3.1. Taxol biosynthesis
Taxol is an expensive anti-cancer drug due to its low availability from
a natural source (about 0.02% of DW) and a high cost of extraction
(Ajikumar et al., 2010). Biosynthesis of
taxol takes place in plants through 19 enzyme-catalyzed steps, starting
from the cyclization of geranylgeranyl diphosphate. Universal
diterpenoid precursor, geranylgeranyl diphosphate, is produced by
condensation of three units of isopentenyl diphosphate (IPP) with
dimethylallyl diphosphate (DMAPP) (Fig.5 ). In plants, these
compounds are produced either by the mevalonic pathway (MVA) in the
cytosol or via the methylerythritol phosphate (MEP) pathway in plastids.
Over the years, the MVA pathway considered the sole source of IPP in all
living organisms. However, inconsistencies in this pathway lead to the
discovery of the MEP pathway in bacteria and plants. The actual origin
of a precursor in the taxol biosynthetic pathway remained unclear. In
the taxol pathway, IPP is synthesized through two distinct routes. In
MVA, IPP is synthesized from acetyl-CoA and an MEP from D-glyceraldehyde
3-phosphate pyruvate from glycolysis. The pathway bifurcates after the
formation of isoprenoid precursor IPP and DMAPP, which then leads to the
formation of geranylgeranyl diphosphate. Subsequently, taxa-4(5),
11(12)-diene, also known as taxadiene, from geranylgeranyl diphosphate
is produced, which is catalyzed by taxadiene synthase (TS) (Fig.
5 ). In the next step, CYP450-dependent hydroxylation at the C-5
position of the taxane ring resulted in the allylic rearrangement of the
4(5)-double bond to the 4(20) positions to yield taxa-4(20),
11-diene-5-ol. Further, hydroxylation of this intermediate resulted in
the production of 10-hydroxytaxa-4(20), 11-diene-5รก-yl-acetate, which
upon subsequently acetylation by 10-deacetylbaccatin
III-10-o-acetyltransferase formed Baccatin III. Consequently, this
compound along with side chains of phenylalanine attached to the taxane
core after several steps of hydroxylation resulted in the formation of
final product taxol (Engels et al., 2008;
Guo et al., 2006;
Howat et al., 2014;
Jennewein and Croteau, 2001).