An obligatory step in the intracellular synthesis of cholesterol is the conversion of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) into mevalonate and CoA by the enzyme HMG-CoA reductase; two molecules of NADPH deliver the four electrons required for this reaction. Human HMG-CoA reductase is a polypeptide chain of 888 amino acids; it includes a membrane-spanning portion (a.a. 1-350) and a catalytic portion (a.a. 460-888), connected by a long linker polypeptide. The crystal structure of the catalytic portion shows a tetramer that consists of two dimers, each of which forms two active sites. The active site structure of the human enzyme differs markedly from that of a bacterial HMG-CoA reductase.
Drugs such as the widely used statins can inhibit the synthesis of mevalonate and thus cause an increased uptake of cholesterol from the bloodstream. Crystallographic studies of the catalytic portion of human HMG-CoA reductase with bound statins show the drugs bound to the active sites of the enzyme. The statin molecules not only occupy the binding site of HMG-CoA, but also prevent the formation of the complete active site configuration necessary for the enzyme's function.
Cholesterol is transported in the bloodstream mostly in the form of low density lipoprotein (LDL), a complex of phospholipids, cholesterol, cholesteryl esters, and apolipoprotein B. The LDL receptor, a membrane anchored protein of 839 amino acids, mediates the uptake of LDL into the cellular interior. This receptor binds LDL at neutral pH with high affinity, and releases it when the pH drops below ~6, as happens in endosomes during endocytosis. A crystallographic study at 3.7 Å resolution of the extracellular portion of the receptor (amino acids 1-699) in its low-pH form shows that two of the seven LDL binding modules form a tight contact with the so-called beta-propeller region of the receptor molecule, thus preventing ligand binding. This suggests a possible mechanism for ligand release by the LDL receptor: at low pH the receptor develops a new binding site for the ligand-binding modules which then competes with the ligand LDL. Mutations that are likely to disturb this mechanism have been found in patients with the genetic disease hypercholesterolemia.