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).