Here is the general scheme for our next biosynthetic pathway.

The chart illustrates why we should be interested in this sequence; the importance of cholesterol itself, and of the compounds derived from cholesterol is obvious.

• Unfortunately we will not follow the scheme all the way to cholesterol, since the single arrow the chart uses to connect lanosterol and cholesterol is quite deceiving

• Nineteen steps are involved, and most of them are not well documented or understood
  

• For example, the three methyl groups marked with green stars are oxidatively removed

Yet another reason is that this is an excellent pathway to use to discuss further the regulation of biosynthetic pathways and enzyme inhibition.

• You all have seen ads on TV for the class of pharmaceuticals called statins; these are inhibitors of a key enzyme in this pathway.

• We'll also look at how Mother Nature regulates cholesterol biosynthesis

Squalene and lanosterol are terpenes, meaning that they are constructed from 5-carbon "isoprene" units; they are in fact triterpenes, according to the strange counting scheme used for terpenes:

Isoprene is 2-methyl-1,3-butadiene; Mother Nature uses it in the form of isopentenyl pyrophosphate:

The units can be connected together in three ways, all of which are observed in nature, although head-to-tail is by far the most common.

When trying to pick out isoprene units in natural products, don't worry about the positions of the double bonds - they are easily isomerized. A fair number of terpenes exist in which methyl groups have migrated, also. Lanosterol is such a molecule.