Biosynthesis of Phytoquinones. Stereospecific Biosynthesis of the Polyprenyl Side Chains of Terpenoid Quinones and Chromanols in Maize Shoots

Abstract

The incorporation of (3RS)‐[2‐14C,(4R)‐4‐3H1]mevalonic acid and (3RS)‐[214‐C,(4S)‐4‐3H1]‐mevalonic acid into plastoquinone, ubiquinone, γ‐tocopherol, α‐tocopherol, α‐tocopherol quinone, β‐carotene, squalene and 3β‐hydroxysterols by maize shoots has been studied. In agreement with the findings of workers using other tissues the atomic 14C/3H ratios for β‐carotene, squalene and 3β‐hydroxysterols labelled from [2‐14C,4R‐3H1]mevalonic acid are found to be 8:6, 6:6 and 5:3. When [2‐14C,4S‐3H1]mevalonic acid is used 3H is found to be entirely absent. All the quinones and chromanols examined gave results which are consistent with the prenyl portions of these molecules being biogenetically all‐trans. Thus using [2‐14C,4R‐3H1]mevalonic acid the atomic 14C/3H ratios of ubiquinone, plastoquinone, phylloquinone, γ‐tocopherol, α‐tocopherol and α‐tocopherol quinone are found to be 9:9, 9:9, 4:4, 4:4, 4:4, and 4:4. Confirmation of these results is provided by the finding that when [2‐14C,4S‐3H1]mevalonic acid is used only 14C radioactivity appears in the assembled molecules. Specific activity data suggest that γ‐tocopherol may be a precursor of α‐tocopherol, and that α‐tocopherol and α‐tocopherol quinone are in equilibrium with each other.

Vitamin E

“…Although the biochemical pathway for the synthesis of tocopherols has been proposed for some time, the inherent difficulties of working with such membrane‐bound enzymes have prevented a good understanding of the biochemical regulation of this pathway from being achieved. The traditional approach of using isolated chloroplasts and labelled compounds to study tocopherol synthesis was highly effective in deciphering the general biosynthetic pathway,58–61 but these studies have provided little insight into the regulation of tocopherol composition or carbon flux through the pathway. For example, virtually nothing is known about the enzymes controlling the availability/production of major pathway precursors (HGA, phytyl‐PP, GGPP, solanyl‐PP), the branch point enzyme in the pathway common to tocopherol/plastoquinone synthesis (the Arabidopsis Pds2 gene product), or the methylase enzymes involved in determining tocopherol composition.…”

Biosynthetis of phytoquinones. Biosynthetic origins of the nuclei and satellite methyl groups of plastoquinone, tocopherols and tocopherolquinones in maize shoots, bean shoots and ivy leaves

“…Threlfall, Griffiths & Goodwin (1967b) and Dada, Threlfall & Whistance (1968) showed that the polyprenyl portions of these compounds are formed from mevalonic acid, and Threlfall, Whistance & Goodwin (1967a) demonstrated that the methyl group of methionine is transferred intact to give the nuclear methyl group of phylloquinone, the nuclear methyl group and two methoxyl groups of ubiquinone and one or more nuclear methyl groups of plastoquinone, y-tocopherol, a-tocopherol and atocopherolquinone. The results for the incorporation of radioactivity from L-[Me-14C,3H]methionine into plastoquinone, tocopherols and tocopherolquinones also led to the conclusion that one nuclear methyl group in each of these compounds is not derived from methionine (see the Results section).…”

Effect of Menadiol Disulfate on the Phylloquinone Content of Wheat (Triticum aestivum)

“…(1966Whistance ct al. ( , 1967 and Whistance and Threlfall (1968) found that radioactivity from shikimic acid-'''C labelled either generally or in the positions 1 and 2 was incorporated into the naphthoquinone nucleus of phylloquinone in maize shoots {Zea mays L.).…”

“…The nuclear methyl group of phylloquinone has been shown to arise from the S-methyl group of methionine in maize shoots and in leaves of Hedera helix L. (Threlfall et al 1968, Whistance and. In maize shoots this methylation involves the transfer of an intact methyl group from methionine, and the reaction appears to take place in the chloroplast ).…”

“…In the same papers evidence was presented which indicated that the radioactivity was present in the phytyl side chain of phylloquinone and that the side chain is biosynthesized within the chloroplast. It has been demonstrated that in maize shoots all the isoprene units in the side chain of phylloquinone are biogenetically trans (Dada et al 1968).…”