Sterols

Sterols are a subgroup of steroids defined by a hydroxyl group at carbon-3 on a four-ring steroid nucleus. They are lipid-soluble molecules embedded in membranes or transported within lipoproteins, where they help regulate fluidity and permeability. Sterols also serve as precursors to other important biomolecules.

The best known sterol in animals is cholesterol. In plants, the major sterols are sitosterol, campesterol, and stigmasterol. Fungi contain ergosterol, while algae and some bacteria also produce sterol-like compounds. These molecules differ in side-chain structure and unsaturation, which influences their membrane properties and biological roles.

Biological roles vary by organism. In animals, cholesterol is essential for animal cell membranes and is the precursor to bile acids, steroid hormones, and vitamin D. Plant sterols contribute to membrane rigidity and signaling and can influence growth and development. Ergosterol in fungal membranes is analogous to cholesterol in animals and is a target for antifungal drugs; azoles inhibit its synthesis, while polyenes disrupt its membrane function.

Biosynthesis of sterols occurs mainly via the mevalonate pathway. HMG-CoA reductase is a key control enzyme; its activity is inhibited by statin drugs to lower cholesterol synthesis in humans. Plants and fungi have similar pathways with species-specific variations. Diet provides sterols from animal products (cholesterol) and plant foods (phytosterols such as sitosterol). Phytosterols can modestly reduce intestinal cholesterol absorption, contributing to lower blood cholesterol in some individuals.

Rare genetic disorders like sitosterolemia cause abnormal accumulation of plant sterols in the body, with potential cardiovascular risk. Overall, sterols are essential, diverse components of cellular membranes and metabolic pathways across life.