Illustration showing the molecular structure of the enzyme proprotein convertase subtilisin/kexin type 9 (PCSK9). PCSK9 binds to the receptor for low-density lipoprotein particles (LDL). LDL is a form of cholesterol found in the body and high amounts in the blood raise the risk of heart disease. The LDL receptor (LDLR), found on cell membranes, binds and initiates uptake of LDL particles from extracellular fluid into cells. As PCSK9 binds to LDLR, it inhibits this process, leading to higher concentrations of LDL cholesterol in the blood. As such, PCSK9 is a target for treatments to lower blood cholesterol, and by extension improve cardiovascular health. PCSK9s structure consists of three domains: a hinge domain rich in the amino acid cysteine (beige), the LDL-binding domain (green) and the prodomain (pink). Between these domains is an allosteric site, which is a structure which changes shape when a specific molecule (in this case LDL) is bound. Atoms in the allosteric site are represented by spheres and rods and are colour-coded: carbon (grey), oxygen (red), nitrogen (blue) and sulphur (yellow).