When the squalene molecule is represented as in Figure 20-35, the relationship of its linear structure to the cyclic structure of the sterols is apparent. All of the sterols have four fused rings (the steroid nucleus) and all are alcohols, with a hydroxyl group at C-3; thus the name "sterol." The action of squalene monooxygenase adds one oxygen atom from O 2 to the end of the squalene chain, forming an epoxide. This enzyme is another mixed-function oxidase (Box 20-1); NADPH reduces the other oxygen atom of O 2 to H 2 O. The double bonds of the product, squalene2,3-epoxide , are positioned so that a remarkable concerted reaction can convert the linear squalene epoxide into a cyclic structure. In animal cells, this cyclization results in the formation of lanosterol , which contains the four rings characteristic of the steroid nucleus. Lanosterol is finally converted into cholesterol in a series of about 20 reactions, including the migration of some methyl groups and the removal of others. Elucidation of this extraordinary biosynthetic pathway, one of the most complex known, was accomplished by Konrad Bloch, Feodor Lynen, John Cornforth, and George Popjak in the late 1950s.
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