ABSTRACT
Cholesterol is a steroid. It is an important constituent of cell membranes, where, in mammals, it modulates their fluidity. Cholesterol is also the precursor of steroid hormones such as progesterone, testosterone and cortisol, and the bile salts. Animals are able to synthesize cholesterol de novo by an elegant series of reactions in which all 27 carbon atoms of cholesterol are derived from acetyl coenzyme A (CoA). The first stage in the synthesis of cholesterol is the formation of isopentenyl pyrophosphate. Squalene is then converted into squalene epoxide in a reaction that uses O2 and nicotinamide adenine dinucleotide phosphate (NADPH). The squalene epoxide cyclizes to form lanosterol, and finally cholesterol is formed from lanosterol by the removal of three methyl groups, the reduction of one double bond by NADPH, and the migration of the other double bond.
