Isoleucyl-transfer ribonucleic acid synthetase from Bacillus stearothermophilus. I. Properties of the enzyme

Purification of isoleucyl-tRNA synthetase from Methanobacterium thermoautotrophicum by pseudomonic acid affinity chromatography

The isoleucyl-tRNA synthetase of the archaebacterium Methanobacterium thermoautotrophicum was purified 1500-fold to electrophoretic homogeneity by a procedure based on affinity chromatography on Sepharose-bound pseudomonic acid, a strong competitive inhibitor of this enzyme. The purified enzyme is a monomer with a molecular mass of 120 kDa. In this respect and in its Km values for the PPi-ATP exchange, and aminoacylation reactions, it resembles the isoleucyl-tRNA synthetases from eubacterial and eukaryotic sources. Its aminoacylation activity is optimal at pH 8.0 and at 55 degrees C. Pseudomonic acid is a strong competitive inhibitor of the aminoacylation reaction with respect to both L-isoleucine (KiIle 10 nM) and ATP (KiATP 20 nM).

The interaction between biologically inactive tRNA conformers and leucyl-tRNA synthetase from rabbit liver

The interaction between tRNA conformers inactive in aminoacylation and leucyl-tRNA synthetase has been investigated. Heat inactivation of the enzyme in the presence of inactive tRNA conformers is shown to lead to a marked increase of inactivation rate while active tRNA conformers, on the other hand, reveal a protecting effect. To study the properties of the enzyme complexed with different tRNA conformers limited proteolysis has been used. Active tRNA conformers are found to protect leucyl-tRNA synthetase against hydrolysis while inactive ones tend to intensify it. Inactive tRNA conformers are also shown to inhibit the aminoacylation of native tRNA in vitro. On the basis of these data biologically inactive conformers of animal tRNA are assumed to form an unproductive complex with leucyl-tRNA synthetase and the structure of the enzyme involved in such interaction is supposed to be more labile and 'extended' than that in complex with active tRNA conformers.

Structural changes of yeast tRNA(Tyr) caused by the binding of divalent ions in the presence of spermine

The existence of specific sites in tRNA for the binding of divalent cations has been seriously questioned by electrostatic considerations [Leroy & Guéron (1979) Biopolymers, 16, 2429-2446]. However, our earlier studies of the binding of Mg2+ and Mn2+ to yeast tRNA(Tyr) have indicated that spermine creates new binding sites for divalent cations [Weygand-Durasević et al. (1977) Biochim. Biophys, Acta, 479, 332-344; Nöthig-Laslo et al. (1981) Eur. J. Biochem. 117, 263-267]. We have now used yeast tRNA(Tyr), spin labeled at the hypermodified purine (i6A-37) in the anticodon loop, to study the effect of spermine on the binding of manganese ions. The presence of eight spermine molecules per tRNA(Tyr) at high ionic strength (0.2 M NaCl, 0.05 M triethanolamine.HCl) and at low temperature (7 degrees C) enhances the binding of manganese to tRNA(Tyr). This effect could not be explained by electrostatic binding. The initial binding of manganese to tRNA(Tyr) affects the motional properties of the spin label indicating a change of the conformation of the anticodon loop. From the absence of the paramagnetic effect of manganese on the ESR spectra of the spin label one can conclude that the first binding site for manganese is at a distance from i6A-37, influencing the spin label motion through a long-range effect. The enhancement of the binding of manganese to tRNA(Tyr) by spermine is lost upon destruction of its specific macromolecular structure and it does not occur in single stranded or in double-stranded polynucleotides. The observed effect can be explained by the binding of Mn2+ to new sites, created by the binding of spermine, which are specific for the macromolecular structure of tRNA.

Conformational changes during enzyme catalysis: role of water in the transition state

The entropy of activation for the synthesis of Ile-tRNA is high and positive. The only likely source of a high delta S is the loss of structured water as the enzyme . substrate complex moves toward the transition state. This requires a change in the orientation or nature of water-organizing residues in the interface between the enzyme . substrate complex and the water. Such changes, which may be some distance from the "active site," are coupled to the active site in such a way that the increased entropy and decreased free energy of the water--enzyme interface is available at the "active site" to reduce the free energy of activation. The effects of Hofmeister anions on KmS and KcatS are consistent with the entropy data.

Arginyl-tRNA synthetase from Escherichia coli K12. Purification, properties, and sequence of substrate addition

Arginyl-tRNA synthetase from Escherichia coli K12 has been purified more than 1000-fold with a recovery of 17%. The enzyme consists of a single polypeptide chain of about 60 000 molecular weight and has only one cysteine residue which is essential for enzymatic activity. Transfer ribonucleic acid completely protects the enzyme against inactivation by p-hydroxymercuriben zoate. The enzyme catalyzes the esterification of 5000 nmol of arginine to transfer ribonucleic acid in 1 min/mg of protein at 37 degrees C and pH 7.4. One mole of ATP is consumed for each mole of arginyl-tRNA formed. The sequence of substrate binding has been investigated by using initial velocity experiments and dead-end and product inhibition studies. The kinetic patterns are consistent with a random addition of substrates with all steps in rapid equilibrium except for the interconversion of the cental quaternary complexes. The dissociation constants of the different enzyme-substrate complexes and of the complexes with the dead-end inhibitors homoarginine and 8-azido-ATP have been calculated on this basis. Binding of ATP to the enzyme is influenced by tRNA and vice versa.

Effects of Ionizing Radiations on the Activity of Valyl-tRNA Synthetase Isolated from Rat Liver and Morris Hepatoma 7777

The effects of ionizing radiations on valyl-tRNA synthetase (L-valine tRNA ligase (AMP), E.C.6.1.1.9.), 100-fold purified from rat liver and Morris hepatoma 7777, are reported. The results showed that: 1) the enzymatic activity is strongly inhibited by radiations; 2) the enzymatic inactivation is much more marked in dilute than in concentrated solutions; 3) the inactivation of enzyme is linked to oxidation of sulphydryl groups indispensable to the catalytic activity of the enzyme; 4) no substantial difference in relation to inactivation by radiation was observed, between rat liver and Morris hepatoma 7777 enzymes.

A comparison of purified valyl-transfer ribonucleic acid synthetase from Bacillus stearothermophilus and from Escherichia coli

The purification of valyl-tRNA synthetase from Bacillus stearothermophilus is described. The protein was greater than 90% homogeneous on polyacrylamide-gel electrophoresis after more than 850-fold purification. It has a molecular weight of 110000, and no evidence was found for the presence of subunit structure. The properties of the purified enzyme were compared with those of purified valyl-tRNA synthetase from Escherichia coli. The thermal stability, pH-stability and dependence of activity on the temperature and pH of the assay are reported. The two enzymes recognize and charge tRNA(Val) from crude tRNA of the mesophile E. coli and of the thermophile B. stearothermophilus, indiscriminately. The gel-filtration method was extended to measure the binding of tRNA to synthetase directly. Binding constants for tRNA(Val) to valyl-tRNA synthetase from B. stearothermophilus were determined between 5 degrees and 60 degrees C.