Question

Why might a compound resembling ADP function as an inhibitor of a protein kinase?

Short answer

Protein kinase gets inhibited through a negative feedback mechanism when ADP concentration is too high.

Step-by-step solution

Concept Introduction 

Protein kinases,as opposed to kinases that modify lipids, carbohydrates,or other substances, preferentially modify other proteins by covalently adding phosphates to them (phosphorylation).

ADP functions as an inhibitor of a protein kinase

Protein kinase is an enzyme that catalyzes the transfer of the phosphate group from ATP to protein; resulting in phosphorylated protein and ADP. When there is too much ADP in the cell, it sends negative feedback to stop the phosphorylation.

If there is a compound resembling ADP, receptors recognize it as ADP causing the inhibition of protein kinase.

Therefore, the negative feedback mechanism inhibits protein kinase.