Mapping the mechanism-based modification sites in L-aspartase from Escherichia coli

Inactivation of the enzyme L-aspartase from Escherichia coli by the substrate analog aspartate beta-semialdehyde has previously been shown to occur by the mechanism-based conversion to the corresponding product aldehyde, followed by covalent modification of cysteine-273 (F. Giorgianni et al. (1995) Biochemistry 34, 3529). Inactivation by the product analog, fumaric acid aldehyde (FAA), has now been examined directly by adding a reduction step to the modification protocol in order to stabilize the resulting enzyme-FAA derivative(s). HPLC and mass spectrometric analyses of proteolytic digests of inactivated L-aspartase have confirmed the modification at cysteine-273, and have also identified an additional modified peptide. The inactivation at this additional site involves a crosslink between cysteine-140 and an adjacent lysine. Site-directed mutagenesis studies have shown that cysteine-140 is a very reactive and accessible nucleophile that is not, however, directly involved in enzyme activity. The adjacent lysine-139 that is modified does appear to play a role in substrate binding. A double mutant in which both of the reactive cysteines have been replaced is almost completely insensitive to modification by these substrate and product analogs.

Elimination of the sensitivity of L-aspartase to active-site-directed inactivation without alteration of catalytic activity

The catalytic activity of the enzyme L-aspartase from Escherichia coli has previously been shown to be sensitive to sulfhydryl reagents. The use of group-specific reagents, and a sequence homology comparison study among the fumarase-aspartase family of enzymes, has not, however, lead to the identification of a specific, essential cysteinyl residue. We have recently shown that L-aspartate-beta-semialdehyde is an alternative substrate for L-aspartase, producing fumaric acid semialdehyde (FAA) which specifically inactivates the enzyme [Schindler, J. F., & Viola, R. E. (1994) Biochemistry 33, 9365]. Proteolytic digests of the resulting inactivated enzyme have now been mapped by HPLC and mass spectrometry. A specific residue (Cys-273) has been determined to be the site of FAA modification. Site-directed mutagenesis of this cysteine in the E. coli enzyme has produced altered enzymes which are considerably less sensitive to active-site-directed inactivation, while retaining full catalytic activity. Thus, cysteine-273 has been identified as an active-site nucleophile that, while not directly involved in catalysis in L-aspartase, is poised to attack an activated double bond in an enzyme-bound product analogue.

Mechanism-based inactivation of L-aspartase from Escherichia coli

The substrate analogue L-aspartate beta-semialdehyde (L-ASA) has been identified as a mechanism-based inactivator of L-aspartase from Escherichia coli. The enzyme catalyzes the deamination of L-ASA to yield fumaric acid semialdehyde (FAA) and NH4+, with the product FAA partitioning between subsequent release or irreversible enzyme inactivation. Complete protection against L-ASA inactivation is observed in the presence of the product fumarate and a divalent metal ion. However, protection against inactivation by the product FAA also requires the presence of an enzyme activator. In addition to functioning as a mechanism-based inactivator, L-ASA has also been shown to serve as an activator of L-aspartase. The mechanism of inactivation by FAA involves the attack of an active site nucleophilic at the alpha-carbon of FAA to yield a stable Michael type enzyme adduct. Subsequent formation of a hydrazone upon treatment of the enzyme adduct with 2,4-dinitrophenylhydrazine confirms the presence of the unreacted aldehydic group of FAA. Examination of a group of product analogues with different substituents has demonstrated a correlation between the electron-withdrawing ability of these functional groups and the rate of inactivation of L-aspartase.

Synthesis of 3-arsonoalanine and its action on aspartate aminotransferase and aspartate ammonia-lyase. Comparison with arsenical analogues of malate and fumarate

DL-3-Arsonoalanine has been synthesized by the Strecker synthesis from the unstable compound arsonoacetaldehyde. It inactivates pig heart cytosolic aspartate aminotransferase and inhibits aspartate ammonia-lyase by competing with aspartate (Ki/Km 0.23). The fumarate analogue (E)-3-arsonoacrylic acid and the malate analogue (RS)-3-arsonolactate also inhibit fumarate hydratase, competing with fumarate (Ki/Km 1.8) and malate (Ki/Km 1.6) respectively. Attempted non-enzymic transamination of 3-arsonoalanine gave elimination of arsenite, in contrast with the transamination of 3-phosphonoalanine, which is either successful or leads to loss of phosphate.

Preparation of active hybrid enzymes composed of the native and chemically inactivated aspartase subunits from Escherichia coli

The hybridization of the native and chemically inactivated aspartase from Escherichia coli was studied. Preparations of the tetrameric enzyme obtained by mixing the native and N-ethylmaleimide (NEM)-inactivated aspartase in 4 M guanidine-HCl followed by 51-fold dilution at room temperature retained catalytic activity. Affinity chromatography on AF-Red TOYO-PEARL separated several active components in the hybridized preparations, and the presence of [14C]NEM-inactivated subunits in the active hybrids was demonstrated. The addition of the native aspartase to Sepharose-bound NEM-inactivated enzyme in 4 M guanidine-HCl resulted in the formation of an immobilized enzyme with enzyme activity. The specific activity of the various hybrids, composed of unmodified and [14C]NEM-inactivated subunits, was roughly proportional to the number of unmodified subunits in each tetramer. Furthermore, when reversible denaturation was conducted on mixtures of the native and NEM-inactivated enzyme at various proportions, the enzyme activity recovered was proportional to the amount of the native enzyme added. These results strongly suggest that each subunit makes an independent contribution to the overall enzyme activity regardless of the presence of other subunits in the same molecule. The theoretical and practical implications of this work are discussed.

L-aspartase from Escherichia coli: substrate specificity and role of divalent metal ions

The enzyme L-aspartase from Escherichia coli has an absolute specificity for its amino acid substrate. An examination of a wide range of structural analogues of L-aspartic acid did not uncover any alternate substrates for this enzyme. A large number of competitive inhibitors of the enzyme have been characterized, with inhibition constants ranging over 2 orders of magnitude. A divalent metal ion is required for enzyme activity above pH 7, and this requirement is met by many transition and alkali earth metals. The binding stoichiometry has been established to be one metal ion bound per subunit. Paramagnetic relaxation studies have shown that the divalent metal ion binds at the recently discovered activator site on L-aspartase and not at the enzyme active site. Enzyme activators are bound within 5 A of the enzyme-bound divalent metal ion. The activator site is remote from the active site of the enzyme, since the relaxation of inhibitors that bind at the active site is not affected by paramagnetic metal ions bound at the activator site.

Fumaraldehydic acid-induced inactivation of aspartase

Fumaraldehydic acid (FAA) induced a time-dependent inactivation of aspartase (L-aspartate ammonia-lyase, EC 4.3.1.1) from Escherichia coli at 30 degrees C and pH 7.4 following pseudo-first order kinetics. The rate of inactivation increased in proportion to the FAA concentration. In addition, the rate of inactivation increased, as the pH was increased. Determination of sulfhydryl groups showed that approximately one among 11 sulfhydryl groups was modified by FAA concomitant with the inactivation. L-Aspartate and fumarate protected the enzyme against FAA-inactivation, when Mg2+ ions were present. Unlike E. coli aspartase, P. fluorescens aspartase was not inactivated by FAA.

Specific inhibition of aspartase by S-2,3-dicarboxyaziridine

Aspartase of Escherichia coli was inhibited in a competitive manner by S-2,3-dicarboxyazirdine (DCAZ), an antibacterial substance against Aeromonas salmonesida. The inhibition constant (Ki) was 55 microM, which was as low as less than one tenth that of the Km value for the substrate, L-aspartate. In view of the fact that both aspartase and fumarase (J. Greenhut et al. (1985) J. Biol. Chem. 260, 6684-6686) were inhibited by DCAZ in competitive manners, common features of the reaction mechanism of the two enzymes were discussed.

Chemical modification of essential histidine residues in aspartase with diethylpyrocarbonate

Aspartase purified from Escherichia coli W cells was inactivated by diethylpyrocarbonate following pseudo-first order kinetics. Upon treatment of the inactivated enzyme with NH2OH, the enzyme activity was completely restored. The difference absorption spectrum of the modified vs. native enzyme preparations exhibited a prominent peak around 240 nm. The pH-dependence of the inactivation rate suggested that an amino acid residue having a pK value of 6.6 was involved in the inactivation. These results indicate that the inactivation was due to the modification of histidine residues. L-Aspartate and fumarate, substrates for the enzyme, and the Cl- ion, an inhibitor, protected the enzyme against the inactivation. Inspection of the spectral change at 240 nm associated with the inactivation in the presence and absence of the Cl- ion revealed that the number of histidine residues essential for the enzyme activity was less than two. Partial inactivation did not result in an appreciable change in the substrate saturation profiles. These results suggest that one or two histidine residues are located at the active site of aspartase and participate in an essential step in the catalytic reaction.