Kinases Are Enzymes That Can Phosphorylate (Transfer Phosphate Groups Onto)macromolecules

Kinases are enzymes that can phosphorylate (transfer phosphate groups onto) macromolecules such as proteins.A particular kinase,Kinase 1 is known to promote cell division.It promotes cell division by phosphorylating Protein X.Phosphorylation of Protein X activates Protein X.Once activated,Protein X stimulates the production of other proteins such as Protein Y and Z that directly promote cell division.In order to function,Kinase 1 requires the presence of Metal A.However,in the presence of Protein A,Kinase 1 is nonfunctional.From the description,what is considered the substrate of Kinase 1?

A) Protein X
B) Protein Y
C) Protein Z
D) Metal A
E) Protein A