(HMG-CoA). The HMG-CoA is then reduced to give meva- 

 lonic acid (Figure 9). Further steps along the biosynthetic 

 path are shown in Figure 10. Pyrophosphorylation and de- 

 carboxylation of mevalonate give isopentenyl-pyrophosphate, 

 the biological isoprene unit. 



According to Lynen, isopentenyl-pyrophosphate units 

 then condense to give, successively, Cio, C15, and C20 com- 

 pounds, as shown in Figure 10. Hydrogenation of the C20 

 compound could presumably lead to phytol, an alcohol that 

 forms the phytyl tail of chlorophyll. Dimerization of the C15 

 compound, farnesyl pyrophosphate, gives squalene, the pre- 

 cursor for steroids. We might expect the C20 compound, 

 geranylgeranyl pyrophosphate, to undergo a similar conden- 

 sation to give C40 compounds, which could in turn be con- 



M£VAlJONAT£ 

 S 

 i 



HgC-OPjHi' 

 ISOPENTENYL-PrROPHOSPHATE 

 (IPP) 



GERANYL - PYKOPHOSPHATE 



^'^^^..-^CHe- 



— SOUALENE 



FARNESYL-PrmPHOSPHATE \ 



STEROIDS 



>'VsX-sA.^H2 



■oPfeHe' 



PHYTOL •^?—GERAUYL-GERAf/rL- PYROPHOSPHATE 





PHYTOENE 



SUCCESSIVE DEHYDRO- 

 GENATICNS. RING 

 CIXSURE. BMEFSZOKHS, 

 ETC. 



CC ANDfi CAROTEte, WTEIN. VKX-AXANTHIN. ETC 



Figure 10. The biosynthesis of carotenoids. 



(* For details see Figures 7 and 9.) 



61 



