Phytyl-diphosphate

An example of a biological Friedel-Crafts reaction occurs during the biosynthesis of phylloquinone, or vitamin Kl( the human blood-clotting factor. Phylloquinone is formed by reaction of 1,4-dihydroxynaphthoic acid with phytyl diphosphate. Phytyl diphosphate first dissociates to a resonance-stabilized allylic carbocation, which then substitutes onto the aromatic ring in the typical way. Several further transformations lead to phylloquinone (Figure 16.10). 

The final step in the biosynthetic pathway to chlorophylls is the esterification of the chlorophyllides with phytyl diphosphate or geranylgeranyl diphosphate (followed by reduction of the three extra double bonds) via the enzyme chlorophyll synthetase. Once again, when zinc or cadmium replaces magnesium, esterification is unaffected. However, when nickel or copper are used, esterification is hindered. Presumably the labile coordinating power of the group Ila and Ilb metals, as opposed to the more inert coordination of transition metals, is involved in the function of the two enzymes, oxidative cyclase and chlorophyll synthetase. 

Vitamin E, as well as vitamin K, plastoquinones and ubiquinones, is formed, in principle, from shikimic acid via 4-hydroxyphenylpyruvic and homogentisic acid. The terpenoid side chain k synthesised from isopentenyl diphosphate, or more precisely from geranylgeranyl diphosphate, which is gradually reduced to phytyl diphosphate. The primary product of biosynthesis is 5-tocopherol other tocopherok are products of its methylation. The biosynthesis of tocotrienok lies in the condensation of homogentisic acid with geranylgeranyl diphosphate. Vitamin E k only synthesised by plants and some cyanobacteria. 

The common intermediate of haem and chlorophyll pigments biosynthesis, protoporphyrin IX, is transformed into chlorophyUde by a sequence of several reaction steps. Chlorophyll a arises from chlorophyHde in a reaction with phytyl diphosphate that is produced by the reduction of geranylgeranyl diphosphate. Oxidation of the C-7 methyl group in chlorophyll a to a formyl group yields chlorophyll b. 

The existence of a chloroplast mevalonate (Mev) pathway, which synthesizes isopentenyl diphosphate (IPP) to form chloroplast isoprenoids (e.g. carotenes, plastoqinone-9 (PQ), toco-pherols and the phytyl moiety of chlorophyll), is still under... 

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