How can ionized amino acid form be important for the catalytic activity?
Enzymatic reactions are chemical reactions. Chemical reactions involve charge ("electron") transfer. Charge transfer occurs more readily if there is a large gradient in charge density between the attacking active site group and the attacked atom of the substrate. Many enzymes (yes, it is really quite common) use ionic forms of aspartate or glutamate as a catalytic amino acid residue.

For example, some enzymes belonging to the alpha/beta-hydrolase fold family (e.g., haloalkane dehalogenases, epoxide hydrolases) use a deprotonated aspartate as the nucleophile (which attacks an electrophilic atom of the substrate); the specific position of the nucleophile within the enzyme and its interaction with other groups in the enzyme enhance the nucleophilic nature of the aspartate, a phenomenon called "oxyanion hole."



Ollis et al. (1992) Protein Eng 5: 197-211.
https://www.ncbi.nlm.nih.gov/pubmed/1409539