D-Aspartic Acid Activating Enzyme (Streptococcus faecalis)

This chapter describes the assay, purification, and properties of D-aspartic acid activating enzyme. The assay for enzyme activity is based on the measurement of the formation of β-D-aspartyl hydroxamate with the FeCl₃ reagent. One unit of enzyme is the amount of enzyme that catalyzes the formation of 1 micromole of β-D-aspartyl hydroxamate per minute under the assay conditions. The unit is defined with reference to a standard of β-DL-aspartyl hydroxamate. Specific activity is expressed as units per milligram of protein. The purified enzyme is strictly specific for D-aspartic acid, giving no hydroxamate formation with L-aspartic acid, D- or L-glutamic acid, oxaloacetate, or DL-malate. The pH optimum for the enzyme is 7.5. Either Mg²⁺ or Mn²⁺ is required for activity. The preparation also contains a D-aspartyltransferase activity, which catalyzes the transfer of D-aspartic acid-¹⁴C either to lipid-P-P-disaccharide pentapeptide or to uridine diphosphate (UDP)-acetylmuramyl pentapeptide. The enzyme is also found in a number of bacteria, which contain D-aspartic acid in the peptidoglycan of the cell wall.