Isoleucyl‐tRNA Synthetase: An Enzyme with Several Catalytic Cycles Displaying Variation in Specificity and Energy Consumption

The translation of DNA via the genetic code into protein is a fundamental process carried out by all living systems. The extreme accuracy with which it is performed requires that the enzymes involved operate with extremely high specificity. It is for this reason that the structure and function of these proteins have been of special interest to molecular biologists and biochemists for the last twenty years. The discrimination between the different amino acids present in the cytosol and their esterification with the correct tRNA is the special task of the aminoacyl‐tRNA synthetases. One of the most difficult discriminations that has to be performed is the specificity for isoleucine. There are many amino acids in the cell that have a structure similar to isoleucine, in particular valine. To solve this problem, the aminoacyl tRNA synthetases have evolved unique proofreading mechanisms. The most detailed investigations of these mechanisms have been on the isoleucyl‐tRNA synthetases from yeast and from Escherichia coli . Both enzymes are single‐chain polypeptides with molecular weights of about 110 000. Sequence homologies with other aminoacyl‐tRNA synthetases seem to occur only within narrow regions, for example in the ATP binding site. The identification of the correct amino acid is achieved in a four‐step process. In the first two steps the hydrophobic interaction of the amino acid side chain with the enzyme is utilized for discrimination. The two hydrolytic proofreading steps that follow make use of the steric hindrance associated with the amino acid side chain. The enzymes are characterized by the special property that slight variations in the reaction conditions cause conformational changes that affect the catalytic cycle. In this way the observed specificity of the enzymes can be changed to almost any value. In general, high specificity is associated with high energy cost, which is reflected in high ATP consumption. Under physiological conditions a medium level of specificity and energy consumption seems to be preferred.