When the receptor is the enzyme: receptor tyrosine kinases
A receptor tyrosine kinase (RTK) skips the G protein middleman: the receptor itself is an enzyme. When a hormone binds, two receptor halves pair up, and each half phosphorylates the other — like two people shaking hands and stamping each other's badges. Those phosphate stamps become docking sites that recruit a crowd of signaling proteins, launching cascades that change metabolism and growth.
The headline example is insulin. Insulin's RTK sets off a chain that, among other things, moves glucose transporters to the cell surface so the cell can take up sugar. Many growth factors use RTKs too, which is why this pathway, when stuck ON by mutation, is a common driver of cancer.
A close cousin: JAK-STAT
Some receptors have no built-in enzyme, so they borrow one. In the JAK-STAT pathway, the receptor clings to a loose kinase called JAK. Hormone binding brings two receptors together; their JAKs phosphorylate each other and then stamp proteins called STATs. The STATs are special: once stamped, they march straight into the nucleus and switch on genes. It is a remarkably short path from the cell surface to the DNA.
All the way to the DNA: nuclear receptors
Fat-soluble hormones — steroids and thyroid hormone — slip through the membrane and bind an intracellular receptor. The hormone-receptor pair becomes a nuclear receptor: a protein that grips a specific stretch of DNA called a hormone response element and turns nearby genes up or down. The cell's response is to make more (or less) of certain proteins.
This explains the slow but lasting nature of steroid and thyroid effects. Building proteins takes hours, and once built they linger. It also explains why these hormones produce broad, programmatic changes — a single nuclear receptor can switch dozens of genes, retuning a cell's whole behavior rather than just toggling one enzyme.
FOUR SIGNALING STRATEGIES, SIDE BY SIDE GPCR : hormone -> G protein -> second messenger -> response (sec-min) RTK : hormone -> receptor IS the kinase -> cascade (min) JAK-STAT : hormone -> borrowed JAK -> STAT -> nucleus -> genes (min-hr) NUCLEAR RCPTR : hormone -> into cell -> bind DNA (HRE) -> genes (hr-days)