Squalene-hopene cyclase

Alicyclobacillus Squalene-hopene cyclase (SC). . raveyllscEkdeByI.. mekirryllheBredBt. . ypdvBdBavvvwa.. rraveylkreBkpdBsB.. kaldwveqhBnpdBgB.. acidocaldarius... 

Fig. 14.13. Enzymatic transformation of acyclic squalene (A) into pentacyclic hopene (I). The squalene-hopene cyclase controls the conformation of the substrate so effectively that only one out of 512 possible stereoisomers is formed.

In this context, Hoshino et al. have recently reported the cyclization of (3S)-2,3-oxidosqualene catalyzed by a Gly600-deletion mutant (AG600SHC) of the enzyme squalene-hopene cyclase (SHC) from Alicyclobacillus acidocal-darius (Scheme 31) [108]. The enzymatic biotransformation gave monocyclic (9) and tricyclic triterpenoids (38 and 44-47), but no detectable bicychc products [108]. Despite the authors biogenetic proposal of a conventional carbocationic pathway, the skeletal profile of products reported closely resembles that expected for a radical-type cyclization. 

The cyclisation is mediated by squalene-hopene cyclase from prokaryotic species, and is analogous in certain aspects to the squalene oxide - lanosterol cyclase from eukaryotic sources. [257]... 

In 1997, Georg E. Schulz in Freiburg succeeded in determining the structure of the membrane-bound squalene-hopene cyclase from the thermophilic microorganism Alicyclobacillus acidocaldar ms (Fig. 3.53 and Fig. 3.54). [258] The enzyme consists of two domains of approximately equal size. Between the two is a cavity with a volume of 1200 A, which contains the active site this is accessible through a non-polar channel in domain 2 (Fig. 3.55). 

Polyene cyclisation in squalene-hopene cyclase. (D aspartic acid, F Phenylalanine, H Histidine,... 

Stefan Neumann and Helmut Simon (from the Technical University of Munich) had discovered already in 1986 that the squalene-hopene cyclase from Alicydo-bacillus acidocaldarius [260] carries out the cyclisation, not only of squalene to hopene andhopanol, but also ofhomofarnesolto A/wfcrojc . [261] It may therefore be assumed, that this latter cydisation proceeds in a very similar way as in case of squalene. 

Fig. 87.15 Squalene/hopene cyclase from A. acidocaldarius (3SQC). (a) Py-Modular structure, (b) top view on the P-domain showing its highly ordered barrel structure, and (c) top view on the irregular barrel structure of the y-domain...

Class 11 terpene cyclases are known for the cycUzation of di-, sester-, and triterpenoids. Prominent examples of triterpene cyclases are the human lanosterol synthase (oxidosqualene cyclase, OSC) and the squalene/hopene cyclase (SHC) firom A. acidocaldarius [1]. Both enzymes have a py-domain architecture (Fig. 87.15a). The p-domain exhibits a highly regular Oe-Oe barrel structure with six inner and six outer helices surrounding a central cavity (Fig. 87.15b) [198,199, 208]. The y-domains may have originated from the p-domains by gene duplicatimi... 

Squalene/hopene cyclase (SHC), 2724 Src/PI3-kinase/Akt, 2366 SRM. See Selected reaction monitoring (SRM)... 

Squalene-hopene cyclase (SHC) enzymes have been used to effect intramolecular biocatalytic Friedel-Crafts reactions of phenyl ethers [27]. The natural substrate for such enzymes, squalene, is a C hexaene, but it has been demonstrated that an SHC from Zymomonas mobilis can accept a... 

SCHEME 32,5 Tandem cycUzation/aryl alkylation catalyzed by squalene hopene cyclase. 

BBE Berberine bridge enzyme BPDO Biphenyl dioxygenase BZDO Benzoate dioxygenase MAO Monoamine oxidase SAM S-adenosybnethionine SHC Squalene-hopene cyclase TDO Toluene dioxygenase... 

Siedenburg, G., Breuer, M., and Jendrossek, D. (2013) Prokaryotic squalene-hopene cyclases can be converted to citronellal cyclases by single amino acid exchange. Appl. Microbiol. Biotechnol, 97, 1571-1580. 

The predicted amino acid sequences for the squalene-hopene and oxidosqualene-lanosterol cyclases are presented in Figure 4. A comparison of these sequences has important implications for the evolution and mode of action of these enzymes. The squalene-hopene cyclase is predicted to consist of 627 amino acids with a molecular... 

Figure 4. Predicted amino acid sequence for the squalene-hopene cyclase from B. acidocaldarius (Top) and the oxidosqualene-lanosterol cyclase from C. albicans (Bottom). Regions of sequence identity are underlined. Hydrophobic regions of Ae C. albicans cyclase are italicized. Tryptophan residues (W) are bold.

The Aromatic Hypothesis. In addition to the specifically conserved tryptophan residues, we had previously noted that the squalene-hopene cyclase and the oxidosqualene-lanosterol cyclase are generally rich in both tprptophan and tyrosine, amino acid residues with electron-rich aromatic sidechains (42), The squ ene-hopene cyclase contains 3.2% tryptophan and 4.0% tyrosine. The median values for representation of these amino acids in E, coli proteins are 1.2% and 2.7%, respectively (43). The levels of tryptophan and tyrosine found in the B. acidocaldarius enzyme exceed those found in > 95% and 85% of E, coli proteins, respectively. The oxidosqualene-lanosterol cyclase contains 3.0% ti tophan and 6.3% tyrosine. When compared to proteins from the yeast S. cerevisiae (for which the me an values for representation of tryptophan and tyrosine are 0.9% and 3.3%, respectively), the C. albicans enzyme i ssesses these amino acids at levels greater than those found in 99% of all S. cerevisiae proteins. 

Seckler B, Poralla K. Characterization and partial purification of squalene-hopene cyclase from Bacillus acidocaldarius. Biochim Biophys Acta, 1986 881 356-363. 

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