II. "DXP" (plastid) pathway ("Rohmer" pathway)

As mentioned in Lecture 17, isoprenoid biosynthesis proceeds either via the "classical" or most well studied, mevalonate pathway (cytosolic) (for the synthesis of sterols, sesquiterpenes, triterpenoids) or via the non-mevalonate (1-deoxy-D-xylulose-5-phosphate, DXP) pathway for plastidic isoprenoids (carotenoids, phytol [side-chain of chlorophylls], plastoquinone, isoprene, monoterpenes and diterpenes).

Both pathways form isopentenyl pyrophosphate (IPP).

IPP forms all other isoprenoids in head-to-tail condensation reactions.

The recently detected plastidic 1-deoxy-D-xylulose-5-phosphate (DXP) pathway of IPP biosynthesis starts from glyceraldehyde-3-phosphate and pyruvate in a transketolase-type, thiamin-dependent reaction as catalyzed by the DXP-synthase. This yields DXP as the first intermediate. After a transposition step and further modifications IPP is formed, the last step being catalyzed by isopentenyl monophosphate kinase. The DXP pathway provides a new insight into isoprenoid metabolism can help to explain many previous experimental observations. The DXP-pathway is also present in algae and cyanobacteria (ß-carotene, phytol) and bacteria.

•  Lange & Croteau (1999, PNAS 96:13714-13719).

The relationship of general carbon metabolism to isoprenoid biosynthesis in plastids (Lichtenthaler, 1999)

Different isoprenoid groups are formed in different subcellular compartments and by different biosynthetic pathways

• III. Monoterpenes

Monoterpene synthases

McConkey, et. al. Paper [2000 Plant Phys. 122:215-223]

 

 

 

 

 



 

 

 

 

 

 

 

In this example limonene synthase catalyzes the formation of limonene predominantly.  The distinct molecules a-pinene, b-pinene and myrcene are also formed as byproducts at lower yields.  These 4 monoterpenes show toxicity toward insects and are all produced by pine trees as major constituents of the resin.  The formation of these products can be rationalized by the mechanisms involved in the cyclization.

Different sesquiterpene cyclases can utilize the same substrate to produce very different reaction products.  The reaction mechanism is similar to GPP cyclases in which the generation of an electron deficient carbon atom is due to the loss of the diphosphate substituent, a very strong electron-withdrawing group.  The resulting carbocation then attacks another carbon atom that is electon-rich by virtue of its association with a double bound [Chappell (1995) Ann. Rev. Plant Physiol. Pl. Mol. Biol. 46:521-47].