Phytoene synthetase

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. ... 

A phytoene synthetase that converts IPP into phytoene has been partially purified from tomato plastids. The complex, which had a molecular weight of ca. 200 000 dalton, was devoid of squalene synthetase activity and required Mn as a cofactor. ATP caused a six-fold stimulation in activity, and, as there was no evidence that it was otherwise involved, it may act as an allosteric effector. ... 

As shown in Table II, application of norflurazon inhibited radioactivity incorporation into p-carotene and phytoene. Neither BAS 13761, a non-bleaching pyri-dazinone, nor oxyfluorfen had an effect on 4 distribution. The inhibitory site of norflurazon was found to be on the dimerization reaction of geranylgeranyl pyrophosphate. The norflurazon inhibition of phytoene synthetase from heterotrophic Phycomyces cannot ex-... 

It is generally believed that carotenoid biosynthesis takes place on a multienzyme complex which is bound to, or may be an integral part of, a membrane. Phytoene synthetase appears to be peripheral to the membrane and is easily dissociated from it by detergent, but the later enzymes, i.e. desaturases, cyclases, hydroxylases, are tightly associated in the membrane and are difficult to solubilize without destroying their activity. The organization of the enzymes, and any associated electron transport systems, in the membrane is crucial for carotenoid biosynthesis to proceed. 

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-... 

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