NDC1 in cyanobacteria is part of the MEN operon and its deletion also leads to loss of phylloquinone ( Fatihi et al., 2015). In Chlamydomonas NDA2 NAD(P)H:quinone oxidoreductase, the homolog of NDC1, is involved in cyclic electron flow and chlororespiration directly transferring electrons from NAD(P)H to plastoquinone ( Desplats et al., 2009). NDC1 is evolutionarily conserved from cyanobacteria.
However, NDC1 was initially also found in mitochondria ( Michalecka et al., 2003), and dual localization in mitochondria and chloroplasts cannot be excluded. 3) whereas the remaining homologs have been localized in mitochondria. NDC1 appears to be localized mostly in chloroplast PG ( Fig. In Arabidopsis, NDC1 is one of seven NDH-2 (Type II NAD(P)H-dependent dehydrogenases). Depending on the prior reduction of demethylphylloquinone by NDC1, MenG methylates demethylphylloquinol to complete the synthesis of phylloquinone. Next, the demethylphylloquinone is reduced by type II NAD(P)H dehydrogenase C1 (NDC1) resulting in demethylphylloquinol. DHNA phytyltransferase (MenA) converts DHNA to 2-phytyl-1,4-naphthoquinone (demethylphylloquinone). PHYLLO, a fusion protein of menaquinone synthesis enzymes (MenF, D, H, C) followed by MenE and MenB convert chorismate into 1,4-dihydroxydihydroxy-2-naphthoate (DHNA). Phylloquinone biosynthetic pathway in plants. Surprisingly, the ndc1 mutant in Arabidopsis mimics the atmenG phenotype and has identical phylloquinone and demethylphylloquinone profiles ( Eugeni Piller et al., 2011).įig. Thus, it appears that the precursor of phylloquinone can functionally replace phylloquinone, at least under standard conditions. The lack of phylloquinone in atmenG has rather minor consequences resulting in reduced levels of Photosystem I and consequently a slight decrease of photosynthetic efficiency upon high light (HL) treatment ( Eugeni Piller et al., 2011 Lohmann et al., 2006). The AtmenG knockout mutant lacks phylloquinone but accumulates the precursor demethylphylloquinone instead. The MenG enzyme, a predicted methylase, catalyzes the final methylation step of the biosynthetic pathway from 2-phytyl-1,4-naphthoquinone (demethylphylloquinone) to phylloquinone ( Fig. This suggests phylloquinone trafficking between PG and the thylakoid membrane. But it has been demonstrated that about one-third of phylloquinone is present in PG ( Lohmann et al., 2006 Spicher & Kessler, 2015). Phylloquinone mainly acts as an electron carrier in Photosystem I in the thylakoid membrane ( Brettel et al., 1986). Plant phylloquinone biosynthesis is also connected via shared intermediates to the metabolism of salicylate, tocopherols, chlorophylls, and in some species to anthraquinones. Phylogenetic reconstructions also demonstrate that the plant genes involved in the formation of phylloquinone display a high degree of evolutionary chimerism owing to multiple events of horizontal gene transfer and gene losses. There is today, tough, compelling evidence that plants have evolved an unprecedented metabolic architecture to synthesize phylloquinone, including extraordinary events of gene fusion, highly divergent enzymes and a separated compartmentalization in chloroplasts and peroxisomes.
It resulted that most of the plant research on phylloquinone focused historically on the study of its function, while very little was done on its metabolism per se. Until recently, the biosynthesis of phylloquinone in plants was considered identical to that of menaquinone (vitamin K 2) in facultative anaerobic bacteria. In humans and other mammals, it is required as a vitamin (vitamin K 1) for blood coagulation and bone metabolism. Phylloquinone (2-methyl-3-phytyl-1,4-naphthoquinone) is a conjugated isoprenoid that serves as a cardinal redox cofactor in plants and some cyanobacteria. Basset, in Advances in Botanical Research, 2011 Abstract
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