Squalene cyclase (SQCY) domain; found in class II terpene cyclases that have an alpha 6 - alpha 6 barrel fold. Squalene cyclase (SQCY) and 2,3-oxidosqualene cyclase (OSQCY) are integral membrane proteins that catalyze a cationic cyclization cascade converting linear triterpenes to fused ring compounds. Bacterial SQCY catalyzes the convertion of squalene to hopene or diplopterol. Eukaryotic OSQCY transforms the 2,3-epoxide of squalene to compounds such as, lanosterol (a metabolic precursor of cholesterol and steroid hormones) in mammals and fungi or, cycloartenol in plants. Deletion of a single glycine residue of Alicyclobacillus acidocaldarius SQCY alters its substrate specificity into that of eukaryotic OSQCY. Both enzymes have a second minor domain, which forms an alpha-alpha barrel that is inserted into the major domain. This group also contains SQCY-like archael sequences and some bacterial SQCY's which lack this minor domain.
Jiyao W et al.(2023). "The conserved domain database in 2023.",