In the Following Schematic Diagram of a Simple Signaling Pathway

Multiple Choice

In the following schematic diagram of a simple signaling pathway, protein Z regulates the activity of protein X, which is an upstream protein kinase, through a negative feedback loop. Which of the following better describes protein Z?

A) It is a protein kinase that is activated by phosphorylation.
B) It is a protein kinase that is inactivated by phosphorylation.
C) It is a protein phosphatase that is activated by phosphorylation.
D) It is a protein phosphatase that is inactivated by phosphorylation.