Lanosterol synthase

1W6J, 1W6K404716987ENSG00000160285ENSG00000281289ENSMUSG00000033105P48449Q8BLN5NM_001001438NM_001145436NM_001145437NM_002340NM_146006NP_001001438NP_001138908NP_001138909NP_002331NP_666118Lanosterol synthase is an oxidosqualene cyclase (OSC) enzyme that converts (S)-2,3-oxidosqualene to a protosterol cation and finally to lanosterol. Lanosterol is a key four-ringed intermediate in cholesterol biosynthesis. In humans, lanosterol synthase is encoded by the LSS gene.1w6j: STRUCTURE OF HUMAN OSC IN COMPLEX WITH RO 48-80711w6k: STRUCTURE OF HUMAN OSC IN COMPLEX WITH LANOSTEROL Lanosterol synthase is an oxidosqualene cyclase (OSC) enzyme that converts (S)-2,3-oxidosqualene to a protosterol cation and finally to lanosterol. Lanosterol is a key four-ringed intermediate in cholesterol biosynthesis. In humans, lanosterol synthase is encoded by the LSS gene. In eukaryotes, lanosterol synthase is an integral monotopic protein associated with the cytosolic side of the endoplasmic reticulum. Some evidence suggests that the enzyme is a soluble, non-membrane bound protein in the few prokaryotes that produce it. Due to the enzyme’s role in cholesterol biosynthesis, there is interest in lanosterol synthase inhibitors as potential cholesterol-reducing drugs, to complement existing statins. Though some data on the mechanism has been obtained by the use of suicide inhibitors, mutagenesis studies, and homology modeling, it is still not fully understood how the enzyme catalyzes the formation of lanosterol. Before the acquisition of the protein’s X-ray crystal structure, site-directed mutagenesis was used to determine residues key to the enzyme’s catalytic activity. It was determined that an aspartic acid residue (D455) and two histidine residues (H146 and H234) were essential to enzyme function. Corey et al. hypothesized that the aspartic acid acts by protonating the substrate’s epoxide ring, thus increasing its susceptibility to intramolecular attack by the nearest double bond, with H146 possibly intensifying the proton donor ability of the aspartic acid through hydrogen bonding. After acquisition of the 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 cysteine residues (C456 and C533) than from the earlier suggested histidine. Epoxide protonation activates the substrate, setting off a cascade of ring forming reactions. Four rings in total (A through D) are formed, producing the 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 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. Lanosterol synthase is a two-domain monomeric protein composed of two connected (α/α) barrel domains and three smaller β-structures. The enzyme 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 protein conformation. In eukaryotes, a 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 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. Lanosterol synthase catalyzes the conversion of (S)-2,3-epoxysqualene to lanosterol, a key four-ringed intermediate in cholesterol biosynthesis. Thus, it in turn provides the precursor to estrogens, androgens, progestogens, glucocorticoids, mineralocorticoids, and neurosteroids. In eukaryotes the enzyme is bound to the cytosolic side of the endoplasmic reticulum membrane. While cholesterol synthesis is mostly associated with eukaryotes, few prokaryotes have been found to express lanosterol synthase; it has been found as a soluble protein in Methylococcus capsulatus.