In the mechanism of the aspartic proteases:

In the mechanism of the aspartic proteases:
 a. Both aspartic acid residues at the active site are always protonated.
 b. Both aspartic acid residues at the active site are always deprotonated
 c. One aspartic acid residue at the active site is always deprotonated, while the other is protonated.
 d. One aspartic acid residue accepts a proton from the substrate, while another aspartic acid residue donates a proton to the product.
 e. Both aspartic acid residues at the active site are converted to asparagines during the catalytic process.