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. Calcineurin requires an additional cofactor to dephosphorylate NFAT
B. Tacrolimus is a competitive inhibitor of calcineurin for NFAT and CDK11
C. NFAT is a substrate of calcineurin, but CDK1 is not
D. Tacrolimus changes the optimum pH for calcineurin
Answer: C
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