Enzymes can be regulated in a multitude of ways. One such way is by covalent modification, in which functional groups are attached to or removed from the enzyme. One such functional group that can be added to an enzyme is a phosphate group. Depending on the enzyme, addition of a phosphate group may increase or decrease that enzyme's activity. Which of the following is the general name of an enzyme that functions to add phosphate groups to its substrate?

a. Oxidoreductases
b. Ligases
c. Hydrolases
d. Isomerases
e. Transferases