Chemical modification of aminoacyl ligases and the effect on formation of aminoacyl-tRNA

An in vivo effect of the metabolites L-alanine and glycyl-L-leucine on the properties of the lysyl-tRNA synthetase from Escherichia coli K-12. II. Kinetic evidence

Wild-type Escherichia coli K-12 was grown in minimal medium alone or with the addition of 20 mM L-alanine or 3 mM glycyl-L-leucine. A lysyl-tRNA synthetase mutant strain was grown in minimal medium containing 20mM L-alanine. The lysyl-tRNA synthetase from these strains was purified to 70-90% of homogeneity. Kinetic studies comparing the effect of thermal and urea inactivation on these different lysyl-tRNA synthetase preparations and measurement of the Michaelis constant for lysine and transfer RNA indicated that growth of Escherichia coli in the presence of alanine and glycyl-L-leucine induces an alteration in the properties of the synthetase. Measurement of the apparent Km for ATP at pH 7.25 indicates lysyl-tRNA synthetase has two two binding sites for this substrate, and further studies indicated a dependence of the apparent Km for lysine on the ATP concentration.

Reversible reaction of cyanate with a reactive sulfhydryl group at the glutamine binding site of carbamyl phosphate synthetase

Carbamyl phosphate synthetase from Escherichia coli reacts stoichiometrically (one to one) with [14C]cyanate to give a 14C-labeled complex which can be isolated by gel filtration. The formation of the complex is prevented if L-glutamine is present or if the enzyme is first reacted with 2-amino-4-oxo-5-chloropentanoic acid, a chloro ketone analog of glutamine which has been shown to react with a specific SH group in the glutamine binding site. The rate of complex formation is increased by ADP and decreased by ATP and HCO3-. The isolated complex is inactive with respect to glutamine-dependent synthetase activity. However, the reaction of cyanate with the enzyme is reversible. The rate of dissociation of the isolated complex is not affected by pH (over the pH range 6-10), is greatly increased by ATP and HCO3-, and is decreased by ADP. The allosteric effectors ornithine and UMP have no effect on either the rate of formation or the rate of dissociation of the complex; however, the apparent affinity of the enzyme for ATP is decreased by UMP and increased by ornithine. The site of reaction of cyanate with carbamyl phosphate synthetase, which is composed of a light and a heavy subunit, is with an SH group in the light subunit to give an S-carbamylcysteine residue. The binding of L-[14C]glutamine to the enzyme and the inhibition of glutamine-dependent synthetase activity by the chloroketone analog are both prevented by the presence of cyanate. The reaction with cyanate is considered to be with the same essential SH group which is located in the glutamine binding site and is alkylated by 2-amino-4-oxo-5-chloropentanoic acid. The bicarbonate-dependent effects of ATP suggest that formation of the activated carbon dioxide intermediate is accompanied by changes in the heavy subunit which functionally alter the properties of the glutamine binding site on the light subunit. The allosteric effects of ornithine and UMP are probably not related to this intersubunit interaction.