Oxidosqualene cyclase

There are two major groups of sterol-producing OSC enzymes:

• Cycloartenol synthase (CAS), found in all plants, which produces primarily cycloartenol
• Lanosterol synthase (LAS), found in all animals and fungi, and occasionally in plants, which produces primarily lanosterol

Sterols and triterpenes are extremely diverse classes of natural products, particularly in plants, which often contain numerous OSC enzymes with different substrate and product specificities;[1] common examples include lupeol synthase and beta-amyrin synthase.[2] OSC enzymes' catalytic mechanism is similar to the prokaryotic squalene-hopene cyclase.[3]

Directed evolution and protein design have been used to identify small numbers of point mutations that alter the product specificities of OSC enzymes, most notably in altering a cycloartenol synthase to produce predominantly lanosterol.[4]

Oxidosqualene cyclase is a monomeric enzyme. Its active site consists of a depression between two barrel domains.[5] The active site is mostly made up of acidic amino acids in the majority of organisms.[6] The residues in the active site make it energetically favorable for oxidosqualene to take on a more folded conformation, which closely resembles its product.[5] This crucially sets the substrate up for the series of reactions that form the rings. Oxidosqualene is located in the cell’s microsome membranes where it can easily harvest its hydrophobic substrate and turn out its hydrophobic product.[7]

Oxidosqualene cyclase is a key enzyme in the cholesterol biosynthesis pathway. It catalyzes the formation of lanosterol, which is then converted through many steps into cholesterol. The body uses cholesterol for temperature regulation. It is also a precursor for testosterone in males and oestradiol in females.[8]

Enzyme Mechanism

The figure shows how, in the cholesterol biosynthesis pathway, squalene is converted to oxidosqualene, which is then converted to lanosterol by oxidosqualene cyclase. Lanosterol is a precursor to cholesterol. This final conversion occurs in many steps.

Mechanistically, the enzyme catalyzes the formation of four rings along the long chain of the substrate (oxidosqualene), producing lanosterol. This cyclization is one of the most complex known enzyme functions[10] and is highly selective.[11] In the enzyme’s active site, a histidine residue activates an aspartic acid residue, which protonates the substrate’s epoxide, setting off a series of carbon-carbon bond formations that form rings.[7][12] Finally, the enzyme deprotonates to yield lanosterol, which has a hydroxyl group instead of an epoxide. This hydroxyl group can be seen in the image above.

High blood cholesterol, also called hypercholesterolemia, significantly increases the risk of stroke, heart attack, and peripheral artery disease. If untreated, it can also lead to plaque accumulation in blood vessels, which is known as atherosclerosis.[13] For this reason, the sterol biosynthetic pathway has long been a target for the drug development industry. Statins, which inhibit HMG-CoA reductase (an enzyme that catalyzes an earlier step in the cholesterol biosynthesis pathway) are commonly prescribed to treat high cholesterol. However, the efficacy and safety of statins has been recently scrutinized in a number of reports.[14][15][16] This is largely because blocking the cholesterol biosynthesis pathway before squalene has been found to disrupt the synthesis of isoprenoids, which are used for the biosynthesis of key molecules in RNA transcription, ATP synthesis, and other essential cell activities.[17] Oxidosqualene cyclase, which is downstream of squalene in the pathway, is an attractive target for inhibition. Many inhibitors have been proposed, among them steroid analogs, phenol-based compounds, benzamide and carboxamide derivatives, and nitrogen-containing heterocyclic compounds. The most effective inhibitors have a hydrogen-bond acceptor at a specific distance away from a hydrophobic region.[6] Inhibitors of oxidosqualene cyclase have shown promise as antimicrobial agents as well, because they’ve been shown to kill off trypanosoma cruzi.[18][19] Trypanosoma cruzi is a parasite transmitted to people by insects, mostly in Latin America. The parasite causes a disease called Chagas disease, in which acute infections around an insect bite can lead to more serious complications, such as decreased heart, esophagus, colon, and even brain function.[20]

Stork, et al. compared the protein sequences of C. albicans oxidosqualene cyclase with the analogous enzyme (squalene cyclase) in two different bacteria and found conserved regions in the former.[21] Rabelo et al. found a conserved active site across seven organisms.[6] It is believed that animal and fungal oxidosqualene cyclases likely evolved from their prokaryotic counterparts.[21]

• Overview of all the structural information available in the PDB for UniProt: P48449 (Lanosterol synthase) at the PDBe-KB.
• Overview of all the structural information available in the PDB for UniProt: P04035 (HMG-CoA reductase) at the PDBe-KB.