The crystallographic structure of the squalene-hopene cyclase dimer, with the membrane position indicated in blue, the two monomers in green and pink, and a substrate mimetic in the central cavity in yellow.[1]
Squalene-hopene cyclase (SHC) (EC5.4.99.17) or hopan-22-ol hydro-lyase is an enzyme in the terpene cyclase/mutase family. It catalyzes the interconversion of squalene into a pentacyclic triterpenes, hopene and hopanol.[2][3][4][5][6] This enzyme catalyses the following chemical reactions.
SHC is important because its products, hopanoids, are very much like sterols in eukaryotes in that they condense lipidmembranes and reduce permeability. In fact, SHC and sterol-producing enzymes (oxidosqualene cyclase) are evolutionarily related to each other.[7] Hopanoids are inferred to provide stability in the face of high temperatures and extreme acidity due to the rigid ring structure.[8] Indeed, up-regulation of SHC occurs in certain bacteria in the presence of hot or acidic environments.[9][10] SHC is found mostly in bacteria, but some eukaryotes, such as fungi and land plants, are also known to possess the enzyme.
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^Hoshino T, Sato T (February 2002). "Squalene-hopene cyclase: catalytic mechanism and substrate recognition". Chemical Communications (4): 291–301. doi:10.1039/b108995c. PMID12120044.
^Hoshino T, Nakano S, Kondo T, Sato T, Miyoshi A (May 2004). "Squalene-hopene cyclase: final deprotonation reaction, conformational analysis for the cyclization of (3R,S)-2,3-oxidosqualene and further evidence for the requirement of an isopropylidene moiety both for initiation of the polycyclization cascade and for the formation of the 5-membered E-ring". Organic & Biomolecular Chemistry. 2 (10): 1456–70. doi:10.1039/b401172d. PMID15136801.
^Ourisson G, Rohmer M, Poralla K (1987). "Prokaryotic hopanoids and other polyterpenoid sterol surrogates". Annual Review of Microbiology. 41: 301–33. doi:10.1146/annurev.mi.41.100187.001505. PMID3120639.