The 18O isotope effect in 13C nuclear magnetic resonance spectroscopy: Mechanistic studies on asparaginase from Escherichia coli

The mechanism of the enzyme asparaginase ( l-asparagine amidohydrolase, EC 3.5.1.1) from Escherichia coli was examined using 13C NMR spectroscopy. The pH-dependent oxygen exchange reactions between water and aspartic acid were followed by use of the 18O isotope-induced shift of the resonance positions of directly bonded 13C nuclei. Both l-1- and l-1,4-[ 13C]aspartic acid were used in experiments with previously 18O-labeled aspartic acid, or in experiments involving the use of 18O-labeled solvent water. Asparaginase catalyzes a relatively efficient exchange between the oxygens of water and those on one carboxyl group of aspartic acid. Exchange at C-4 occurs rapidly but, within experimental error, no exchange at C-1 could be detected. These and related experiments involving the position of 18O incorporation during hydrolysis of aspartic acid β-methyl ester are all consistent with possible acyl-enzyme mechanisms involving C-4, but do not support a free aspartic acid anhydride mechanism.