Phytoene from Mycobacterium

The last step in the biosynthesis of phytoene is the head-to-head condensation of two molecules of GGPP (Fig. 8). In most respects, this reaction is analogous to the condensation of two molecules of famesyl pyrophosphate to form squalene in the sterol biosynthetic pathway. Studies on the latter reaction have provided a great deal of insight into the mechanism of phytoene biosynthesis and are included in the discussion below where relevant. Both reactions involve the formation of a cyclopropylcarbinyl pyrophosphate intermediate. In the biosynthesis of phytoene this compound is pre-phytoene pyrophosphate, the C40 analogue of presqualene pyrophosphate. A proposed mechanism for the formation of prephytoene pyrophosphate is shown in Fig. 9 (Beytia and Porter, 1976). Prephytoene pyrophosphate has been isolated from a Mycobacterium preparation and synthesized chemically. Both the natural and synthetic compounds have been converted to phytoene by a cell-fiee system from Mycobacterium (Altman et al., 1972). Prephytoene pyrophosphate was also formed when GGPP was incubated with yeast squalene synthetase (Qureshi et al., 1972, 1S>73), but in the pres-... 

The intermediacy of prephytoene pyrophosphate (89) was first shown by Altman and coworkers who identified this compound as a product of the incubation of geranylgeranyl pyrophosphate (66) with an extract of photoinduced Mycobacterium sp. The direct incorporation of geranylgeranyl pyrophosphate (66) into phytoene (88) has been observed in various systems including a soluble tomato plastid enzyme systemchloroplasts isolated from Phaseolus vulgaris cell-free extracts of P. blakesleeanus and... 

Mycobacterium sp.. However, the only relatively well characterized enzyme system is isolated from the soluble fraction of acetone powder of tomato fruit plastids. The tomato plastid enzyme has been partially purified and carries out the direct conversion of both isopentenyl pyrophosphate and geranylgeranyl pyrophosphate (66) to phytoene (88). The molecular weight of phytoene synthetase was estimated to be 200 000 daltons. ... 

The only cofactor required for this condensation is a divalent cation, either Mg or Mn ", and the activity can be inhibited by iodoacetamide, N-ethylmaleimide and p-hydroxymercuribenzoate. Cell-free extracts catalyzing phytoene formation have also been isolated from pea fruits spinach leaves Mycobacterium, Halobacterium cutirub-... 

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