You are considering the four enzymes below as candidates for iteration saturation mutagenesis (ISM). Enzyme A: structure unknown, substrate known, catalytic domain identified, but important residues have not been characterized, fluorescence multi-step assay developed for detecting product formation.
Enzyme B: structure known, substrate unknown, catalytic residues identified.
Enzyme C: structure/substrate known, catalytic residues identified biochemically, fluorescence-based single-step assay for detecting product formation
Enzyme D: structure/substrate known, catalytic residues identified biochemically, fluorescence-based multi-step assay developed for detecting product formation.

Which of the enzymes above would be the best candidate for ISM?

a. Enzyme A
b. Enzyme B
c. Enzyme C
d. Enzyme D