Vitamin K in photosynthetic organisms.

Recently, great progress in vitamin K research in plants was made, especially on vitamin K epoxide reductase (VKOR) homolog, new functions of VK in plant plasma membrane and VK1 biosynthesis. Although vitamin K (VK) was discovered in the 1930s, only recently has significant progress in the research on this lipid-soluble vitamin been made both in mammals and in photosynthetic organisms. For example, the molecular structure and catalytic mechanisms of the cyanobacterial vitamin K epoxide reductase (VKOR) homolog have been resolved. It has been recently shown that all organisms from bacteria to higher plants share common, phylogenetically related VKOR-type enzymes, engaged in the formation of disulfide bridges in various target proteins. This activity is related to the phylloquinone (VK1)-dependent regulation of the STN7 kinase, which is responsible for optimal energy distribution between two photosystems. Moreover, new functions of VK1 and menaquinone-4 (VK2) in plant plasma membrane electron transport chain were identified. Recently, the plastid-localized biosynthetic pathway of VK1 has been nearly completely recognized. However, there are also indications of an alternative, plasma membrane-targeted VK1 biosynthesis pathway.