Final answer:
The pathway that activates DAG and elevates Ca2+ involves phospholipase C, which is activated by a ligand-receptor interaction. It cleaves PIP2 to form DAG and IP3; DAG activates PKC and IP3 prompts Ca2+ release from the endoplasmic reticulum, modulating various cellular responses.
Step-by-step explanation:
The pathway that ultimately activates DAG (diacylglycerol) and elevates Ca2+ involves the enzyme phospholipase C. When a ligand binds to a cell surface receptor, it activates phospholipase C through a G protein. Phospholipase C then cleaves a membrane-bound phospholipid, PIP2, into two molecules: DAG, which remains in the plasma membrane, and inositol triphosphate (IP3). DAG activates protein kinase C (PKC), which then phosphorylates serine and threonine residues in its target proteins. At the same time, IP3 diffuses into the cytoplasm and binds to ligand-gated calcium channels on the endoplasmic reticulum, prompting them to release sequestered Ca2+ into the cytosol. The increase in cytosolic Ca2+ levels further activates enzymes, some by binding to calmodulin or calmodulin-like subunits, driving a series of cellular responses.