A bimodular oxidoreductase mediates the specific reduction of phylloquinone (vitamin K₁) in chloroplasts.

Plants and certain species of cyanobacteria are the only organisms capable of synthesizing phylloquinone (vitamin K₁ for vertebrates), which they use as an electron carrier during photosynthesis. Recent studies, however, have identified a plastidial pool of non-photoactive phylloquinone that could be involved in additional cellular functions. Here, we characterized an Arabidopsis bimodular enzyme--the At4g35760 gene product--comprising an integral domain homologous to the catalytic subunit of mammalian vitamin K₁ epoxide reductase (VKORC1, EC 1.1.4.1) that is fused to a soluble thioredoxin-like moiety. GFP-fusion experiments in tobacco mesophyll cells established that the plant protein is targeted to plastids, and analyses of transcript and protein levels showed that expression is maximal in leaf tissues. The fused and individual VKORC1 domains were separately expressed in yeast, removing their chloroplast targeting pre-sequence and adding a C-terminal consensus signal for retention in the endoplasmic reticulum. The corresponding microsomal preparations were equally effective at mediating the dithiotreitol-dependent reduction of phylloquinone and menaquinone into their respective quinol forms. Strikingly, unlike mammalian VKORC1, the Arabidopsis enzyme did not reduce phylloquinone epoxide, and was resistant to inhibition by warfarin. The isoprenoid benzoquinone conjugates plastoquinone and ubiquinone were not substrates, establishing that the plant enzyme evolved strict specificity for the quinone form of naphthalenoid conjugates. In vitro reconstitution experiments established that the soluble thioredoxin-like domain can function as an electron donor for its integral VKORC1 partner.

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