Isoprenoids occur in all eukaryotes. Despite the astonishing diversity of isprenoid molecules that are produced, there is a great deal of similarity in the mechanisms by which different species synthesize them. In fact, the initial phase of isoprenoid synthesis (the synthesis of isopentenyl pyrophosphate) appears to be identical in all of the species in which this process has been investigated. Thus, some early steps of isporenoid pathway could be used for genetic modification.

Starting with the simple compounds acetyl-CoA, glyceraldehyde-3-phopsphate, and pyruvate, which arise via the central pathawys of metabolism, the key intermediate isopentenyl diphosphate is formed by two independent routes. It is then converted by bacteria, fungi, plants and animals into thousands of different naturally occuring products. In fungi, carotenoids are derived by sequnce reactions via the mevalonate biosynthetic pathway. The main product 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) is finaly reduced to the mevalonic acid. This two-step reduction of HMG-CoA to mevalonate is highly controlled and is also a major control factor of sterol synthesis (Metzler, 2003). From prenyl diphosphates of different chain lengths, specific routes branch off into various terpenoid end products (Figure 9).