Subtilisin Is a Bacterial Serine Protease That Is Commonly Used

Subtilisin is a bacterial serine protease that is commonly used as an additive in laundry detergent. In its native state, it has a methionine that becomes oxidized, surprisingly leading to enzyme inactivation. To prevent inactivation, subtilisin could be engineered by mutating methionine residues to ___________.
I. isoleucine
II. alanine
III. cysteine
IV. phenylalanine

A) I
B) II
C) III
D) II, III and IV
E) I, II and IV