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 changes the optimum pH for calcineurin
C. NFAT is a substrate of calcineurin, but CDK1 is not
D. Tacrolimus is a competitive inhibitor of calcineurin for NFAT and CDK11
Answer: C
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