phospholipase C
Phospholipase C is a molecular pair of scissors. When the cell needs to start the calcium branch of signaling, this enzyme snips a specific fatty molecule in the membrane in two, and each half becomes a working second messenger.
Specifically, the relevant form (phospholipase C-beta, activated by Gq) hydrolyzes the membrane lipid PIP2 (phosphatidylinositol 4,5-bisphosphate) into inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 is water-soluble and diffuses away to release calcium from intracellular stores; DAG is fatty and stays in the membrane to activate protein kinase C. A related form, phospholipase C-gamma, is switched on instead by kinase-linked receptors such as growth-factor receptors.
Because phospholipase C is the gatekeeper of the IP3/DAG/calcium pathway, every Gq-coupled drug response — alpha-1-mediated vasoconstriction, M3-mediated glandular secretion, histamine H1 effects — passes through it. Few clinical drugs target the enzyme directly, but blocking the upstream receptor is a common and effective way to shut the whole branch down.
One enzymatic cut yields two different messengers at once — IP3 going inward to mobilize calcium and DAG staying put to activate protein kinase C — an economical way to launch two arms of a response together.