Phytoene biosynthesis: Possible mechanisms for the coupling of geranylgeranyl pyrophosphate

Abstract

A cell-free enzyme system, prepared from Phycomyces blakesleeanus mutant albino 10, converted ( 14C)geranylgeranyl pyrophosphate into phytoene. Two mechanisms proposed for the coupling of two molecules of geranylgeranyl pyrophosphate to form phytoene were investigated, and it was also established that phytoene waas the first C 40 polyene formed and not lycopersene (C 40 analogue of squalene). Free geranyllinaloyl pyrophosphate was not detected when the cell-free enzyme system was incubated with ( 14C)geranylgeranyl pyrophosphate. A labelled enzyme-bound intermediate, isolated from the reaction mixture was converted to phytoene when incubated with the cell-free enzyme system. The two proposed coupling mechanisms are discussed in the light of these results.