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?

Question

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

Clarify Question
What is the key concept addressed by the question?
What type of thinking is required?

  Gather Content
  What do you already know about phosphates and enzymestructure? What other information is related to the question?

  Choose Answer Do you have all necessary information to analyze how tacrolimus works on NFAT and CDK1?

  Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

B.  NFAT is a substrate of calcineurin, but CDK1 is not

Clarify Question
What is the key concept addressed by the question?
        · The question asks you determine the likely explanation for why the drug tacrolimus works in cells to inhibit the enzyme calcineurin, prevent dephosphorylation of NFAT, and maintain phosphorylation of CDK1.
What type of thinking is required?
        · You are being asked to break down, or analyze, a possible means by which the drug tacrolimus works on calcineurin phosphatase enzyme.  

  Gather Content
  What do you already know about phosphates and enzymestructure? What other information is related to the question?
        · Since calcineurin is an enzyme, that means it works to convert substrates to products – just like any other enzyme. In the case of this enzyme, calcineurin dephosphorylates other proteins. The trick to answering this question correctly is to try and decipher how NFAT and CDK1 relate to     calcineurin.
        · Let’s consider the options. How would calcineurin function if NFAT were a cofactor? A substrate? How might tacrolimus work if it were a competitive inhibitor of calcineurin? Is it possible for tacrolimus to change the operating pH of calcineurin? If you take it one step at a time, what is the likely role of calcineurin on NFAT and CDK1?

  Choose Answer Do you have all necessary information to analyze how tacrolimus works on NFAT and CDK1?
        · It is unlikely that tacrolimus would affect calcineurin’spH.Enyzmes operate in very specific pH ranges and if tacrolimus affected pH, other enzymes besides calcineurin would also be affected.  
        · The question mentions nothing about any cofactors needed for calcineurin to dephosphorylate NFAT.
        · If tacrolimus were a competitive inhibitor of calcineurin, it would interact with the enzyme as a substrate but prevent calcineurin’sdephosphorylation activity, leading to greater phosphorylation of NFAT and CDK1. The question indicates that tacrolimus acts to prevent dephosphorylation of NFAT but not CDK1, so that answer is not plausible.
        · That leaves NFAT as a substrate of calcineurin, which makes sense when you consider that adding tacrolimus prevents NFAT being dephosphorylated by calcineurin. CDK1 is unaffected, as indicated in the question.

  Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?         · This question asked you to determine the likely explanation for relationships between calcineurin, NFAT, and CDK1 proteins based on tacrolimus treatment.
        · If you got the answer correct, good job! If you got an incorrect answer, were you able to decipher that tacrolimus resulted in less calcineurin activity and therefore less NFAT dephosphorylation? Were you able to correlate the activity of calcineurin and NFAT together? Were you infer that their relationship would only be possible if NFAT were a substrate for calcineurin?