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Nucleic Acids Research, 1980, Vol. 8, No. 17 4021-4040
© 1980

ENZYMOLOGY

Serine activation is the rate limiting step of tRNASer aminoacylation by yeast seryl tRNA synthetase

Leif Dibbelt, Ulrich Panchmann1 and Hans G. Zachau

Institut für Physiologische Chemie, Physikalische Biochemie und Zellbiologie der Universität München, GFR

Received May 19, 1980.

Using the quenched flow technique the mechanism of seryl tRNA synthetase action has been investigated with respect to the presteady state kinetics of individual steps. Under conditions where the strong binding sites of the enzyme are nearly saturated and the steady state turnover number is about 1 s–1, rate constants of four different processes have been determined: steps connected with substrate associations are relatively slow (12 s–1 for the entire process); activation of serine is the rate determining step (about 1.2 s–1 in presence of tRNASer); whereas the transfer of seryl onto RNASer (35 s–1) and the dissociation of seryl tRNASer (70 s–1) are fast. Similar kinetic parameter seem to hold also for the steady state reactions. This conclusion is based on a detailed study of the substrate, product, and Mg2+ concentration dependence of the transfer reaction. The results also indicate that a second serine binding site is operative. Since the transfer of serine from a preformed adenylate complex onto tRNASer is fast, seryl adenylate seems to be a kinetically competent intermediate of the aminoacylation reaction although, of course, alternative mechanisms cannot be excluded.

1 Present address: Krankenhaus Seefeld, Hauptstraße 23, 8031 Seefeld, Federal Republic of Germany.


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