Preliminary crystallographic studies of Y25F mutant of periplasmic Escherichia coli L-asparaginase

Periplasmic Escherichia coli L-asparaginase II with Y25F mutation in the active-site cavity has been obtained by recombinant techniques. The protein was crystallized in a new hexagonal form (P6(5)22). Single crystals of this polymorph, suitable for X-ray diffraction, were obtained by vapor diffusion using 2-methyl-2,4-pentanediol as precipitant (pH 4.8). The crystals are characterized by a = 81.0, c = 341.1 A and diffract to 2.45 A resolution. The asymmetric unit contains two protein molecules arranged into an AB dimer. The physiologically relevant ABA'B' homotetramer is generated by the action of the crystallographic 2-fold axis along [1, -1, 0]. Kinetic studies show that the loss of the phenolic hydroxyl group at position 25 brought about by the replacement of Y with F strongly impairs kcat without significantly affecting Km.

Cloning and expression of L-asparaginase gene in Escherichia coli

The L-asparaginase (ASN) from Escherichia coli AS1.357 was cloned as a DNA fragment generated using polymerase chain reaction technology and primers derived from conserved regions of published ASN gene sequences. Recombinant plasmid pASN containing ASN gene and expression vector pBV220 was transformed in different E. coli host strains. The activity and expression level of ASN in the engineering strains could reach 228 IU/mL of culture fluid and about 50% of the total soluble cell protein respectively, more than 40-fold the enzyme activity of the wild strain. The recombinant plasmid in E. coli AS1.357 remained stable after 72 h of cultivation and 5 h of heat induction without selective pressure. The ASN gene of E. coli AS1.357 was sequenced and had high homology compared to the reported data.

Construction and structural modeling of a single-chain Fv-asparaginase fusion protein resistant to proteolysis

In this study, we construct a fusion protein composed of L-asparaginase (ASNase; from Escherichia coli AS 1.357) and a protective single-chain Fv (scFv), which was selected from a phage-display scFv library from our previous studies. The antibody moiety of the fusion protein was fused to the N-terminus of the enzyme moiety via a linker peptide, (Gly(4)Ser)(6). Recombinant plasmid pET-SLA was constructed to express scFv-ASNase fusion to high levels in E. coli and the expressed product was found to form inclusion bodies. We obtained a soluble fusion protein by refolding and purification. The soluble fusion protein exhibited about 82% of the enzymatic activity of the native ASNase at the same molar concentration, and had a K(m) value similar to that of the native enzyme for the substrate L-asparagine. Importantly, the fusion protein was more stable than native ASNase. In addition: (1) following treatment with trypsin, alpha-chymotrypsin, and rennet, at 37 degrees C for 30 min, scFv-ASNase fusion retained 94.0%, 88.8%, and 84.5% of its original activity, respectively, whereas native ASNase became inactive; and (2) ScFv-ASNase fusion had a much longer in vitro half-life (9 h) in serum than the native enzyme (2 h). The three-dimensional structure of the fusion protein was obtained by modeling with the Homology and Discover modules of the INSIGHT II software package. On the basis of the structural evidence and biochemical properties, we propose that the scFv moiety of the fusion protein may confer ASNase moiety resistance to proteolysis as a result of both steric hindrance and a change in the electrostatic surface of the enzyme.

Crystallization and preliminary crystallographic studies of a new L-asparaginase encoded by the Escherichia coli genome

A new Escherichia coli L-asparaginase belonging to the class of Ntn amidohydrolases has been crystallized using the vapour-diffusion method and PEG 4000 as the precipitant. The crystals belong to the orthorhombic space group P2(1)2(1)2(1) (unit-cell parameters a = 50. 3, b = 77.6, c = 148.2 A) and diffract to 1.65 A resolution. The structure has been solved by molecular replacement using aspartylglucosaminidase from Flavobacterium meningosepticum as the search model. The asymmetric unit contains four protein chains composed into a dimer of alphabeta heterodimers, where the subunits alpha and beta are the product of autoproteolytic cleavage of the immature protein.

Nitrogen regulation of Saccharomyces cerevisiae invertase. Role of the URE2 gene

The regulation of extracellular enzymes is of great biotechnological interest. We studied the regulatory role of the URE2 gene on the periplasmic invertase of Saccharomyces cerevisiae, because its periplasmic asparaginase is regulated by the URE2/GLN3 system. Enzymatic activity was measured in the isogenic strains P40-1B, the ure2 mutant P40-3C, and the P40-3C strain transformed with the pIC-CS plasmid carrying the URE2 gene. The assays were performed using midlog and stationary phase cells and nitrogen-starved cells from these growth phases. During exponential growth, the level of invertase in both wild-type and ure2 mutant cells was comparable. However, the invertase activity in ure2 mutant cells from stationary phase was sixfold lower than in the wild-type cells. When P40-3C cells were transformed with the pIC-CS plasmid, the wild-type phenotype was restored. On nitrogen starvation in the presence of sucrose, the invertase activity in wild-type cells from midlog phase decreased three times, whereas in stationary cells, the activity decreased eight times. However, invertase activity doubled in ure2 mutant cells from both phases. When these cells were transformed with the aforementioned plasmid, the wild-type phenotype was restored, although a significant invertase decrease in stationary cell was not observed. These results suggested that the URE2 protein plays a role in invertase activity.

Characterization of l-asparaginase fused with a protective ScFv and the protection mechanism

A fusion protein of the protective scFv linked to the C-terminus of ASNase via (Gly(4)Ser)(6) peptide was constructed. The ASNase-scFv fusion protein expressed in Escherichia coli exists mainly in the form of inclusion bodies, and a small amount of it was soluble. The soluble form was purified by four-step purification and it has been demonstrated that ASNase-scFv fusion exists as a dimer. By assay of the stability against proteolysis, the ASNase-scFv fusion was found to be more stable than native ASNase but less stable than scFv-ASNase fusion. The results of immunological assay indicated that the immunogenicity of the fusion proteins increased while their binding capacity with the anti-ASNase serum decreased by comparison to the native ASNase. Moreover, here the comparison of the basic physical and chemical properties of the ASNase-scFv fusion, scFv-ASNase fusion, and native ASNase is presented. Based on the structural evidence and the biochemical analysis described in this paper, the protection mechanism proposed in our previous study was further supported. The scFv moiety of the fusion protein may confer the ASNase moiety resistance to proteolysis as a result of both steric hindrance such as blocking the cleavage sites of trypsin and a change in the electrostatic potential surface of the enzyme.

The L-asparagine operon of Rhizobium etli contains a gene encoding an atypical asparaginase

The L-asparagine operon of Rhizobium etli was cloned and sequenced. Sequence analysis showed four adjacent open reading frames which were designated as ansR, ansP, ansA and ansB. The ansR and ansP genes encoded proteins similar to a transcriptional repressor and an L-asparagine permease, respectively. By Tn5 mutagenesis and complementation analysis we identified the ansA product as a thermolabile asparaginase, and the ansB product as an aspartase. An asparagine-inducible transcript covering ansP, ansA and ansB was detected by reverse transcription (RT)-PCR, indicating that these genes are organized in an operon. Introduction of the R. etli ans operon into Sinorhizobium meliloti induced growth with asparagine as the sole carbon and nitrogen source, suggesting that the ans operon plays the same physiological role in both bacteria. The product of the R. etli ansA gene showed no sequence similarity with previously reported microbial asparaginases, this protein seems to be an atypical asparaginase which evolved apart from bacterial and yeast asparaginases.

Selecting and expressing protective single-chain Fv fragment to stabilize L-asparaginase against inactivation by trypsin

Four non-inhibitory specific single-chain Fv (sc Fv) fragments directed against L-asparaginase (ASNase) of Escherichia coli were selected from a synthetic phage-display scFv library. The scFv46 fragment could enhance the resistance of ASNase to trypsin proteolysis, with 70% of the initial ASNase activity present after the ASNase-scFv46 complex had been treated with trypsin for 30 min at 37 degrees C, whereas little residual activity was detected without the scFv46 fragment. The scFv46 gene was cloned to an expression vector pET-21a and expressed at high levels (about 45% of total cell protein) in E. coli BL21 (DE3) as inclusion bodies. The refolded and purified scFv46 fragment was proved to protect ASNase, and the protective effect was further confirmed by SDS/PAGE. It was found that under optimum conditions of molar ratio of scFv to ASNase, incubation time and temperature, the residual activity of the ASNase-scFv46 complex could reach about 78% after treatment with trypsin for 30 min at 37 degrees C. The results demonstrated that scFv fragments prepared by phage-antibody library technology could be used to protect target proteins.

Identification of an asparagine amidohydrolase from the filarial parasite Dirofilaria immitis

The nematode cuticle is a complex extracellular structure which is secreted by an underlying syncytium of hypodermal cells. Recent studies have demonstrated that the cuticle of parasitic nematodes is a dynamic structure with important absorptive, secretory, and enzymatic activities. In addition, the cuticle serves as a protective barrier against the host. A 48-h third stage larval Dirofilaria immitis cDNA library was immunoscreened with sera raised against larval cuticles. One clone, L3MC4 that reacted strongly with the anti-cuticle antisera was sequenced. The composite cDNA sequence comprises 2073 bp coding for a full-length protein of 590 amino acids. GenBank analysis showed that DiAsp had significant similarity to a Caenorhabditis elegans gene-product (54% identity) and to other asparaginases at the amino acid level. Escherichia coli-expressed recombinant DiAsp (rDiAsp) catalysed the hydrolysis of asparagine to aspartate and ammonia. Antibodies raised against D. immitis larval cuticles reacted with rDiAsp in immunoblots. This is the first report of identification of a cDNA clone encoding an asparaginase enzyme from a parasitic nematode.

Why a "benign" mutation kills enzyme activity. Structure-based analysis of the A176V mutant of Saccharomyces cerevisiae L-asparaginase I

A conservative and apparently harmless A176V mutation in intracellular S. cerevisiae L-asparaginase (ScerAI) completely abolishes the enzyme activity. Sequence and structural comparisons with type II bacterial L-asparaginases show that the mutated residue is in a very conservative region and plays a vital role in the cohesion of functional tetramers of these enzymes through participation in side-chain...main-chain (Ser) Oy...O (Ala) hydrogen bonds across the tetramer interface. The fact that bacterial L-asparaginases of type I show less conservation in this region suggests that they may have different quaternary structure while adopting the subunit fold and intimate dimer architecture of type II enzymes. A comparison of all available sequences of microbial L-asparaginases confirms that separate intra- and extra-cellular enzymes evolved in prokaryotes and eukaryotes independently. However, an analysis of the available complete genome sequences reveals a surprising fact that Haemophilus influenzae possesses only a type II asparaginase while the archaebacterium Methanococcus jannaschii has a type I gene, but not a type II.

Regulation of the ndh gene of Escherichia coli by integration host factor and a novel regulator, Arr

The ndh gene of Escherichia coli encodes the non-proton-translocating NADH dehydrogenase II. Expression of the ndh gene is subject to a complex network of regulatory controls at the transcriptional level. Under anaerobic conditions ndh is repressed by the regulator of fumarate and nitrate reduction (FNR). However, in the absence of FNR, ndh expression is activated by the amino acid response regulator (Arr) during anaerobic growth in rich medium. Expression of the ndh gene varies during the growth cycle in response to the intracellular concentration of the heat-stable DNA-binding protein, Fis. In this work two additional heat-stable proteins, integration host factor (IHF) and the histone-like protein HU were found to interact with the ndh promoter. IHF was shown to bind at three sites centred at +26, -17 and -58 in the ndh promoter (Kd = 10(-8) M), to prevent open-complex formation and to repress ndh transcription in vitro. Studies with an ndh-lacZ fusion confirmed that IHF represses ndh expression in vivo. Two putative binding sites for Arr, which overlap the two FNR boxes in the ndh promoter, were identified. Studies with the FNR-activated and amino-acid-inducible asparaginase II gene (ansB) showed that IHF and a component of the Arr-containing fraction (but not HU) interact with the corresponding ansB promoter.

Asparaginase II of Saccharomyces cerevisiae. GLN3/URE2 regulation of a periplasmic enzyme

The production of some extracellular enzymes is known to be negatively affected by readily metabolized nitrogen sources such as NH4+ although there is no consensus regarding the involved mechanisms. Asparaginase II is a periplasmic enzyme of Saccharomyces cerevisiae encoded by the ASP3 gene. The enzyme activity is not found in cells grown in either ammonia, glutamine, or glutamate, but it is found in cells that have been subjected to nitrogen starvation or have been grown on a poor source of nitrogen such as proline. In this report it is shown that the formation of this enzyme is dependent upon the functional GLN3 gene and that the response to nitrogen availability is under the control of the URE2 gene product. In this respect the expression of ASP3 is similar to the system that regulates the GLN1, GDH2, GAP1, and PUT4 genes that codes for glutamine synthetase, NAD-linked glutamate dehydrogenase, general amino-acid permease, and high affinity proline permease, respectively.

Isolation and characterization of Rhizobium etli mutants altered in degradation of asparagine

Rhizobium etli mutants unable to grow on asparagine as the nitrogen and carbon source were isolated. Two kinds of mutants were obtained: AHZ1, with very low levels of aspartase activity, and AHZ7, with low levels of asparaginase and very low levels of aspartase compared to the wild-type strain. R. etli had two asparaginases differentiated by their thermostabilities, electrophoretic mobilities, and modes of regulation. The AHZ mutants nodulated as did the wild-type strain and had nitrogenase levels similar to that of the wild-type strain.

A covalently bound catalytic intermediate in Escherichia coli asparaginase: crystal structure of a Thr-89-Val mutant

Escherichia coli asparaginase II catalyzes the hydrolysis of L-asparagine to L-aspartate via a threonine-bound acyl-enzyme intermediate. A nearly inactive mutant in which one of the active site threonines, Thr-89, was replaced by valine was constructed, expressed, and crystallized. Its structure, solved at 2.2 A resolution, shows high overall similarity to the wild-type enzyme, but an aspartyl moiety is covalently bound to Thr-12, resembling a reaction intermediate. Kinetic analysis confirms the deacylation deficiency, which is also explained on a structural basis. The previously identified oxyanion hole is described in more detail.

Transcriptional co-activation at the ansB promoters: involvement of the activating regions of CRP and FNR when bound in tandem

Previous work with semi-synthetic promoters containing a single CRP binding site centred at 41.5 bp from the transcription start site has demonstrated enhanced transcription (synergism) when a second binding site, for CRP or FNR, is placed upstream at around -91 bp. The ansB promoter in Escherichia coli is co-activated in a co-dependent manner by one dimer each of CRP and FNR protein whose binding sites are at around -91 and -41 bp, respectively, from the transcription start site. Similarly, the homologous ansB promoter in Salmonella is co-activated by two dimers of CRP which function synergistically. The binding sites at the E. coli promoter have been changed by mutation to provide a number of active promoter derivatives carrying other combinations of FNR and CRP binding sites. The co-dependent versus synergistic interaction of these activators and their requirement for known activating regions have been examined. The results demonstrate that FNR can co-activate when located upstream at around -91 bp in combination with either FNR or CRP downstream. When FNR occupies the downstream site the promoter is co-dependent on an upstream activator, but not when CRP occupies this site. Activating region 1 in CRP (defined by substitutions at residue H159) and its putative equivalent in FNR (defined by substitutions at S73) are mainly required in the upstream activator; the putative equivalent in FNR of activating region 3 of CRP (defined by substitutions at G85 and K52, respectively) is mainly required in the dimer which binds downstream. Activating region 1 of FNR is required only in the downstream subunit of the upstream activator in a promoter which is co-dependent on two FNR dimers. These data suggest that both bound upstream and downstream activators interact with RNA polymerase to promote transcription, and that co-dependence is determined by the nature of the activator plus the promoter context.

Engineering resistance to trypsin inactivation into L-asparaginase through the production of a chimeric protein between the enzyme and a protective single-chain antibody

We have demonstrated that a trypsin sensitive enzyme such as L-asparaginase can be rendered trypsin resistant by genetically fusing its gene with that of a single-chain antibody derived from a preselected monoclonal antibody capable of providing protection against trypsin. The chimeric L-asparaginase retained 75% of its original activity upon exposure to trypsin, whereas the native unprotected L-asparaginase control was totally inactivated.

Repression of the L-asparaginase gene during nodule development in Lupinus angustifolius

Upon the establishment of an effective nitrogen-fixing symbiosis in amide-transporting plants the enzymatic activity and transcript levels of L-asparaginase are dramatically decreased. This decrease in L-asparaginase activity is essential for the correct functioning of the Rhizobium-legume symbiosis in lupin in which asparagine, synthesized from recently fixed nitrogen, is exported to aerial parts of the plant for use in growth and development. Concomitant with this decrease in L-asparaginase transcript a DNA-binding protein was detected in the nodules. This binding protein was not detectable in ineffective nodules, in nodules treated with nitrate, or in root tips, mature roots, developing flowers or developing seeds. The DNA-binding activity was shown to interact with a 59 bp sequence proximal to the transcription start site. Within this sequence a CTAAAAT direct repeat and a ACTGT/TGTCA incomplete inverted repeat were implicated in the binding of protein to the DNA by DNase I protection experiments. Competitive binding studies with synthesized binding sites were consistent with the CTAAAAT/TGTCA sequence pair proximal to the transcription start site having the highest affinity for the DNA-binding protein. We postulate that this DNA-binding protein is associated with repression of L-asparaginase gene expression in mature lupin root nodules.

Erwinia chrysanthemi L-asparaginase: epitope mapping and production of antigenically modified enzymes

This study shows that the antigenicity of Erwinia chrysanthemi L-asparaginase can be reduced by site-directed mutagenesis. Ten B-cell epitopes of the enzyme were identified using synthetic hexapeptides and polyclonal antisera from rabbits and mice. The region 282GIVPPDEELP292 near the C-terminus was an immunodominant epitope. Binding of two hexapeptides (283IVPPDE288 and 287DEELPG292) to the antibodies was dependent on Pro285, and Pro286, since their replacement by almost any other amino acid resulted in reduced binding. The other residues were less important for binding the antibodies, as binding was relatively unaffected by amino acid substitutions. Three site-directed mutant enzymes, P285T (proline-285-->threonine etc.), P286Q and E288A, were expressed in Escherichia coli. The purified enzymes had subunit M(r) values of 35,000. The pI values of P285T, P286Q and the wild-type enzymes were 8.6, and that for the mutant E288A was 9.2. The kcat. and Km values for the mutants P286Q and E288A with L-asparagine and L-glutamine were comparable with those of the wild-type enzyme. The Km values for the mutant P285T with both substrates was similar to that of the wild-type enzyme, whereas the kcat. was reduced by 2-fold with L-asparagine and by 4-fold with L-glutamine. The change proline-->threonine reduced the antigenicity of the enzyme by 8-fold, as shown in sandwich e.l.i.s.a.s. using monoclonal antibodies raised against the wild-type enzyme.

The ASP1 gene of Saccharomyces cerevisiae, encoding the intracellular isozyme of L-asparaginase

Saccharomyces cerevisiae produces two L-asparaginases (ASPs), intracellular ASP I and cell-wall ASP II. In this report, the ASP-I-encoding gene, ASP1, has been identified by homology cloning based on the structures of ASPs from other organisms. Its deduced protein product has a subunit M(r) of 41,414, and shows substantial sequence homology to the bacterial amidohydrolase family. The product of the S. cerevisiae ASP3 gene, a further member of this family, encoding the nitrogen catabolite-regulated cell-wall ASP II, has 46% overall sequence identity to ASP1. Duplication of ancestral asparaginase genes, resulting in separate intra- and extracellular isozymes, appears to have occurred independently in the prokaryotic and eukaryotic lineages. Exact physical mapping of the new cloned ASP1 gene locates it 73% of the distance from the left telomere of chromosome IV, at a position precisely matching the known genetic map location of ASP1. This, along with the structural features of the clone, confirms that ASP1 is the structural gene encoding cytoplasmic ASP I in S. cerevisiae. Sequence analysis of the ethylmethanesulfonate-induced asp1-12 allele of strain XE101-1A revealed a C-->T transition altering Ala176 to Val. This residue lies within a highly conserved region, and the results suggests a critical function for Ala176 in ASP function. Expression of ASP1 and other recombinant ASPs may allow access to improved products for use in the chemotherapy of leukaemia.

Regulation of the ansB gene of Salmonella enterica

The expression of L-asparaginase II (encoded by ansB) in Salmonella enterica was found to be positively regulated by the cAMP receptor protein (CRP) and anaerobiosis. The anaerobic regulation of the S. enterica ansB gene is not mediated by the anaerobic transcriptional activator FNR. This is unlike the situation of the ansB gene of Escherichia coli, which is dependent on both CRP and FNR. To investigate this fundamental difference in the regulation of L-asparaginase II expression in S. enterica, the ansB gene was cloned and the nucleotide sequence of the promoter region determined. Sequence analysis and transcript mapping of the 5' promoter region revealed a single transcriptional start point (tsp) and two regulatory sites with substantial homology with those found in E. coli. One site, centred -90.5 bp from the tsp, is homologous to a hybrid CRP/FNR ('CF') site which is the site of CRP regulation in the E. coli promoter. The other site, centred 40.5 bp upstream of the tsp, is homologous to the FNR binding site of the E. coli promoter. Significantly, however, a single base-pair difference exists in this site, at a position of the related CRP and FNR DNA-binding site consensus sequences known to be involved in CRP versus FNR specificity. Site-directed mutagenesis indicates that this single difference, relative to the homologous E. coli site, results in a CRP binding site and the observed FNR-independent ansB expression in S. enterica.(ABSTRACT TRUNCATED AT 250 WORDS)

Co-dependent positive regulation of the ansB promoter of Escherichia coli by CRP and the FNR protein: a molecular analysis

Transcription of the ansB gene, encoding L-asparaginase II, is positively regulated by cAMP receptor protein (CRP) and by the product of the fnr gene, the FNR protein. These global regulatory proteins mediate the expression of ansB in Escherichia coli in response to carbon source and to anaerobiosis, respectively, and are required concurrently for optimal ansB expression. The mechanism whereby CRP and FNR interact co-operatively with the ansB promoter to achieve transcription has not previously been established. We have utilized an ansB'-'lacZ fusion, in conjunction with deletion analysis and site-directed mutagenesis, to identify two sites which interact with these regulatory proteins in the ansB promoter. The first is an FNR site, centred 41.5 bp upstream of the major transcriptional start site. The second site, located 28 bp upstream of the FNR site, is the site of CRP regulation. This site is homologous to both the CRP and FNR binding-site consensus sequences and may respond to both CRP and FNR. The concurrent requirement for CRP and FNR for optimal expression of ansB may be explained if, first, essentially no transcription occurs unless the FNR is bound at the downstream site, and, second, the level of transcription when FNR alone is present is enhanced when CRP binds at the upstream site.

Probing the role of threonine and serine residues of E. coli asparaginase II by site-specific mutagenesis

Site-specific mutagenesis has been used to probe amino acid residues proposed to be critical in catalysis by Escherichia coli asparaginase II. Thr12 is conserved in all known asparaginases. The catalytic constant of a T12A mutant towards L-aspartic acid beta-hydroxamate was reduced to 0.04% of wild type activity, while its Km and stability against urea denaturation were unchanged. The mutant enzyme T12S exhibited almost normal activity but altered substrate specificity. Replacement of Thr119 with Ala led to a 90% decrease of activity without markedly affecting substrate binding. The mutant enzyme S122A showed normal catalytic function but impaired stability in urea solutions. These data indicate that the hydroxyl group of Thr12 is directly involved in catalysis, probably by favorably interacting with a transition state or intermediate. By contrast, Thr119 and Ser122, both putative target sites of the inactivator DONV, are functionally less important.

Molecular cloning of the gene encoding developing seed L-asparaginase from Lupinus angustifolius

A genomic sequence encoding Lupinus angustifolius L-asparaginase has been obtained, and is the first report of this gene from a plant source. The 3.2 kb of DNA sequenced contains a 1136 bp 5' flanking sequence, four exons and three introns. Intron-exon borders were mapped by comparing the genomic sequence with that of a L. arboreus cDNA. Primer extension analysis revealed transcription start sites 16 bp and 13 bp 5' of the initiating ATG for L. angustifolius and L. arboreus, respectively. The 5' flanking region contained sequences associated with seed-specific expression.

Site-specific mutagenesis of Escherichia coli asparaginase II. None of the three histidine residues is required for catalysis

Site-specific mutagenesis was used to replace the three histidine residues of Escherichia coli asparaginase II (EcA2) with other amino acids. The following enzyme variants were studied: [H87A]EcA2, [H87L]EcA2, [H87K]EcA2, [H183L]EcA2 and [H197L]EcA2. None of the mutations substantially affected the Km for L-aspartic acid beta-hydroxamate or impaired aspartate binding. The relative activities towards L-Asn, L-Gln, and l-aspartic acid beta-hydroxamate were reduced to the same extent, with residual activities exceeding 10% of the wild-type values. These data do not support a number of previous reports suggesting that histidine residues are essential for catalysis. Spectroscopic characterization of the modified enzymes allowed the unequivocal assignment of the histidine resonances in 1H-NMR spectra of asparaginase II. A histidine signal previously shown to disappear upon aspartate binding is due to His183, not to the highly conserved His87. The fact that [H183L]EcA2 has normal activity but greatly reduced stability in the presence of urea suggests that His183 is important for the stabilization of the native asparaginase tetramer. 1H-NMR and fluorescence spectroscopy indicate that His87 is located in the interior of the protein, possibly adjacent to the active site.