4. G protein effector adenylyl cyclase (or AC); phospholipase C, A2, or D CGMP phosphodiesterase A G protein effector enzyme may act to increase or decrease the concentration of second messenger. Certain ion channels are directly modulated by G Proteins4. G protein effector • Certain ion channels are directly modulated by G Proteins. • adenylyl cyclase (or AC); phospholipase C, A2 , or D; cGMP phosphodiesterase. • A G protein effector enzyme may act to increase or decrease the concentration of second messenger