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Histidyl-tRNA synthetase from Salmonella typhimurium: specificity in the binding of histidine analogues.

The topography of the active site of histidyl-tRNA synthetase has been investigated by determining Ki values for a variety of structural analogues of histidine, using the ATP-PPi exchange and tRNA aminoacylation reactions. Using these kinetic constants it has been possible to have a measure of the relative binding affinity of the enzyme for the histidine analogues. The following conclusions have been drawn: (a) the enzyme is stereospecific in the formation of aminoacyl-tRNA complexes, since the D-isomer of histidine does not influence the two reactions; (b) the carboxyl group is not required for binding; (c) bulky derivatives of the carboxyl group prevent the molecules from binding to the enzyme; (d) the amino group permits a good binding affinity; (e) the length of the ring side chain plays a very important role as point of attachment to the enzyme; (f) the kinds of heteroatoms on the ring determine the inhibitory properties of the analogues.