100) Tacrolimus (FK-506) is a drug that inhibits an enzyme called calcineurin. C

Question

100) 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?

100) A) NFAT is a substrate of calcineurin, but CDK1 is not

B) Tacrolimus is a competitive inhibitor of calcineurin for NFAT and CDK11

C) Tacrolimus changes the optimum pH for calcineurin

D) Calcineurin requires an additional cofactor to dephosphorylate NFAT

Explanation / Answer

Choice A is correct.

Reason: According to the information, tacrolimus inhibits NFAT but not CDK1. The information also suggests that tacrolimus functions by inhibiting the enzyme calcineurin. Together, these two sets of information sugget that calcineurin requires a specific substrate for deactivation by tacrolimus. Thus, NFAT is supposedly a specific substrate for calcineurin but CDK1 is not. Thus, the enzyme substrate binding will not take place in presence of CDK1 and thus no inhibition of CDK1 phosphorylation will take place by tacrolimus.

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