Kinetics of Peptide Synthesis by the Leucyl-tRNA Synthetase from Bacillus stearothermophilus

Abstract

The kinetic parameters of peptide synthesis by the leucyl-tRNA synthetase from Bacillus stearothermophilus were compared with those of leucyl-tRNA formation and of hydroxamate formation. Km values for ATP were almost the same among these three reactions. In contrast to the low Km value obtained for leucine in leucyl-tRNA formation (6.2μM), the values for peptide synthesis and hydroxamate formation were higher, 1.3mM and 2.2mM, respectively. Although peptide synthesis had Michaelis type kinetics in respect to the dependence on nucleophile concentration and had a Km for nucleophiles in the range of 150-200mM, the velocity of hydroxamate formation had second order kinetics against hydroxylamine concentration. These results indicate that the reaction mechanism of peptide formation was different from either hydroxamate formation or aminoacyl-tRNA formation. Due to the difficulty in accessibility to nucleophiles of the aminoacyl adenylate stabilized by the enzyme, the initial velocity of peptide synthetic reaction was surprisingly low (2×10^-3 sec^-1). Some multivalent metal ions such as Fe²⁺, Fe³⁺, Al³⁺ Sn²⁺, and Sn⁴⁺ accelerated the reaction. When 2'-deoxy ATP was used in place of ATP, similar enhancement of the reaction was observed. In either case, an increase in the Km value for the nucleotide was observed, indicating the significant involvement of the adenylate-enzyme interaction in the reactivity of the aminoacyl adenylate against nucleophiles.