The reaction we are examining:

• Squalene is oxidized to the oxirane (epoxide) by an oxidase enzyme.

• The oxirane, in order to undergo cyclization, must adopt the conformation shown

• Is this conformational change enzyme-induced, or does the enzyme simply bind an existing alternative conformation?
  

• The cyclase enzyme initiates a series of steps:
  • The oxirane is opened

  • The resulting carbocations are attacked by double bonds.

  • Does the enzyme do the whole job at once or sequentially?
  

• Does the methyl migration occur as shown below, by two 1,2-methyl shifts, or by a single 1,3-shift?
  

• The cyclization here is the animal version

• Plants skip the oxidation step, and cyclize through an all-chair conformation leading to a pentacyclic species:
  

Until the summer of 2004, the crystal structures that were available all were of the plant enzyme. In August, 2004, a structure was published for the human oxidosqualene cyclase.

The two types of enzymes have about 25% sequence identity, both have several regions rich in aromatics, and both have aspartates as essential residues.

Sequence Alignment of Squalene Hopene Cyclase (1sqc) and Human Oxidosqualene Cyclase (1w6j)

The backbones align with an rmsd of 1.63 A:

Backbone Alignment of Squalene Hopene Cyclase (1sqc) and Human Oxidosqualene Cyclase (1w6j)

The plant enzyme has so far been more extensively studied. We will look first at it, and then at the human enzyme.