Aminoacyl-tRNA Synthetase

Different Approaches to the Same Problem

In this picture, five complexes of an aminoacyl-tRNA synthetase with tRNA are shown, aligned so that the tRNA molecules (shown in red) are in the same orientation. Notice that the enzymes approach the tRNA from different angles. The isoleucine (entry 1ffy), valine (entry 1gax) and glutamine (entry 1euq) enzymes cradle the tRNA, gripping the anticodon loop (at the bottom in each tRNA), and placing the amino-acid acceptor end of the tRNA in the active site (at the top right in each tRNA). These all share a similar protein framework, known as "Type I," approaching the tRNA similarly and adding the amino acid to the last 2' hydroxyl group in the tRNA. The phenlyalanine (entry 1eiy) and threonine (entry 1qf6) enzymes are part of a second class of enzymes, known as "Type II." They approach the tRNA from the other side, and add the amino acid to the other free hydroxyl on the last tRNA base.