receptor tyrosine kinase
A receptor tyrosine kinase is a surface receptor that is also its own enzyme. Most surface receptors hand the signal off to a separate worker, but this receptor does both jobs itself: it catches the hormone outside and immediately starts adding chemical tags on the inside.
When a hormone binds, two receptor molecules usually pair up, and their inner portions phosphorylate each other on the amino acid tyrosine — hence the name. These phosphorylated tyrosines become docking spots that recruit signaling proteins, launching cascades that control growth, metabolism, and gene expression. The insulin receptor is the classic endocrine example.
Receptor tyrosine kinases tend to drive longer-term programs like growth and nutrient storage rather than the fast second-messenger bursts of GPCRs. A caveat worth noting: because these receptors govern cell growth and survival, mutations that lock them permanently “on” are a common cause of cancer, which is why many cancer drugs are kinase inhibitors.
Insulin binds its receptor on muscle and fat cells, the receptor's tyrosine kinase activity fires, and downstream signals move GLUT4 transporters to the surface so glucose can enter.
The insulin receptor, a classic receptor tyrosine kinase, enabling glucose uptake.
Whereas protein kinase A and protein kinase C add phosphate to serine or threonine, receptor tyrosine kinases specifically tag the amino acid tyrosine — a distinction that routes signals along different pathways.