RESUMO
By means of atomic absorption spectroscopy up to 0.9 Zn2+ atom per molecule of bovine tryptophanyl-tRNA-synthetase (E. C. 6.1.1.2) was found. Treatment of the enzyme with orthophenanthroline (Zn2+-chelating agent) or prolonged dialysis leading to the removal of bound Zn2+ causes inactivation of the enzyme whereas the addition of Zn2+ reactivates it. Kinetic analysis of the inhibiting action of orthophenanthroline at various concentrations of tryptophan, ATP and tRNA leads to the conclusion that removal of Zn2+ prevents the binding of the ATP molecule to tryptophanyl-tRNA-synthetase. By means of chemical modification it is shown that exposed histidine residues and the carboxylic groups of the enzyme participate in Zn2+ binding. According to circular dichroism data removal of Zn2+ has no influence on the secondary structure although some local alterations of the ternary structure are revealed.
