PCSK9 (proprotein convertase subtilisin/kexin type 9) is a protein produced mostly by the liver that controls blood cholesterol levels.
It binds to LDL receptors on liver cells, causing those receptors to break down.
Fewer receptors mean less LDL (“bad”) cholesterol is cleared from the blood.
Its net effect is to reduce LDLR recycling, raise plasma LDL-C, and — via MHC class I degradation — dampen tumor immune surveillance.
PCSK9 controls how many LDL receptors stay active on cell surfaces.
High-function mutations cause very high cholesterol, while low-function mutations lead to low cholesterol and lower heart disease risk.
PCSK9 Inhibitors block the PCSK9 protein.
This saves LDL receptors so the liver can clear more bad cholesterol.
PCSK9 (Proprotein Convertase Subtilisin/Kexin Type 9) PCSK9 is the ninth member of a family of serine endoproteinases (the proprotein convertases), but it is unique in that it performs its enzymatic cleavage only once — on itself — and then acts as a non-enzymatic “escort” protein that directs cell-surface receptors to lysosomal degradation.
Its dominant, best-characterized role is in cholesterol homeostasis, but it has emerging roles in inflammation, viral infection, and tumor immune evasion.
Gain-of-function (GOF) mutations in PCSK9 cause autosomal dominant hypercholesterolemia (by accelerating LDLR degradation), while loss-of-function (LOF) mutations produce lifelong hypocholesterolemia and striking protection from atherosclerotic cardiovascular disease.
The most famous example involves two individuals with compound LOF mutations who had extremely low LDL-C from birth and no cardiovascular events despite other risk factors — a “natural experiment” that validated PCSK9 as a drug target. PCSK9, along with LDLR, APOB, and more recently APOE, is now one of the established genes associated with familial hypercholesterolemia.
PCSK9 is synthesized in the endoplasmic reticulum (ER) of hepatocytes as a zymogen and undergoes autocatalytic (intramolecular) cleavage of its prodomain.
This is the only proteolytic event it performs; after cleavage, the prodomain remains bound and masks the catalytic site, rendering PCSK9 catalytically inactive against other substrates.
Mature PCSK9 is secreted into the plasma by the liver.
Circulating PCSK9 is the form responsible for its LDL-raising effect.as it binds to the EGF-A (epidermal growth factor-like repeat A) domain of the LDL receptor (LDLR) on the hepatocyte surface.
This binding locks LDLR in an open/extended conformation, preventing the normal pH-dependent conformational change, result: fewer LDLRs on the hepatocyte surface, reduced hepatic clearance of LDL particles, and elevated plasma LDL-cholesterol (LDL-C).
By downregulating hepatic LDLR, PCSK9 raises circulating LDL-C, lipoprotein(a), and triglyceride-rich lipoproteins, increasing cardiovascular risk.
Inhibiting PCSK9 restores LDLR surface expression and enhances LDL-C clearance.
PCSK9 also has roles in heart failure and vascular inflammation beyond its LDL-C effect.
PCSK9 promotes degradation of MHC class I on tumor cells, reducing intratumoral infiltration of cytotoxic T cells.
PCSK9 inhibition can enhance antigen presentation and tumor immune checkpoint regulation.
PCSK9 has been implicated in modulating viral infection pathways.
PCSK9 inhibitors (monoclonal antibodies such as evolocumab and alirocumab, and the siRNA-based inclisiran) block the PCSK9-LDLR interaction, producing substantial LDL-C reductions (often 50-60%) on top of statin therapy.
PCSK9 is a self-cleaved, catalytically inactive serine protease that functions as a chaperone, escorting the LDL receptor (and other receptors) to lysosomal degradation.
Persons carrying loss of function of proprotein convertase subtilisin kexin type 9 have reduced levels of low density lipoprotein cholesterol and fewer atherosclerotic cardiovascular disease events than persons without such variants.