Hk. Lichtenthaler et al., 2 INDEPENDENT BIOCHEMICAL PATHWAYS FOR ISOPENTENYL DIPHOSPHATE AND ISOPRENOID BIOSYNTHESIS IN HIGHER-PLANTS, Physiologia Plantarum, 101(3), 1997, pp. 643-652
In the early times of isoprenoid research, a single pathway was found
for the formation of the C-5 monomer, isopentenyl diphosphate (IPP), a
nd this acetate/mevalonate pathway was supposed to occur ubiquitously
in all living organisms. Now, 40 years later, a totally different IPP
biosynthesis route has been detected in eubacteria, green algae and hi
gher plants. In this new pathway glyceraldehyde 3-phosphate (GAP) and
pyruvate are precursors of isopentenyl diphosphate, but not acetyl-CoA
and mevalonic acid. In green tissues of three higher plants it was sh
own that all chloroplast-bound isoprenoids (beta-carotene, phytyl chai
ns of chlorophylls and nona-prenyl chain of plastoquinone-9) are forme
d via the GAP/pyruvate pathway, whereas the cytoplasmic sterols are fo
rmed via the acetate/mevalonate pathway. Also, isoprene, emitted by va
rious plants at high light conditions by action of the plastid-bound i
soprene synthase, is formed via the new GAP/pyruvate pathway. Thus, in
higher plants, there exist two separate and biochemically different I
PP biosynthesis pathways: (1) the novel alternative GAP/pyruvate pathw
ay apparently bound to the plastidic compartment and (2) the classical
cytoplasmic acetate/mevalonate pathway. This new GAP/pyruvate pathway
for IPP formation allows a reasonable interpretation of previous odd
results concerning the biosynthesis of chloroplast isoprenoids, which,
so far, had mainly been interpreted assuming compartmentation differe
nces. The novel GAP/pyruvate pathway for IPP formation in plastids app
ears as a heritage of their prokaryotic, endosymbiotic ancestors.