Protein kinase is an enzyme that promotes phosphorylation of target proteins at specific sites, whereas protein phosphatase catalyzes the removal of a phosphate from a phosphorlated protein. phosphorylation and dephosphorylaytion can function as an on-off switch for a protein's activity, most likely through:

A) the removal of one or more peptides
B) A change in the active site structure
C) A change in the optimal temperature at which a chemical reaction will occur
D) A change in the optimal pH at which a reaction will occur ​