Role of chlorophyll degradation during the biosynthesis of tocopherol (vitamin E) and phylloquinone (vitamin K)
Romer et al. study the origin of the lipid side chain phytol in tocopherol and phylloquinone synthesis in Arabidopsis.
https://doi.org/10.1093/plcell/koad316
By J. Romer, K. Gutbrod and P. Dörmann
Background: Isoprenoid lipids in Arabidopsis chloroplasts include chlorophyll, carotenoids, tocopherols (vitamin E) and phylloquinone (vitamin K). Their biosynthesis depends on the availability of the isoprenoid precursors geranylgeranyl-diphosphate and phytyl-diphosphate. The geranylgeranyl moiety can first be incorporated into chlorophyll where it is reduced to the phytyl group and subsequently hydrolyzed. Phytol kinase (VTE5) phosphorylates phytol, and phytol-phosphate kinase (VTE6) produces phytyl-diphosphate which is employed for tocopherol and phylloquinone synthesis. While the vte5 mutant of Arabidopsis contains reduced amounts of tocopherol, the vte6 mutant is deficient in tocopherol and phylloquinone.
Questions: Why do tocopherol and phylloquinone synthesis differentially depend on phytol phosphorylation by phytol kinase (VTE5)? Does another kinase contribute to phytol phosphorylation?
Findings: Arabidopsis contains one paralog to VTE5, FOLK (farnesol kinase), which is involved in farnesol phosphorylation. We show here that FOLK also phosphorylates phytol. While the vte5 folk double mutant is completely devoid of tocopherol, it still accumulates residual amounts of phylloquinone. The amounts of chlorophyll and carotenoids are not affected in the vte5 folk mutant. Ectopic expression of FOLK partially complements tocopherol deficiency in vte5. Therefore, VTE5 and FOLK are essential to provide phytol-phosphate for tocopherol synthesis, but alternative pathways exist to provide the phytyl group during phylloquinone synthesis.
Next steps: Further research is required to elucidate the contribution of the different pathways to geranylgeraniol and phytol metabolism for isoprenoid lipid synthesis in chloroplasts.
Reference:
Jill Romer, Katharina Gutbrod, Antonia Schuppener, Michael Melzer, Stefanie J. Müller-Schüssele, Andreas J. Meyer, Peter Dörmann (2024). Tocopherol and phylloquinone biosynthesis in chloroplasts requires the phytol kinase VTE5 and the farnesol kinase FOLK. https://doi.org/10.1093/plcell/koad316
