Phytoene formation

Misawa, N. et al.. Expression of a tomato cDNA coding for phytoene synthase in Escherichia coli, phytoene formation in vivo and in vitro, and functional analysis of the various truncated gene products, J. Biochem. (Tokyo) 116, 980, 1994. 

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-... 

In the second step of phytoene formation, inorganic pyrophosphate is expelled and a proton is eliminated. Pyridine nucleotide cofactors are not involved, as is the case in triterpene biosynthesis (Fig. 26.3) (Poulter, 1990). 

The biosynthesis of phytoene proceeds without reduction of prephytoene. The phytoene molecule therefore contains a central double bond. Phytoene synthases from different organisms either form cis- or trans-phytoene. Formation of cis-phytoene requires the stereospecific loss of the Hb in the postulated intermediate I, whereas removal of yields irans-phytoene (Fig. 95). 

Most of the natural carotenoid pigments are tetraterpenes, being formed from two C20 geranylgeranyl pyrophosphate units through phytoene 4.122). The mechanism of phytoene formation has much in common with squalene biosynthesis. In particular, head-on fusion of the two C20 units occurs via prephytoene pyrophosphate 4.120), analogous to presqualene pyrophosphate 4.48) formation from two... 

Prephytoene pyrophosphate as an intermediate of the sequence was established by Gregornis and Rilling (ref. 13). Porter s group investigated the phytoene synthase complex which was obtained from a relatively crude chromoplast preparation from tomato fruits and from an acetone powder prepared thereof. The complex was partially purified and a molecular mass of about 166 000 Da was determined (ref. 14). The dissociation of an isomerase and prenyltransferase activity was achieved (molecular masses about 34 000 Da and 64 000 Da, respectively) (ref. 15). Furthermore, a strict dependency on Mn was established which is known for preny1transferases from other sources. A severalfold stimulation of phytoene formation by ATP was observed, although the hydrolysis of ATP is not required within this enzymatic sequence (ref. 16). 

Biosynthesis and Metabolism.—Pathways and Reactions. Two reviews of carotenoid biosynthesis discuss, respectively, the early steps and the later reactions." The former paper deals with the mechanism of formation of phytoene and the series of desaturation reactions by which phytoene is converted into lycopene, and also describes in detail the biosynthesis of bacterial C30 carotenoids. The second paper" presents details of the mechanism and stereochemistry of cyclization and the other reactions that involve the carotenoid C-1 —C-2 double bond and the later modifications, especially the introduction of oxygen functions. 

Scheme 7.5 Formation of some aroma compounds after oxidative cleavage of a acyclic carotenoids (e.g., lycopene, phytofluene and phytoene) and b cyclic carotenoids (e.g. a-carotene and / -caro-tene)...

Significant amounts of several carotenoid decomposition products were also identified in this study. Toluene, a-ionone and B-ionone are well-known decomposition products of carotenoids. In corn grain, the two most abundant carotenoids are lutein and phytoene (10). The formation of isophorone from lutein by a free radical mechanism was reported in an earlier publication (4), and phytoene... 

First, we will take up cyclopropyl group formation by the rearrangement of carbon skeletons via cationic intermediates encountered in various mono- and sesquiterpenes, and also examine the illudin biosynthesis where contraction of a cyclobutyl cation to a cyclopropane has been invoked. We will then discuss the head-to-head condensation of isoprenoid alcohols at the C15 or C20 level to generate the cyclopropyl intermediates, presqualene pyrophosphate and prephytoene pyrophosphate, on the way to the C30 and C40 polyene hydrocarbons, squalene and phytoene respectively. Conversion of 2,3-oxidosqualene via common intermediate protosterol cation to cycloartenol or lanosterol represents an important pathway in which the angular methyl group participates in the three-membered ring formation. The cyclopropanation outcome of this process has been carefully studied. 

The condensation of geranylgeranyl pyrophosphate (66) may lead to the formation of 15-cis-phytoene (88a) or all-trans-phytoene (88b), depending on the stereochemistry of... 

A multienzyme aggregate of four identical dehydrogenases (copies of the car B gene product) that act sequentially in the conversion of phytoene into lycopene has been proposed to account for the formation of acyclic carotenoids in P. blakesleeanus... 

Pathways. Studies of carotenoid transformations that take place when a mutant strain, PGl, of the green alga Scenedesmus obliquus is transferred from dark to light conditions have indicated that the transformations 15-cw-phytoene (180) 15-c/5-phytofluene (181) - 15-cis- -carotene (182) -> trans-C-caro-tene (183) (Scheme 7) take place in the biosynthesis of the normal cyclic carotenoids. The results were also in agreement with the formation of the xanthophylls lutein (16) and zeaxanthin (174) from the corresponding carotenes. 

The biosynthesis of carotenoids in plants has been reviewed extensively in recent years and is only briefly described here (Britton, 1988 Bartley and Scolnik, 1994 Sandmann, 1994). The committed step to carotenoid synthesis is the formation of the first compound phytoene by the head-to-head condensation of two molecules of GGDP by phytoene synthase. Phytoene is subjected to a series of four sequential desaturation reactions, by two separate enzymes to yield lycopene, which has eleven conjugated double bonds. Lycopene is then cyclized to /3-carotene by two /3-cyclizations or to a-carotene... 

Biosynthesis The biosynthesis of the C. proceeds in chloroplasts or chromoplasts in analogy to other iso-prenoids by way of 3-isopentenyl diphosphate ( active isoprene ). The colorless 15-cis-phytoene (C40) is formed from 2 molecules of geranylgeranyl diphosphate, further transformations furnish, wiaall-trans- -carotene (colored), all-trans- ycopenc. Subsequent cy-clization reactions allow the formation of other carotinoids. Xanthophvlls are formed from C. by incorporation of oxygen... 

Although not as widely employed as palladium, nickel catalysts may be used to effect carbon-carbon bond formation. In addition to the example illustrated in Scheme 24, the magnesium anion derived from (all-E)-geranylgeranyl phenyl sulphone (103) by sequential treatment with -BuLi and /-PrMgBr in hexane/THF dimerizes on heating with a catalytic amount of nickel(n) acetylacetonate [Ni(acac)2], to give a 1 1 fE/Z)-mixture of phytoene (44) in 74% yield [52] (Scheme 25). 

Formation of Geranylgeranyl Pyrophosphate Formation of Prephytoene Pyrophosphate Formation of Phytoenes Acyclic Carotenoids Alicyclic Carotenoids Oxygenated Carotenoids Site of Synthesis Chemosystematic Studies Carotenoids in Algae Cartenoids in Fungi Biological Activity... 

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