SITE-SPECIFIC MUTAGENESIS OF ESCHERICHIA-COLI ASPARAGINASE-II - NONE OF THE 3 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 K(m) for L-aspartic acid beta-hydroxamate or impaired aspartate binding. The relative activities towards L-Asn, L-Gln, and I-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 H-1-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. H-1-NMR and fluorescence spectroscopy indicate that His87 is located in the interior of the protein, possibly adjacent to the active site.