Figure 1

Structures of Vitamin K and biosynthesis in green plastids. (A) Chemical structure of Vitamin K₁ (Phylloquinone) and Vitamin K2 (menaquinone/MK₄). Phylloquinone is present as a bound co-factor in photosystem 1 (PS1) of most cyanobacteria and plants. Hydroxyphylloquinone has been shown to replace phylloquinone in PS1 of the green alga Chlamydomonas reinhardtii and is reported in this work as the major naphtoquinone of the glaucophyte Cyanophora paradoxa. Vitamin Ks are composed of a 2-methyl-1,4-naphthoquinone ring coupled to various types of polyprenyl chains. This chain consists of phytyl in phylloquinones which is a reduced form of the geranyl geranyl diphosphate found in menaquinone-4 (MK 4). Menaquinones are characterized by the presence of polyunsaturated polyprenyl chains of various lengths synthesized by polymerization of the MEP pathway 5 carbon products IPP and DMAPP in bacteria. MK4 is characterized by a degree of polymerization of 4 and thus contains 20 carbon units. Menaquinones shuttle electrons between different respiratory complexes in anaerobic respiration or aerobic respiration in a microaerophilic environment. Phylloquinone is unable to supply this particular function. Mutants defective for vitamin K synthesis in algae and cyanobacteria substitute phylloquinone/menaquinone by plastoquinone for PSI function and thereby become highly light sensitive. (B) Synthesis of phylloquinone in plants. The end product of the shikimate pathway (chorismate) is converted in 4 steps in the chloroplast to O-succinyl benzoate (OSB) by a large protein called “phyllo” consisting of a fusion of the MenF, MenD, MenC and MenH gene products derived from the bacterial Men pathway of menaquinone synthesis. OSB diffuses out to the peroxisome where it is converted to DHNA (1.4-dihydroxy-2-naphthoate) in 3 steps (catalyzed by MenE, MenB and DHNAT) and diffuses back to the plastid. The isoprenoid polyprenyl (phytyl) tail is added to DHNA by the MenA gene product. The DHNA ring is then reduced by the NDC1 NADPH dehydrogenase and methylated by the MenG gene product. In bacteria MK4 is synthesized by transfer to the quinone core of geranylgeranyl diphosphate (GGPP). Synthesis of GGPP in bacteria and plants involves the transfer of 3 IPP molecules onto a single DMAPP acceptor by geranylgeranyl diphosphate synthase (GGPPS). Phylloquinone instead of MK4 is synthesized specifically in cyanobacteria and plants because geranylgeranyl diphosphate is reduced by GGPP reductase (GGPPR) prior to its transfer to the quinone core by the Men A prenyl transferase. IPP and DMAP are the end products of the 8 steps MEP pathway in cyanobacteria and Archaeplastida.