Kinetic mechanism of threonyl-tRNA synthetase from human placenta

Using purified enzyme and homologous tRNA, we have investigated the order of substrate binding and product release for the human placental threonyl-tRNA synthetase by isotope exchange, initial velocity, dead-end inhibition and product inhibition studies. The kinetic patterns obtained from these studies are consistent with a unique Bi Uni Uni Bi ping-pong mechanism. The order of addition of the first two substrates ATP and threonine is random, while the release of products follows an obligatory sequence, with AMP as the last product to dissociate from the enzyme.