In mammals, including humans, nitric oxide ($NO$) is an important cellular signaling molecule involved in many physiological processes. Research into its function led to the 1998 Nobel Prize for discovering its role in cardiovascular function. One specific role of nitric oxide in cardiac function is the dilation of blood vessels, a process called vasodilation. Vasodilation of the arteries lowers blood pressure and decreases the force that the heart muscle needs to exert to pump blood.
The cell signaling mechanism begins when $NO$ diffuses into the smooth muscle cells of the blood vessel and activates Guanylate Cyclase which, in turn, stimulates the Guanylate Cyclase to generate cyclic GMP (guanosine monophosphate) from Guanosine Triphosphate (GTP). Protein Kinase G phosphorylates several proteins that regulate calcium concentrations and, in this case, causes alterations in thin and thick muscle filaments that result in smooth muscle relaxation. This signaling mechanism is illustrated below:
Signal transduction pathways are composed of a complex series of biochemical reactions and can be initiated two different ways.
Which of the following statements BEST explains the primary function of each molecule involved in nitric oxide cell signaling?