Oxidosqualene cyclase

Oxidosqualene cyclase is a monomeric enzyme. Its active site consists of a depression between two barrel domains. The active site is mostly made up of acidic amino acids in the majority of organisms. The residues in the active site make it energetically favorable for oxidosqualene to take on a more folded conformation, which closely resembles its product. == Function ==

Oxidosqualene cyclases produce a wide range of sterols and triterpenes. Products produced by other types of the enzyme include cycloartenol, lupeol, beta-amyrin, and many more. == Regulation ==

The enzyme’s genetic expression in animals is regulated by sterol regulatory element binding protein (SREBP-2), a molecule which also regulates the expression of other enzymes in the cholesterol biosynthesis pathway. The SREBP-2 transcription factor increases enzymatic activity upstream of OSC, such as that of HMG-CoA reductase and squalene synthase, to increase flux through sterol synthesis process. == Mechanism ==

Mechanistically, the enzyme oxidosqualene:lanosterol cyclase 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 and is highly selective. The reaction can be characterized by two main mechanistic series: an epoxide ring opening followed by a series of ring closures and a series of 1,2-hydride and 1,2-methyl shifts. These mechanistic steps are catalyzed by a collection of amino acids in the active site. Ring closure In the enzyme’s active site, a histidine residue activates an aspartic acid residue (which has been identified as Asp455), More specifically, this ring closure occurs ring by ring (labeled A-D in the mechanism below). It is widely accepted that the epoxide protonation is concurrent with the A ring closure. The cation is then transferred from C10 to C8 to form the B ring. Molecular dynamics simulations have proved critical to understanding the conformational changes that oxidosqualene-lanosterol intermediates undergo. Researchers have identified Phe696 as a critical amino acid for controlled and specific hydride shifting in this mechanism (steps 5-6), as it speeds up the shift by lowering the energy barrier of the respective transition states. Final de-protonation Finally, the enzyme de-protonates to yield lanosterol, which has a hydroxyl group instead of an epoxide. This hydroxyl group can be seen in the image above. The literature has shown that two amino acids in OSC are critical for this de-protonation to occur correctly and uniformly. Tyrosine at the 503rd position works in tandem with histidine at the 203rd position to de-protonate the hydrogen at the carbon adjacent to the carbocation in intermediate 8. This is accomplished as shown in the proposed mechanism below. The crystal structure of the OSC-lanosterol complex confirms that Tyr503 and His232 are in optimal positions for this final de-protonation step. == Disease relevance ==

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. 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. Research is being done for other compounds which block different steps in the biosynthesis of cholesterol, including the reaction performed by oxidosqualene cyclase which cyclizes squalene to form lanosterol. 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. Another parasite, Leishmania donovani, is the causative agent for leishmaniasis, a similar tropical disease that is spread by sand flies and disproportionately affects rural areas. GSK2920487A, a cyclic and aromatic small molecule, has been shown to significantly inhibit oxidosqualene cyclase and decrease the survival of the L. donovani. While there is concern that parasites such as T. cruzi and L. donovani are able to scavenge the sterol compounds that oxidosqualene cyclase catalyzes the formation of, the potency of chemical inhibition against this vital enzyme demonstrate the potential of OSC as a therapeutic target. == Evolution ==

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. Rabelo et al. found a conserved active site across seven organisms. It is believed that animal and fungal oxidosqualene cyclases likely evolved from their prokaryotic counterparts. == References ==