Differences in the Thermal Inactivation Properties of Lysyl and Arginyl Transfer Ribonucleic Acid Synthetases of Bakers' Yeast

Abstract

Abstract An investigation was made of the thermal inactivation of lysyl-tRNA synthetase (l-lysine:tRNA ligase (AMP), EC 6.1.1.6) 400-fold purified from bakers' yeast. During heat inactivation at pH 7.2, 45°, the enzyme was stabilized by the presence of l-lysine or by ATP and MgCl2. The concentration of l-lysine giving half-maximal protection of the enzyme was decreased by the presence of ATP and MgCl2, or tRNA and MgCl2. Similarly, polyuridylate and polycytidylate enhanded the protection of the synthetase by l-lysine. No stabilization was given by any polynucleotide in the absence of l-lysine. Adenosine 5'-phosphate, adenosine 2'(3')-monophosphate, adenosine, 5-hydroxy-dl-lysine, and ethylene glycol each gave some stabilization of the enzyme in certain conditions. In a comparative study it was found that partially purified preparations of arginyl-tRNA synthetase (l-arginine:tRNA ligase (AMP), EC 6.1.1) from bakers' yeast showed a different pattern of susceptibility to heat from that shown by the lysine enzyme. Low concentrations of l-arginine were less effective than tRNA and MgCl2 in stabilization of arginyl-tRNA synthetase from thermal inactivation at 50°. The greatest protection was observed in the presence of l-arginine, tRNA, and MgCl2, whereas l-arginine, ATP, and MgCl2 gave lower protection. The characteristics of heat inactivation of the yeast aminoacyl-tRNA synthetases specific for lysine and arginine could be interpreted in terms of substrate-induced conformational changes in the enzyme proteins.