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?
  
  

• The cyclization here is the animal version

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

Human oxidosqualene cyclase has 104 more residues than the bacterial squalene cyclase;

• The two types of enzymes have about 25% sequence identity

• Both have several regions rich in aromatics

• 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)

Divergent evolution?

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