In this system, the interconversion between these two states is performed by two enzymes; thus one example would be a protein Z that exists in a phosphorylated and in an unphosphorylated form and a corresponding kinase Y and a phosphatase X interconvert the two forms.
In this system, the interconversion between these two states is performed by two enzymes with opposing effect. One example would be a protein Z that exists in a phosphorylated form Z P and in an unphosphorylated form Z; the corresponding kinase Y and phosphatase X interconvert the two forms.