Arginyl-tRNA synthetase from Escherichia coli, purification by affinity chromatography, properties, and steady-state kinetics

A Blue Sephadex G-150 affinity column adsorbs the arginyl-tRNA synthetase of Escherichia coli K12 and purifies it with high efficiency. The relatively low enzyme content was conveniently purified by DEAE-cellulose chromatography, affinity chromatography, and fast protein liquid chromatography to a preparation with high activity capable of catalyzing the esterification of about 23,000 nmol of arginine to the cognate tRNA per milligram of enzyme within 1 min, at 37 degrees C, pH 7.4. The turnover number is about 27 s-1. The purification was about 1200-fold, and the overall yield was more than 30%. The enzyme has a single polypeptide chain of about Mr 70,000 and binds arginine and tRNA with 1:1 stoichiometry. For the aminoacylation reaction, the Km values at pH 7.4, 37 degrees C, for various substrates were determined: 12 microM, 0.9 mM, and 2.5 microM for arginine, ATP, and tRNA, respectively. The Km value for cognate tRNA is higher than those of most of the aminoacyl-tRNA synthetase systems so far reported. The ATP-PPi exchange reaction proceeds only in the presence of arginine-specific tRNA. The Km values of the exchange at pH 7.2, 37 degrees C, are 0.11 mM, 2.9 mM, and 0.5 mM for arginine, ATP, and PPi, respectively, with a turnover number of 40 s-1. The pH dependence shows that the reaction is favored toward slightly acidic conditions where the aminoacylation is relatively depressed.     

Purification of wheat germ RNA ligase. II. Mechanism of action of wheat germ RNA ligase

The mechanism of action of purified wheat germ RNA ligase has been examined. ATP was absolutely required for the ligation of substrates containing 5'-OH or 5'-P and 2',3'-cyclic P or 2'-P termini. Ligation of 1 mol of 5'-P-2',3'-cyclic P-terminated poly(A) was accompanied by the hydrolysis of 1 mol of ATP to 1 mol each of AMP and PPi. Purified RNA ligase catalyzed an ATP-PPi exchange reaction, specific for ATP and dATP, and formed a covalent enzyme-adenylate complex that was detected by autoradiography following incubation with [alpha-32P]ATP and separation of the products by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A protein doublet with a molecular weight of approximately 110 kDa, the major product detected by silver staining, was labeled in these reactions. Isolated E-AMP complex was dissociated by the addition of ligatable poly(A), containing 5'-P-2',3'-cyclic P termini, to yield AMP and by the addition of PPi to yield ATP. The unique feature of the reactions leading to an exchange reaction between ATP and PPi and to the formation of an E-AMP complex was their marked stimulation (up to 400-fold) by the addition of RNA. This property distinguishes the wheat germ RNA ligase from other known RNA and DNA ligases which catalyze ATP-PPi exchange reactions and form E-AMP complexes in the absence of substrate. Thus, RNA appears to function in two capacities in the wheat germ system: as a cofactor, to stimulate the reaction of the enzyme with ATP, and as an authentic substrate for ligation.     

Enzymatic characterisation of the multifunctional enzyme delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase from Streptomyces clavuligerus.

delta-(L-alpha-Aminoadipyl)-L-cysteinyl-D-valine (ACV) synthetase, the multienzyme catalyzing the formation of ACV from the constituent amino acids and ATP in the presence of Mg2+ and dithioerythritol, was purified about 2700-fold from Streptomyces clavuligerus. The molecular mass of the native enzyme as determined by gel filtration chromatography is 560 kDa, while that determined by denaturing gel electrophoresis is 500 kDa. The enzyme is able to catalyze pyrophosphate exchange in dependence on L-cysteine and L-valine, but no L-alpha-aminoadipic-acid-dependent ATP/PPi exchange could be detected. Other L-cysteine- and L-valine-activating enzymes present in crude extracts were identified as aminoacyl-tRNA synthetases which could be separated from ACV synthetase. The molecular mass of these enzymes is 140 kDa for L-valine ligase and 50 kDa for L-cysteine ligase. The dissociation constants have been estimated, assuming three independent activation sites, to be 1.25 mM and 1.5 mM for cysteine and ATP, and 2.4 mM and 0.25 mM for valine and ATP, respectively. The enzyme forms a thioester with alpha-aminoadipic acid and with valine in a molar ratio of 0.6:1 (amino acid/enzyme). Thus, the bacterial ACV synthetase is a multifunctional peptide synthetase, differing from fungal ACV synthetases in its mechanism of activation of the non-protein amino acid.     

Glutamine-dependent asparagine synthetase from Lupinus luteus

Asparagine synthetase (glutamine-hydrolyzing [l-aspartate: l-glutamine amido-ligase (AMP-forming), E.C. 6.3.5.4] was purified over 500-fold from cotyledon extracts of 1-week-old yellow lupin seedlings. The enzyme was labile and required protection by high levels of thiols; glycerol and the substrates also stabilized it. The reaction products were shown to be asparagine, AMP, PPi and glutamate. The limiting K_m values were for aspartate 1·3 mM, for MgATP 0·14 mM and for glutamine 0·16 mM. Positive homotropic cooperativity was observed for MgATP only, and gel filtration studies indicated that the substrate-free enzyme (MW 160 000) associated to a dimer (MW 320 000 in the presence of MgCl₂ and ATP. The purified enzyme, which had some glutaminase activity, catalyzed an aspartate- and glutamine-independent ATP-PPi exchange reaction at a rate 5–7-fold higher than the rate of asparagine synthesis. Initial velocity studies and exchange data indicated an overall ping-pong mechanism. Compared to similar enzymes isolated from mammalian tumor cells, the lupin enzyme appears to be unique with respect to MW, reaction mechanism and regulatory properties. The allosteric properties observed suggest an important role for this enzyme in the regulation of asparagine biosynthesis.     

Tyrosyl-tRNA synthetase from wheat germ

Tyrosyl-tRNA synthetase (TyrRS) was purified 5,000-fold from wheat germ extract by ultracentrifugation, precipitation with ammonium acetate, and column chromatography. Under denaturing conditions the enzyme ran as a single band on SDS-polyacrylamide electrophoresis with an apparent Mr of 55,000. The native molecular weight determined by gel filtration was 110,000, suggesting a quaternary structure of an alpha 2 type for native TyrRS. Purified enzyme activity, based on the aminoacylation reaction, was studied in terms of Mg2+, ATP, pH, and KCl dependence. Optimum concentrations were 6 mM Mg2+, 4 mM ATP, and 200 mM KCl at pH 8. The Km values for ATP, tyrosine, and tRNA were 40, 3.3, and 1.5 microM, respectively. The instability of the TyrRS activity and the methods used for stabilizing it are discussed. In wheat germ extract we found a second tyrosylating activity that works with Escherichia coli tRNA, but not with wheat germ tRNA. We believe that this enzyme is the mitochondrial tyrosyl-tRNA synthetase of wheat germ.     

A novel reaction catalyzed by unadenylylated glutamine synthetase from Escherichia coli. AMP-dependent synthesis of pyrophosphate and L-Glutamate from orthophosphate and L-glutamine

The unadenylylated, manganese form of glutamine synthetase (L-glutamate: ammonia ligase (ADP forming), EC 6.3.1.2 from Escherichia coli catalyzes a novel, AMP-dependent (reversible) synthesis of pyrophosphate and L-glutamate from orthophosphate and L-glutamine: Formula (See Text). The hydrolysis of the L-glutamine amide bond is coupled to the stoichiometric synthesis of pyrophosphate, although as PPi accumulates, additional hydrolysis of L-glutamine occurs in a secondary reaction catalyzed by the [manganese x enzyme x AMP x PPi] complex. The synthesis of PPi probably occurs at the subunit catalytic site in the positions normally occupied by the beta, gamma-phosphates of ATP. To promote PPi synthesis, AMP apparently binds to the subunit catalytic site rather than to the allosteric inhibitor site; equilibrium binding results suggest that Pi directs the binding of AMP to the active site. In this reaction, Mg2+ will not substitute for Mn2+, and adenylylated glutamine synthetase is inactive. Pyrophosphate is synthesized by the unadenylylated, manganese enzyme at approximately 2% of the rate of that of ATP in the reverse biosynthetic reaction. If P1 is replaced by arsenate, the enzymatic rate of the AMP-supported hydrolysis of L-glutamine is 100-fold faster than is PPi synthesis and is one-half the rate of the ADP-supported, irreversible arsenolysis of L-glutamine. This latter activity also is supported by GMP and IMP, suggesting that the catalytic site of glutamine synthetase has a rather broad specificity for the nucleotide base. The reactions supported by AMP directly relate to the mechanism of glutamine synthetase catalysis.     

Kinetic mechanism of glutaminyl-tRNA synthetase from human placenta

The order of interaction of substrates and products with human placental glutaminyl-tRNA synthetase was investigated in the aminoacylation reaction by using the steady-state kinetic methods. The initial velocity patterns obtained from both the glutamine-ATP and glutamine-tRNA substrate pairs were intersecting, whereas ATP and tRNA showed double competitive substrate inhibition. Dead-end inhibition studies with an ATP analog, tripolyphosphate, showed uncompetitive inhibition when tRNA was the variable substrate. The product inhibition studies revealed that PPi was an uncompetitive inhibitor with respect to tRNA. The noncompetitive inhibition by AMP versus tRNA was converted to uncompetitive by increasing the concentration of glutamine from 0.05 to 0.5 mM. These and other kinetic patterns obtained from the present study, together with our earlier finding that this human enzyme catalyzed the ATP-PPi exchange reaction in the absence of tRNA, enable us to propose a unique two-step, partially ordered sequential mechanism, with tRNA as the leading substrate, followed by random addition of ATP and glutamine. The products may be released in the following order: AMP, PPi and then glutaminyl-tRNA. The proposed mechanism involves both a quarternary complex including all three substrates and the intermediary formation of an enzyme-bound aminoacyl adenylate, common to the usual sequential and ping-pong mechanisms, respectively, for other aminoacyl-tRNA synthetases.     

On the roles of magnesium and spermidine in the isoleucyl-tRNA synthetase reaction. Analysis of the reaction mechanism by total rate equations

The reaction of isoleucyl-tRNA synthetase from Escherichia coli B was analysed by deriving total steady-state rate equations for the ATP/PPi exchange reaction and for the aminoacylation of tRNA, and by fitting these rate equations to series of experimental results. The analysis suggests that (a) a Mg2+ inhibits the aminoacylation of tRNA but not the activation of the amino acid. In the chosen mechanism, this enzyme-bound Mg2+ is required at the activation step. (b) Another Mg2+ is required at ATP, but the MgATP apparently can be replaced by the spermidine.ATP complex. Spermidine.ATP is a weaker substrate. The role of spermidine.ATP is especially suggested by the relative rates of the aminoacylation of tRNA when the spermidine and magnesium concentrations are varied. The aminoacylation measurements still suggest that (c) two (or more) Mg2+ are bound to the tRNA molecule and are required for enzyme activity at the transfer step, and that these Mg2+ can be replaced by spermidines.     

Evidence for two types of complexes formed by yeast tyrosyl-tRNA synthetase with cognate and non-cognate tRNA. Effect of ribonucleoside triphosphates

Polyacrylamide gel electrophoresis at pH 8.3 was used to detect and quantitate the formation of the yeast tyrosyl-tRNA synthetase (an alpha 2-type enzyme) complex with its cognate tRNA. Electrophoretic mobility of the complex is intermediate between the free enzyme and free tRNA; picomolar quantities can be readily detected by silver staining and quantitated by densitometry of autoradiograms when [32P]tRNA is used. Two kinds of complexes of Tyr-tRNA synthetase with yeast tRNA(Tyr) were detected. A slower-moving complex is formed at ratios of tRNA(Tyr)/enzyme less than or equal to 0.5; it is assigned the composition tRNA.(alpha 2)2. At higher ratios, a faster-moving complex is formed, approaching saturation at tRNA(Tyr)/enzyme = 1; any excess of tRNA(Tyr) remains unbound. This complex is assigned the composition tRNA.alpha 2. The slower, i.e. tRNA.(alpha 2)2 complex, but not the faster complex, can be formed even with non-cognate tRNAs. Competition experiments show that the affinity of the enzyme towards tRNA(Tyr) is at least 10-fold higher than that for the non-cognate tRNAs. ATP and GTP affect the electrophoretic mobility of the enzyme and prevent the formation of tRNA.(alpha 2)2 complexes both with cognate and non-cognate tRNAs, while neither tyrosine, as the third substrate of Tyr tRNA synthetase, nor AMP, AMP/PPi, or spermidine, have such effects. Hence, the ATP-mediated formation of the alpha 2 structure parallels the increase in specificity of the enzyme towards its cognate tRNA.     

Mechanistic and kinetic study of the ATP-dependent DNA ligase of Neisseria meningitidis

The gene from Neisseria meningitidis serogroup A, encoding a putative, secreted ATP-dependent DNA ligase was cloned and overexpressed, and the soluble protein was purified. Mass spectrometry indicated that the homogeneous protein was adenylated as isolated, and sedimentation velocity experiments suggested that the enzyme exists as a monomer in solution. The 31.5 kDa protein can catalyze the ATP-dependent ligation of a singly nicked DNA duplex but not blunt-end joining. The first step of the overall reaction, the ATP-dependent formation of an adenylated ligase, was studied by measuring the formation of the covalent intermediate and isotope exchange between [alpha-32P] ATP and PPi. Mg2+ was absolutely required for this reaction and was the best divalent cation to promote catalysis. Electrophoretic gel mobility shift assays revealed that the enzyme bound both unnicked and singly nicked double stranded DNA with equivalent affinity (Kd approximately 50 nM) but cannot bind single stranded DNA. Preadenylated DNA was synthesized by transferring the AMP group from the enzyme to the 5'-phosphate of a 3'-dideoxy nicked DNA. The rate of phosphodiester bond formation at the preadenylated nick was also Mg(2+)-dependent. Kinetic data showed that the overall rate of ligation, which occurred at 0.008 s(-1), is the result of three chemical steps with similar rate constants (approximately 0.025 s(-1)). The Km values for ATP and DNA substrates, in the overall ligation reaction, were 0.4 microM and 30 nM, respectively.     

Characterization of a proteolipid complex of aminoacyl-tRNA synthetases and transfer RNA from rat liver.

A high molecular weight complex containing aminoacyl-tRNA synthetases, peptidyl acetyltransferase, lipids and tRNA has been isolated from the 250,000 x g postmitochondrial supernatant from rat liver cells. Aminoacyl-tRNA synthetase activity directed towards arginine, aspartate, glutamine, glutamate, glycine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, and tyrosine is present. An endogenous pool of aminoacyladenylates is indicated by an ATP-32PPi exchange catalyzed by the native complex, which shows a dramatic increase after addition of ATP. Lysine is the only amino acid which greatly increases the exchange rate catalyzed by the native complex in vitro, whereas components of the denatured complex activate all the 13 amino acids in the presence of ATP. Six of the eight lipid fractions were glycolipids; cholesterol and cholesterol esters were absent. The extracted RNA has many characteristics of tRNA. These findings provide evidence for the organization of aminoacyl-tRNA synthetases in a complex with peptidyl acetyltransferase that also contains lipids and tRNA and that can be readily isolated from the cytosol of rat liver cells.     

Purification and partial characterization of ubiquitin-activating enzyme from Saccharomyces cerevisiae

Ubiquitin-activating enzyme was purified from the yeast Saccharomyces cerevisiae by covalent affinity chromatography on ubiquitin-Sepharose followed by HPLC anion-exchange chromatography. Enzyme activity was monitored by the ubiquitin-dependent ATP: 32PPi exchange assay. The purified enzyme has a specific activity of 1.5 mumol 32PPi incorporated into ATP.min-1.mg-1 at 37 degrees C and pH 7.0 under standard conditions for substrate concentrations as described by Ciechanover et al. (1982) J. Biol. Chem. 257, 2537-2542. The catalytic activity showed a maximum at pH 7.0. Its molecular weight both in non-denaturing and in SDS-gel electrophoresis was estimated to be 115 kDa, suggesting a monomeric form. The isoelectric point determined by gel electrofocusing was approximately 4.7. Two protein bands differing slightly in electrophoretic mobility could be distinguished when SDS gels were loaded with very small amounts of purified E1 and immunoblotted, the one with higher molecular weight being clearly predominant. The same two bands were also found in anti-E1 immunoblots of crude yeast lysates prepared under broad protease inhibition.     

Partial reactions of aminoacyl-tRNA synthetases as functions of pH.

The effect of pH on the properties of the partial reactions of arginyl-tRNA synthetase of E. coli has been investigated. V max of pyrophosphorolysis of arginyl adenylate has a pH optimum at pH 6.1, whereas V max of the transfer of arginine to tRNA has a pH optimum of 8.2. These values correlate with the pH optima of the ATP:PPi exchange and the overall esterification reaction, respectively. Only the pyrophosphorolysis reaction requires a divalent cation; transfer proceeds in the presence of EDTA. Inorganic pyrophosphate inhibits the transfer reaction to an extent independent of the concentration of tRNA; the maximum inhibition is a function of pH, corresponding to the relative rate of pyrophosphorolysis of the common intermediate compared with the rate of transfer. These results show that different groups on the enzyme participate in the rate-limiting steps of the two partial reactions and that these partial reactions have properties consistent with their participation in the overall esterification of arginine with tRNA.     

Transfer RNA pyrophosphorolysis with CTP(ATP):tRNA nucleotidyltransferase. A direct route to tRNAs modified at the 3' terminus

The pyrophosphorolysis of tRNA by yeast CTP-(ATP):tRNA nucleotidyltransferase has been studied in an effort to define the behavior of the enzyme and the experimental parameters that lead to net loss of the 3'-terminal nucleotide or to nucleotide exchange. It was found that removal of AMP from the terminus of tRNA proceeded optimally at 1.0 mM PPi; incorporation of 2'- or 3'-dAMP was also studied and shown to proceed optimally at a 6.0 mM concentration of deoxynucleoside triphosphate. CTP was shown to inhibit the pyrophosphorolysis and nucleotide exchange observed when starting from intact tRNA, but apparently not by inhibiting removal of CMP from tRNA missing the 3'-terminal adenosine moiety. The optimized conditions for nucleotide exchange were used for the preparative conversion of tRNAs to species terminating in 2'- and 3'-deoxyadenosine.     

Arginyl-tRNA synthetase from baker's yeast. Purification and some properties.

Arginyl-tRNA synthetase from baker's yeast (Saccharomyces cerevisiae, strain 836) was obtained pure by a large-scale preparative method, which involves four chromatographic columns and one preparative polyacrylamide gel electrophoretic step. The enzyme has a high specific activity (9000 U/mg) and consists of a single polypeptide chain of molecular weight approximately 73000 as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulphate. Amino acid analysis of the enzyme permitted calculation of the absorption coefficient of arginyl-tRNA synthetase (A(1 mg/ml 280 nm)=1.26). Concerning kinetic parameters of the enzyme we found the following Km values: 0.28 muM, 300 muM, 1.5 muM for tRNA(Arg III), ATP and arginine in the aminoacylation reaction, and 1400 muM, 2.5 muM, and 50 muM for ATP, arginine and PP(i) in the ATP-PP(i) exchange reaction. Arginyl-tRNA synthetase required tRNA(Arg III) to catalyse the ATP-PP(i) exchange reaction.     

Purification and characterization of phosphatidylinositol kinase from Saccharomyces cerevisiae

The membrane-associated phospholipid biosynthetic enzyme phosphatidylinositol kinase (ATP:phosphatidylinositol 4-phosphotransferase, EC 2.7.1.67) was purified 8,000-fold from Saccharomyces cerevisiae. The purification procedure included Triton X-100 solubilization of microsomal membranes, DE-52 chromatography, hydroxylapatite chromatography, octyl-Sepharose chromatography, and two consecutive Mono Q chromatographies. The procedure resulted in the isolation of a protein with a subunit molecular weight of 35,000 that was 96% of homogeneity as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Phosphatidylinositol kinase activity was associated with the purified Mr 35,000 subunit. Maximum phosphatidylinositol kinase activity was dependent on magnesium ions and Triton X-100 at pH 8. The true Km values for phosphatidylinositol and MgATP were 70 microM and 0.3 mM, and the true Vmax was 4,750 nmol/min/mg. The turnover number for the enzyme was 166 min-1. Results of kinetic and isotopic exchange reactions indicated that phosphatidylinositol kinase catalyzed a sequential Bi Bi reaction mechanism. The enzyme bound to phosphatidylinositol prior to ATP and phosphatidylinositol 4-phosphate was the first product released in the reaction. The equilibrium constant for the reaction indicated that the reverse reaction was favored in vitro. The activation energy for the reaction was 31.5 kcal/mol, and the enzyme was thermally labile above 30 degrees C. Phosphatidylinositol kinase activity was inhibited by calcium ions and thioreactive agents. Various nucleotides including adenosine and S-adenosylhomocysteine did not affect phosphatidylinositol kinase activity.     

Crystal structure of tryptophanyl-tRNA synthetase complexed with adenosine-5' tetraphosphate: evidence for distributed use of catalytic binding energy in amino acid activation by class I aminoacyl-tRNA synthetases

Tryptophanyl-tRNA synthetase (TrpRS) is a functionally dimeric ligase, which specifically couples hydrolysis of ATP to AMP and pyrophosphate to the formation of an ester bond between tryptophan and the cognate tRNA. TrpRS from Bacillus stearothermophilus binds the ATP analogue, adenosine-5' tetraphosphate (AQP) competitively with ATP during pyrophosphate exchange. Estimates of binding affinity from this competitive inhibition and from isothermal titration calorimetry show that AQP binds 200 times more tightly than ATP both under conditions of induced-fit, where binding is coupled to an unfavorable conformational change, and under exchange conditions, where there is no conformational change. These binding data provide an indirect experimental measurement of +3.0 kcal/mol for the conformational free energy change associated with induced-fit assembly of the active site. Thermodynamic parameters derived from the calorimetry reveal very modest enthalpic changes, consistent with binding driven largely by a favorable entropy change. The 2.5 A structure of the TrpRS:AQP complex, determined de novo by X-ray crystallography, resembles that of the previously described, pre-transition state TrpRS:ATP complexes. The anticodon-binding domain untwists relative to the Rossmann-fold domain by 20% of the way toward the orientation observed for the Products complex. An unexpected tetraphosphate conformation allows the gamma and deltad phosphate groups to occupy positions equivalent to those occupied by the beta and gamma phosphates of ATP. The beta-phosphate effects a 1.11 A extension that relocates the alpha-phosphate toward the tryptophan carboxylate while the PPi mimic moves deeper into the KMSKS loop. This configuration improves interactions between enzyme and nucleotide significantly and uniformly in the adenosine and PPi binding subsites. A new hydrogen bond forms between S194 from the class I KMSKS signature sequence and the PPi mimic. These complementary thermodynamic and structural data are all consistent with the conclusion that the tetraphosphate mimics a transition-state in which the KMSKS loop develops increasingly tight bonds to the PPi leaving group, weakening linkage to the Palpha as it is relocated by an energetically favorable domain movement. Consistent with extensive mutational data on Tyrosyl-tRNA synthetase, this aspect of the mechanism develops high transition-state affinity for the adenosine and pyrophosphate moieties, which move significantly, relative to one another, during the catalytic step.     

The three cysteine residues of cytoplasmic aspartyl-tRNA synthetase from Saccharomyces cerevisiae are not essential for its activity

Cytoplasmic aspartyl-tRNA synthetase from Saccharomyces cerevisiae is a dimer made up of identical subunits (Mr 63,000) each of these containing three cysteines (residues 255, 512 and 519 in the amino acid sequence). Thiol-specific probes were used to label these cysteines and study the resulting effect of the modification on the kinetic parameters of both the ATP/PPi exchange and tRNA aminoacylation reactions. Using the classical techniques of protein chemistry it was shown that none of the three cysteines was labelled with iodoacetic acid, whilst N-ethylmaleimide and 5,5'-dithiobis(2-nitrobenzoate) reacted with Cys512 and Cys255, respectively. Only the latter modification was accompanied by a decrease in the rates of both enzyme activities whilst the Km values for the various substrates remained unaffected. Site-directed mutagenesis was also used to replace each of the three cysteines by other residues, either individually or simultaneously. For these experiments the enzyme was expressed in Escherichia coli using an expression vector bearing the structural gene in which the first 13 codons were replaced by the first 14 of the CII lambda gene. The resulting substitution in the amino-terminal part of the expressed enzyme had no effect on the kinetic parameters, compared to those of the enzyme purified from S. cerevisiae. Taking into account the consequences of such substitutions, as well as those of chemical modifications on the two reactions catalysed by the enzyme. ATP/PPi exchange and tRNA aminoacylation, it could be concluded that none of these three cysteines plays any essential role in either substrate binding or catalysis.     

Human thymidylate kinase. Purification, characterization, and kinetic behavior of the thymidylate kinase derived from chronic myelocytic leukemia.

Thymidylate kinase derived from the blast cells of human chronic myelocytic leukemia was purified 2186-fold to near homogeneity by means of alcohol precipitation, alumina-Cgamma gel fractionation, calcium phosphate gel fraction, ultrafiltration, and affinity column chromatography. The molecular weight was estimated by glycerol gradient centrifugation to be 50,000. This enzyme had an optimal activity at pH 7.1 and required a divalent cation in order to catalyze the reaction. Mg2+ and Mn2+ were found to be the preferential divalent cations. The activation energy was estimated to be 19.1 kcal/mol at pH 7.2. Initial velocity study suggested that the reaction followed a sequential mechanism. Mg2+ ATP had a Km of 0.25 mM and dTMP had a Km of 40 micrometer. The enzyme was unstable even at 4 degrees. In the presence of ATP or dTMP the enzyme maintained its activity. Purine triphosphate nucleosides were found to be better phosphate donors than the pyrimidine triphosphate nucleosides. ATP and dATP had a lower Km and a higher Vmax than GTP and dGTP. dTMP was the only preferred phosphate receptor among all the monophosphate nucleotides tested dTTP and IdUTP competed with both substrates and inhibited the reaction with a Ki of 0.75 mM and 1.1 mM, respectively.