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?