protein kinase
/ PROH-teen KY-nayss /
Think of a light switch on the wall. Flip it up and the lamp turns on; flip it down and it turns off. Now imagine the cell needs a way to flip thousands of tiny molecular switches reliably. Its favorite trick is to stick a small chemical tag onto a protein to flip it, and to peel the tag off to flip it back. A protein kinase is the worker that flips switches on — it sticks the tag.
A protein kinase is an enzyme that attaches a phosphate group to another protein, a step called phosphorylation. The phosphate comes from ATP, the cell's energy currency. Adding this small, negatively charged tag changes the target protein's shape, and that shape change usually turns the protein on or off — activating an enzyme, opening a channel, or making a protein grab a new partner. The reverse job, removing the phosphate to flip the switch back, is done by a different enzyme called a phosphatase. Kinases and phosphatases working as a pair give the cell a fast, reversible on/off control over almost any protein.
Protein kinases are the workhorses of signal transduction: a huge share of internal signaling is just one kinase switching on the next. This makes them powerful but dangerous — a kinase stuck permanently 'on' can drive uncontrolled growth, which is why many cancer drugs are kinase inhibitors. The human genome encodes over five hundred kinases, a sign of how central this single trick is. A common misconception is that phosphorylation always activates a protein; in fact adding a phosphate can just as easily switch a protein off — it depends entirely on the protein.
Many '-tinib' cancer drugs, such as imatinib (Gleevec), are kinase inhibitors. In chronic myeloid leukemia a mutation creates a kinase that is permanently switched on, telling the cell to keep dividing; imatinib plugs that kinase and stops the runaway signal at its source.
A stuck-on kinase can drive cancer — and be shut down by a targeted drug.
Adding a phosphate does not always switch a protein on — for some proteins it switches them off. A phosphatase does the reverse, removing the tag, so the on/off control is fully reversible.