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

Protein kinase is an enzyme that adds phosphate groups to molecules in order to alter their function. ADP, or adenosine diphosphate, has the ability to gain one phosphate group and can be acted on by protein kinase. If a molecule similar in structure to ADP were to come to protein kinase, it would bind to its active site and inhibit the binding of other molecules to the enzyme; therefore, impairing its function.