Kinetics of formation of tryptophanyl-adenylate by tryptophanyl-tRNA synthetase from beef pancreas

On the mechanism of tRNATrp aminoacylation catalysed by beef tryptophanyl-tRNA synthetase using presteady-state kinetics

The dimeric tryptophanyl-tRNA synthetase from beef pancreas has been found to activate 2 tryptophans/mol enzyme [Eur. J. Biochem. (1982) 128, 389-398]. By using quenched-flow and stopped-flow methods under presteady-state conditions, we show that only one enzyme subunit operates at a time in the aminoacylation of tRNATrp and that the transfer reaction is not the rate-limiting step in the overall aminoacylation process.

Reaction of tryptophanyl-tRNA synthetase from beef pancreas with periodate-oxidized ATP

Tryptophanyl-tRNA synthetase from beef pancreas reacts with periodate-oxidized ATP according to biphasic kinetics. A rapid phase involves two groups of the protein, presumably lysine side-chains. The slow phase corresponds to the reaction of a larger number of groups. The time-course of the partial losses of the ATP-PPi isotopic exchange and of the aminoacylation activities of the enzyme follow the labelling of the two fast-reacting groups. However, the ability of the enzyme to form a bis(tryptophanyladenylate)-enzyme complex is not lost after reaction of these two groups with the reagent. The affinity for ATP is also unaffected by this initial labelling of the protein, as seen from the Km values of this substrate in the ATP-PPi isotopic exchange reaction. These data suggest that, in this fast initial reaction, oxidized ATP reacts neither with specific ATP-binding groups of the enzyme nor with any major catalytic residue of the tryptophan-activation site. In contrast with this first step, the further slow labelling of lysine residues leads to a disappearance of the aminoacylation ability of the enzyme, while it does not further affect the ATP-PPi exchange activity. The behaviour of beef tryptophanyl-tRNA synthetase during derivatization with oxidized ATP is therefore at variance with that which has been described for the homologous E. coli enzyme.

Pyrophosphate-caused inhibition of the aminoacylation of tRNA by the leucyl-tRNA synthetase from Neurospora crassa

Inorganic pyrophosphate inhibits the aminoacylation of tRNALeu by the leucyl-tRNA synthetase from Neurospora crassa giving very low Kapp.i, PPi values of 3-20 microM. The inhibition by pyrophosphate, together with earlier kinetic data, suggest a reaction mechanism where leucine, ATP and tRNA are bound to the enzyme in almost random order, and pyrophosphate is dissociated before the rate-limiting step. A kinetic analysis of this mechanism shows that the measured Kapp.i values do not give the real dissociation constant but it is about 0.4 mM. Other dissociation constants are 90 microM for leucine, 2.2 mM for ATP and 1 microM for tRNALeu. At the approximate conditions of the living cell (2 mM ATP, 100 microM leucine and 150 microM PPi) the leucyl-tRNA synthetase is about 85% inhibited by pyrophosphate.

Kinetic anticooperativity in pre-steady-state formation of tryptophanyl adenylate by tryptophanyl-tRNA synthetase from beef pancreas. A consequence of the tryptophan anticooperative binding

The kinetics of formation of tryptophanyl adenylate by tryptophanyl-tRNA synthetase from beef pancreas has been followed by stopped-flow, using the quenching of fluorescence of the enzyme linked to the amino acid activation reaction. Both subunits of this alpha 2 enzyme catalyze the adenylate formation. At saturation with substrates the rate constant of the activation reaction is the same for both subunits. The same behaviour is observed for the pyrophosphorolysis reaction. Both subunits exhibit the same affinity for ATP-Mg in the forward reaction and the same affinity for magnesium pyrophosphate in the backward reaction. On the contrary the formation of tryptophanyl adenylate follows biphasic kinetics when tryptophan concentration is much below saturation. This is independent of ATP-Mg concentration and is the consequence of different affinities of the two subunits for tryptophan as already observed by Graves et al. (1979, Eur. J. Biochem. 96, 509-518) in equilibrium dialysis experiments. A monoadenylate-enzyme complex on one subunit has been prepared. This complex made possible the study of the formation of the second adenylate on the other subunit. The formation of this second adenylate followed first-order kinetics at all ATP-Mg and tryptophan concentrations. The tryptophan concentration dependence of the rate of formation of this second adenylate leads to a Michaelis constant close to the dissociation constant of the low affinity tryptophan site of the enzyme. No isomerization step could be evidenced. The experiments were carried out under two conditions corresponding to those used by Merault et al. (1978. Eur. J. Biochem. 87, 541-550) in the steady state of the tRNATrp aminoacylation reaction (10 mM total magnesium in 100 mM KCl and 1 mM free magnesium ions, both at pH 8.0.25 C). No great difference either in the mechanism or in the dissociation and rate constants was observed but an inhibitory effect of KCl. It is concluded that the enzyme is symmetrical as far as the ATP-Mg and the magnesium pyrophosphate sites are concerned and that the rate of the activation reaction reflects the anticooperative occupancy of the tryptophan sites carried by the two subunits.