PDBsum entry 1sqc

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Isomerase PDB id
Protein chain
619 a.a. *
Waters ×38
* Residue conservation analysis
PDB id:
Name: Isomerase
Title: Squalene-hopene-cyclase from alicyclobacillus acidocaldarius
Structure: Squalene-hopene cyclase. Chain: a. Engineered: yes
Source: Alicyclobacillus acidocaldarius. Organism_taxid: 405212. Cell_line: jm105. Atcc: 27009. Cellular_location: membrane. Expressed in: escherichia coli k12. Expression_system_taxid: 83333. Expression_system_cell_line: jm105. Other_details: thermostable, acidophilic
2.85Å     R-factor:   0.167     R-free:   0.243
Authors: K.U.Wendt,G.E.Schulz
Key ref:
K.U.Wendt et al. (1997). Structure and function of a squalene cyclase. Science, 277, 1811-1815. PubMed id: 9295270 DOI: 10.1126/science.277.5333.1811
01-Sep-97     Release date:   17-Dec-97    
Go to PROCHECK summary

Protein chain
Pfam   ArchSchema ?
P33247  (SQHC_ALIAD) -  Squalene--hopene cyclase
631 a.a.
619 a.a.
Key:    PfamA domain  Secondary structure  CATH domain

 Enzyme reactions 
   Enzyme class 2: E.C.  - Squalene--hopanol cyclase.
[IntEnz]   [ExPASy]   [KEGG]   [BRENDA]
      Reaction: Hopan-22-ol = squalene + H2O
= squalene
+ H(2)O
   Enzyme class 3: E.C.  - Squalene--hopene cyclase.
[IntEnz]   [ExPASy]   [KEGG]   [BRENDA]

      Reaction: Squalene = hop-2229-ene
= hop-22(29)-ene
Note, where more than one E.C. class is given (as above), each may correspond to a different protein domain or, in the case of polyprotein precursors, to a different mature protein.
Molecule diagrams generated from .mol files obtained from the KEGG ftp site
 Gene Ontology (GO) functional annotation 
  GO annot!
  Cellular component     membrane   2 terms 
  Biological process     hopanoid biosynthetic process   1 term 
  Biochemical function     lyase activity     4 terms  


DOI no: 10.1126/science.277.5333.1811 Science 277:1811-1815 (1997)
PubMed id: 9295270  
Structure and function of a squalene cyclase.
K.U.Wendt, K.Poralla, G.E.Schulz.
The crystal structure of squalene-hopene cyclase from Alicyclobacillus acidocaldarius was determined at 2.9 angstrom resolution. The mechanism and sequence of this cyclase are closely related to those of 2,3-oxidosqualene cyclases that catalyze the cyclization step in cholesterol biosynthesis. The structure reveals a membrane protein with membrane-binding characteristics similar to those of prostaglandin-H2 synthase, the only other reported protein of this type. The active site of the enzyme is located in a large central cavity that is of suitable size to bind squalene in its required conformation and that is lined by aromatic residues. The structure supports a mechanism in which the acid starting the reaction by protonating a carbon-carbon double bond is an aspartate that is coupled to a histidine. Numerous surface alpha helices are connected by characteristic QW-motifs (Q is glutamine and W is tryptophan) that tighten the protein structure, possibly for absorbing the reaction energy without structural damage.
  Selected figure(s)  
Figure 1.
Fig. 1. The proposed reaction steps in squalene-hopene cyclases involving carbocationic intermediates. The general acid B[1]:H protonates (H) squalene at C3, whereas the general base B[2] deprotonates at C29 of the hopenyl cation. In a side reaction, the cation is hydroxylated^ forming hopan-22-ol.
Figure 5.
Fig. 5. The color-coded surface representations (30) with nonpolar (yellow), positive (blue), and negative (red) areas. (A) View similar to Fig. 2 but rotated around a vertical axis and^ sliced. The cutting plane (checked) opens the large internal cavity with the bound inhibitor LDAO. The nonpolar channel runs to the^ left, opening into a nonpolar plateau. The channel constriction (C) appears closed, but it is mobile enough to be readily opened. At the upper left, hopane (two views) is shown at scale. (B) View similar to Fig. 2 directly onto the 1600 Å2 nonpolar plateau with the channel entrance (E) at its center and two nonpolar side chains pointing to the outside. This is the only large nonpolar region on the surface.
  The above figures are reprinted by permission from the AAAs: Science (1997, 277, 1811-1815) copyright 1997.  
  Figures were selected by the author.  

Literature references that cite this PDB file's key reference

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PDB codes: 3p5p 3p5r
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PDB code: 3b96
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PDB code: 1ump
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Biosynthesis and accumulation of sterols.
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Insight into steroid scaffold formation from the structure of human oxidosqualene cyclase.
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PDB codes: 1w6j 1w6k
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Oxidosqualene cyclase second-sphere residues profoundly influence the product profile.
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Site-directed mutagenesis experiments on the putative deprotonation site of squalene-hopene cyclase from Alicyclobacillus acidocaldarius.
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Protein prenyltransferases.
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PDB codes: 1n1b 1n1z 1n20 1n21 1n22 1n23 1n24
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Structure, mechanism and function of prenyltransferases.
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Conversion of a plant oxidosqualene-cycloartenol synthase to an oxidosqualene-lanosterol cyclase by random mutagenesis.
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Functional analyses of Tyr420 and Leu607 of Alicyclobacillus acidocaldarius squalene-hopene cyclase. Neoachillapentaene, a novel triterpene with the 1,5,6-trimethylcyclohexene moiety produced through folding of the constrained boat structure.
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Crystal structure of cis-prenyl chain elongating enzyme, undecaprenyl diphosphate synthase.
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Eubacterial diterpene cyclase genes essential for production of the isoprenoid antibiotic terpentecin.
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Catalytic function of the residues of phenylalanine and tyrosine conserved in squalene-hopene cyclases.
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The most recent references are shown first. Citation data come partly from CiteXplore and partly from an automated harvesting procedure. Note that this is likely to be only a partial list as not all journals are covered by either method. However, we are continually building up the citation data so more and more references will be included with time. Where a reference describes a PDB structure, the PDB codes are shown on the right.