steroid biosynthesis cholesterol biosynthesis UPa:UPA00063 GO:0006695 Cholesterol biosynthesis springs from a six-carbon intermediate called mevalonate. Cholesterol is found in all eukaryotic cells. Its primary function is to regulate membrane fluidity by altering the packing density of the polar membrane lipids. It is also a precursor of several hormones. Its synthesis from acetyl CoA can be separated into 5 stages, all occuring in the cytosol: - mevalonate biosynthesis, - phosphorylation and decarboxylation of mevalonate: isopentenyl-PP biosynthesis and conversion to dimethylallyl-PP - squalene biosynthesis - conversion of squalene to 7-dehydrocholesterol - cholesterol biosynthesis: cholesterol from 7-dehydrocholesterol Cholesterol is used in the body as a precursor of many other important molecules: bile acids (salts), sex steroid hormones (testosterone, estrogen), adrenal steroid hormones (cortisol, aldosterone, vitamin D). pathway cholesterol metabolism sterol biosynthesis