Aminoacyl-tRNA synthetases characteristics

Each aminoacyl-tRNA synthetase is specific for one amino acid and one or more isoaccepting tRNA. All 20 aRS s can be categorized into one of the two classes and their characteristics are summarized in Table 13.7. 

TABLE 13.7 Principal characteristics of the two classes of aminoacyl-tRNA synthetases... 

On the other hand, regularities in translation are dependent to a great extent upon the activity of the aminoacyl-tRNA synthetases (Strehler et al., 1971) and upon the ratio of different tRNA fractions (Osterman, 1971). It was found that the ratio of these fractions was correlated with the amino acid constitution of the proteins characteristic of this type of cell. This type of correlation was obtained for tRNA in the silk secretory glands of Bombix mori and the reticulocytes of different animals (Lee and Ingram, 1967 Litt and Kabat, 1972 Smith et al., 1974). 

The biosynthesis of a variety of biologically active peptides proceeds nucleic acid-free on protein templates (IK Peptide synthetases generally activate an acceptor amino acid by formation of amino-acyl adenylates or phosphates, which will be stabilized in an enzyne-aminoacylation step, similar as in tRNA-aminoacylation. Reaction with a donor peptide, which may be covalently bound, leads to a specific chain elongation. While small peptides like glutathione are formed by "one-step"-synthetases, more complex structures like gramicidin S are produced by multienzvme systems, which may contain multifunctional polypeptides. Characteristic features of such systems are 1.)activation as aminoacyl adenylates, 2.) aminoacylation of enzyme thiol-groups, 3.) covalently bound peptide intermediates and 4.) a specific intrinsic transport mechanism similar to the biosynthesis of fatty acids. 

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