Arginyl-tRNA synthetase

Examination of this question with the tobacco hornworm, an insect known to be canavanine-sensitive (this insect normally feeds on canavanine-free plants) revealed that it readily incorporates [ C]canavanine into its newly synthesized proteins. Caryedes brasiliensis. however, very effectively avoids the production of such radiolabeled proteins. When the arginyl- RNA synthetase activity of these insects was compared, tobacco hornworm larvae readily activated canavanine while the larvae of the bruchid beetle possess an arginyl- tRNA synthetase with a marked ability to discriminate between arginine and its structural analogue (22). 

Note Here, Arg represents arginyl-tRNA synthetase, and so forth. The classification applies to all organisms for which tRNA synthetases have been analyzed and is based on protein structural distinctions and on the mechanistic distinction outlined in Figure 27-14. 

Delagoutte, B., Moras, D., and Cavarelli, J. (2000). tRNA aminoacylation by arginyl-tRNA synthetase Induced conformations during substrates binding. EMBO J. 19, 5599—5610. 

The influence of three factors, namely pH, enzyme concentration and amino acid concentration, on the esterification of tRNA arginyl-tRNA synthetase is to be studied by counting the radioactivity of the final product, using 14C-labelled arginine. The higher is the count, the better are the conditions. 

Toxicity On account of its structural similarity to l- arginine, C. is considered to be its antimetabolite and causes poisoning symptoms in mammals and plants C. inhibits biosynthesis, uptake, and transport of arginine. It can be bound to tRNA by arginyl-tRNA synthetase and be incorporated in place of arginine in proteins. This changes the tertiary and quaternary structure of the protein . C. exhibits diuretic activity . 

The potent antimetabolic properties of canavanine result primarily from its ability to function as a highly effective antagonist of arginine (2) metabolism because of its structural similarity to that acid. This structural similarity accounts for the ability of arginyl-tRNA synthetase to activate and attach canavanine to the tRNA that normally carries arginine to the protein assembly site (Rosenthal, 1991). Canavanine is incorporated into the proteins of canavanine-sensitive organisms, but it is rarely, if ever, encountered in the proteins of the plants that produce it (Rosenthal, 1988, 1991). 

The bruchid beetle Caryedes brasiliensis, for instance, a specialized insect living on canavanine-containing seeds, possesses an arginyl-tRNA synthetase that, in contrast to the enzymes of most other organisms, discriminates against canavanine. Moreover the beetle is able to degrade cana-vanine to canaline and urea and to hydrolyze the latter compound to ammonia. 

One of the explanations for the toxic properties of some nonprotein amino acids is their ability to form anomalous proteins. When the structure of a particular compound is structurally analogous to its protein constituent, the nonprotein amino acid may be incorporated. The well-known case of canavanine substitution for arginine in insect proteins is due to the inability of most insects arginyl-tRNA synthetases to discriminate the two compounds (20). When canavanine containing food is consumed, proteins with disrupted tertiary structure are built and the insects may not survive. 

Enzymes involved in protein synthesis also possess potential, e.g. aminoacyl-tRNA-synthetase-aminoadenylate complex 1791, the arginyl-tRNA protein arginyl-transferase11801, and nonribosomal poly- or multienzyme complexes1721 which require ATP or GTP to activate the carboxyl group of an amino acid, and seem to accept various amino acid nucleophiles for peptide bond formation. Flowever, the application of these enzyme systems for generally practical peptide bond formation is rather limited. 

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