Tacrolimus (FK-506) Is a Drug That Inhibits an Enzyme Called

Tacrolimus (FK-506) is a drug that inhibits an enzyme called calcineurin. Calcineurin is a protein phosphatase. This is an enzyme that dephosphorylates (removes phosphate groups from) proteins. When added to cells, tacrolimus can inhibit the dephosphorylation of a protein called NFAT, but it cannot prevent the dephosphorylation of a protein called CDK1. What is the most likely explanation for this finding?

A) NFAT is a substrate of calcineurin, but CDK1 is not.
B) Tacrolimus changes the optimum pH for calcineurin.
C) Tacrolimus is a competitive inhibitor of calcineurin for NFAT, but not for CDK1.
D) Calcineurin requires an additional cofactor to dephosphorylate NFAT.