzyme binds to tRNA via its major groove. Class II phenylalanyl-tRNA synthetase (PheRS), which charges amino acids onto the 2 -hydroxyl group of A76 of tRNA , is the only known exception to this rule. 

Calicheamicin (3) binds in an extended conformation to the minor groove of DNA, placing the terminal ene-diyne aglycon deep into the groove (Figure 2a). 

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