274 '"'• "^^'^ GOODWIN 



In steroid biosynthesis isopentenyl pyrophosphate is first isomerized 

 to /3^-dimethylallyl pyrophosphate which acts as a starter for polymeriza- 

 tion. This compound then reacts with two molecules of isopentenyl 

 pyrophosphate as indicated in Fig. 2, eventually yielding the 15-C com- 

 pound farnesyl pyrophosphate which dimerizes to yield the hydrocarbon 

 squalene, the acyclic steroid precursor. By analogy the corresponding 

 compounds concerned with carotenoid biosynthesis would be the 

 20-C compound geranylgeranyl pyrophosphate and lycopersene respect- 

 ively; neither has yet been unequivocally identified. If we accept the 

 assumption that this lycopene derivative is the basic 40-C carotenoid 

 precursor then three main problems arise which are unique to the bio- 

 synthesis of carotenoids: (a) what is the pathway of dehydrogenation of 



y \ /\ 



"OH 



Lycopene Rhodopin [Dehydrorhodopin]* 



I 



'OMe HO"^ \, / ^OMe HO 



Spirilloxanthin 



[Dehydrorhodovibrin] * Rhodovibrin 



* These compounds have not yet been identified. 



Fig. 4. The conversion of lycopene into spirilloxanthin in RJiodospirilliim 

 rubrinn. 



the 40-C-precursor ? {b) how do the acyclic (lycopene) and cyclic (^-caro- 

 tene) derivatives arise .'' and (f) how do the oxygenated derivatives (xantho- 

 phylls) arise ? 



With regard to the first tw'o queries lycopene almost certainly arises by 

 the sequential dehydrogenation of phytoene via the route indicated in 

 Fig. 3. The pathway of /S-carotene synthesis is much less certain because 

 of the doubts about the point at which cyclization takes place. My own 

 view, which has been given in detail previously [4], is that cyclization is an 

 early event in the conversion of the basic 40-C precursor into ^-carotene. 



Hydroxylated xanthophylls are formed in the photosynthetic bacteria 

 by the insertion of oxygen following the completion of the parent hydro- 

 carbon. For example, in Rhodospirillum riibrum the formation of spiril- 

 loxanthin from lycopene has been proved, and the pathway is probably 

 that indicated in Fig. 4 [i, 3]. There are no reports of experiments indicat- 



