Phytyl group

Fig. 19.18 Structure of chlorophyll. The long alkyl chain at the bottom is the phytyl group.

The absorption peak in acetone is at 635 ran (15,800 cm ). Chlorophyll c found in diatoms, brown algae (Phaeophyta), and dinoflagellates (Fig. 1-9) lacks the phytyl group. Chlorophyll d contains a formyl group on ring I.261... 

The phytyl group appears also as a side chain in vitamin K3 (Section 26-2B). 

The earliest steps (MVA to GGPP) for polyisoprenoid biosynthesis are identical for all plants and animals (12,13). They involve the well-known diterpene pathway, MVA — MVAP — MVAPP — IPP + DMAPP — GPP — FPP — GGPP. The enzymes catalyzing these steps have been studied extensively, especially from animals (liver) and yeast, and to a more limited extent from higher plants. In some cases the enzymes have been purified to homogeneity most have been only partially purified. In both plants and animals a major branch at FPP leads to the production of squalene and the steroids. In plants, three major branches occur at GGPP, of which one leads to the carotenoids via phyto-ene, a second to the phytyl group of chlorophyll, and a third to the GAs. 

A synthesis of (S)-cx-tocotrienol, in which the phytyl group of tocopherol is replaced by a famesyl side chain, (six steps, 19% ee = 98%) is based on the enzymatic desymmetrisation of the achiral 2,2-di(hydroxymethyl)chroman 16 <02TL7971>. 

The term vitamin E refers to two groups of compounds, the tocophenols and the tocotrienols. The structures of these compounds appear in Figure 9.90. All forms of the vitamin contain two parts, a "head" and a "tail." The head consists of an aromatic ring structure, called chroman or chromanol, and is the site of antioxidant action. The tail of tocopherols is a phytyl group, while the tail of tocotrienols is a polyisoprenoid group. The tail of vitamin K setv es to anchor the vitamin in lipid membranes, in the lipids of adipose tissue, and in the lipid surface and core of the lipoproteins. 

A good example of compartmentation, although not yet completely understood, is the apparent segregation of isoprenoid biosynthesis in plants (123). [2- 14C]-Mevalonate was easily incorporated in squalene, phytosterols, P-amyrin and ubiquinone but not in the plastid constituents P-carotene, chlorophyll phytyl group or plastoquinone. The opposite result was obtained with 14C02. 

The phytyl group it is replaced by Cj5 famesyl or olher. pol3 renyl group in Chlorobium chlorophyll... 

Chlorophyll (E140) itself which exists as the compounds a and b (where the 3-methyl group in a has been replaced by a formyl substituent) is a less satisfactory colourant for commodity purposes than one of its derivatives lacking the phytyl group prepared by semi-synthesis (hydrolysis and coppering), namely copper chlorophyllin. 

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