Structural Studies of Aminoacyl-tRNA Synthetases

Abstract

Everyone admits that aminoacyl-tRNA synthetases are an important and interesting family of enzymes. By 1965, sequence and crystallographic studies had made it clear that the serine proteases were a family of enzymes with considerable sequence homologies and an amazing evolutionary conservation of tertiary structure. Dramatic differences in substrate specificity were grafted onto a common catalytic mechanism by only one or two amino acid changes in the substrate binding sites. If this were true of an apparently peripheral cluster of digestive enzymes, must it not be even more true of a family of enzymes that has a common catalytic mechanism and is under enormous selective pressure to conserve precise substrate specificities?

At that time, it had become clear that only a combination of the protein chemistry and X-ray crystallography of several aminoacyl-tRNA synthetases could provide definitive answers to such a question. However, unlike the pancreatic proteinases, these enzymes were not available in quantity in a pure form, since they represent a relatively minor fraction of the protein in most cells. The majority of early studies used animal enzymes, notably the tryptophanyl enzyme from beef pancreas, but as Escherichia coli had become the organism of choice for studying protein synthesis, workers naturally turned to purifying the aminoacyl-tRNA synthetases from this organism.

Purifying enzymes tends to be a frustrating occupation of low prestige, but it is remarkable how pioneering work in the field guides the future direction of biochemistry. The early purification studies of Berg and his collaborators (Baldwin and Berg 1966) led...