Lanosterol synthase

Lanosterol synthase is an oxidosqualene is_associated_with::cyclase (OSC) is_associated_with::enzyme that converts (S)-2,3-oxidosqualene to a protosterol cation and finally to is_associated_with::lanosterol. is_associated_with::Lanosterol is a key four-ringed intermediate in is_associated_with::cholesterol biosynthesis. In humans, lanosterol synthase is encoded by the LSS is_associated_with::gene.

In is_associated_with::eukaryotes, lanosterol synthase is an is_associated_with::integral monotopic protein associated with the is_associated_with::cytosolic side of the is_associated_with::endoplasmic reticulum. Some evidence suggests that the is_associated_with::enzyme is a soluble, non-membrane bound is_associated_with::protein in the few is_associated_with::prokaryotes that produce it.

Due to the enzyme’s role in is_associated_with::cholesterol biosynthesis, there is interest in lanosterol synthase inhibitors as potential cholesterol-reducing drugs, to complement existing is_associated_with::statins.

Mechanism


Though some data on the mechanism has been obtained by the use of suicide inhibitors, mutagenesis studies, and is_associated_with::homology modeling, it is still not fully understood how the is_associated_with::enzyme catalyzes the formation of is_associated_with::lanosterol.

Initial epoxide protonation and ring opening
Before the acquisition of the protein’s is_associated_with::X-ray crystal structure, is_associated_with::site-directed mutagenesis was used to determine residues key to the enzyme’s catalytic activity. It was determined that an is_associated_with::aspartic acid residue (D455) and two is_associated_with::histidine residues (H146 and H234) were essential to enzyme function. Corey et al. hypothesized that the aspartic acid acts by protonating the substrate’s is_associated_with::epoxide ring, thus increasing its susceptibility to is_associated_with::intramolecular attack by the nearest is_associated_with::double bond, with H146 possibly intensifying the proton donor ability of the aspartic acid through is_associated_with::hydrogen bonding. After acquisition of the is_associated_with::X-ray crystal structure of the enzyme, the role of D455 as a proton donor to the substrate’s epoxide was confirmed, though it was found that D455 is more likely stabilized by hydrogen bonding from two is_associated_with::cysteine residues (C456 and C533) than from the earlier suggested histidine.

Ring formation cascade
Epoxide protonation activates the substrate, setting off a cascade of ring forming reactions. Four rings in total (A through D) are formed, producing the is_associated_with::cholesterol backbone. Though the idea of a concerted formation of all four rings had been entertained in the past, kinetic studies with (S)-2,3-oxidosqualene analogs showed that product formation is achieved through discrete is_associated_with::carbocation intermediates (see Figure 1). Isolation of monocyclic and bicyclic products from lanosterol synthase mutants has further weakened the hypothesis of a concerted mechanism. Evidence suggests that epoxide ring opening and A ring formation is concerted, though.

Enzyme structure
Lanosterol synthase is a two-domain monomeric protein composed of two connected (α/α) barrel domains and three smaller β-structures. The enzyme is_associated_with::active site is in the center of the protein, closed off by a constricted channel. Passage of the (S)-2,3-epoxysqualene substrate through the channel requires a change in is_associated_with::protein conformation. In is_associated_with::eukaryotes, a is_associated_with::hydrophobic surface (6% of the total enzyme surface area) is the ER membrane-binding region (see Figure 2).

The enzyme contains five fingerprint regions containing is_associated_with::Gln-Trp motifs, which are also present in the highly analogous bacterial enzyme squalene-hopene cyclase. Residues of these fingerprint regions contain stacked sidechains which are thought to contribute to enzyme stability during the highly exergonic cyclization reactions catalyzed by the enzyme.

Catalysis of lanosterol formation
Lanosterol synthase catalyzes the conversion of (S)-2,3-epoxysqualene to is_associated_with::lanosterol, a key four-ringed intermediate in is_associated_with::cholesterol biosynthesis. Thus, it in turn provides the precursor to is_associated_with::estrogens, is_associated_with::androgens, is_associated_with::progesterones, and is_associated_with::glucocorticoids. In is_associated_with::eukaryotes the enzyme is bound to the is_associated_with::cytosolic side of the is_associated_with::endoplasmic reticulum membrane. While is_associated_with::cholesterol synthesis is mostly associated with is_associated_with::eukaryotes, few is_associated_with::prokaryotes have been found to express lanosterol synthase; it has been found as a soluble protein in is_associated_with::Methylococcus capsulatus.

Catalysis of epoxylanosterol formation
Lanosterol synthase also catalyzes the cyclization of 2,3;22,23-diepoxysqualene to 24(S),25-epoxylanosterol, which is later converted to 24(S),25-epoxycholesterol. Since the enzyme affinity for this second substrate is greater than for the monoepoxy (S)-2,3-epoxysqualene, under partial inhibition conversion of 2,3;22,23-diepoxysqualene to 24(S),25-epoxylanosterol is favored over is_associated_with::lanosterol synthesis. This has relevance for disease prevention and treatment (see Disease Relevance, below).

Enzyme inhibitors as cholesterol-lowering drugs
Interest has grown in lanosterol synthase inhibitors as drugs to lower blood cholesterol and treat is_associated_with::atherosclerosis. The widely popular is_associated_with::statin drugs currently used to lower LDL (low-density lipoprotein) cholesterol function by inhibiting is_associated_with::HMG-CoA reductase activity. Because this enzyme catalyzes the formation of precursors far upstream of (S)-2,3-epoxysqualene and cholesterol, is_associated_with::statins may negatively influence amounts of intermediates required for other biosynthetic pathways (e.g. synthesis of is_associated_with::isoprenoids, is_associated_with::coenzyme Q). Thus, lanosterol synthase, which is more closely tied to cholesterol biosynthesis than is_associated_with::HMG-CoA reductase, is an attractive drug target.

Lanosterol synthase inhibitors are thought to lower is_associated_with::LDL and is_associated_with::VLDL cholesterol by a dual control mechanism. Studies in which lanosterol synthase is partially inhibited have shown both a direct decrease in is_associated_with::lanosterol formation and a decrease in is_associated_with::HMG-CoA reductase activity. The is_associated_with::oxysterol 24(S),25-epoxylanosterol, which is preferentially formed over is_associated_with::lanosterol during partial lanosterol synthase inhibition, is believed to be responsible for this inhibition of is_associated_with::HMG-CoA reductase activity.

Evolution
It is believed that oxidosqualene cyclases (OSCs, the class to which lanosterol cyclase belongs) evolved from bacterial squalene-hopene cyclase (SHC), which is involved with the formation of is_associated_with::hopanoids. is_associated_with::Phylogenetic trees constructed from the amino acid sequences of OSCs in diverse organisms suggest a single common ancestor, and that the synthesis pathway evolved only once. The discovery of is_associated_with::steranes including is_associated_with::cholestane in 2.7-billion year-old shales from is_associated_with::Pilbara Craton, is_associated_with::Australia, suggests that is_associated_with::eukaryotes with OSCs and complex steroid machinery were present early in earth’s history.