Pre-phytoene pyrophosphate

Presqualene pyrophosphate (32), a compound whose structure has caused considerable controversy in the past, has been isolated from intact rat liver and a yeast microsomal system. Previously, (32) had been detected only in systems which have been starved of NADH and hence the new findings demonstrate that (32) is not an artefact. Despite earlier evidence that lycopersene is a precursor of phytoene, a recent stereochemical analysis of phytoene synthesis makes this appear to be unlikely, and a mechanism has been proposed for the synthesis of cis- and tmnj -phytoene directly from pre-phytoene pyrophosphate (33) (Scheme 7). This mechanism is similar to one proposed for squalene synthesis. ... 

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 enzymes of carotenoid biosynthesis are nuclear encoded but the sites of biosynthesis are the chloroplasts or (in the case of the pigments) the chromoplasts, which are non-chlorophyll-containing plastids. Biosynthesis is initiated by the condensation of two molecules of GGPP (6) which, unusually, occur in a head-to-head fashion under the action of the enzyme phytoene synthase (PSY) (Figure 3.22) (Dogbo et a/. 1988). A cyclopropane ring-containing intermediate, pre-phytoene pyrophosphate (62), has been implicated in the reaction this reaction closely parallels that which occurs in the synthesis of chrysanthemic add (see Section 3.4.1) and squalene (vide infra). PSY purified from Capsicum chromoplasts was found to have an absolute requirement for Mn " " for activity (most terpene synthases use Mg " " to bind pyrophosphate). 

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