What enzyme does a G protein activate?

A G protein activates adenylyl cyclase, which is an important enzyme in the signaling pathways of cells. When a ligand binds to a G protein-coupled receptor (GPCR), it causes the G protein to exchange GDP for GTP and become activated. The activated G protein then interacts with adenylyl cyclase, stimulating the conversion of ATP to cyclic AMP (cAMP). cAMP acts as a second messenger that can lead to various cellular responses, such as the activation of protein kinases, which further propagate the signaling cascade.

In this context, adenylyl cyclase plays a critical role in mediating the effects of hormones and neurotransmitters that utilize GPCRs. Other enzymes mentioned in the choices, such as protein kinases and phospholipase, are indeed involved in other signaling pathways but are not directly activated by G proteins in the same manner as adenylyl cyclase. DNA polymerase is primarily involved in DNA replication and is not part of the G protein signaling mechanism.