**2. Structure of vitamin K**

Both vitamin K forms have 2‐methyl‐1,4‐naphthoquinone, also called menadione or vitamin K3, as a common ring structure. However, they differ from each other in the length and degree of saturation of the polyisoprenoid side chain attached to the 3‐position.

Phylloquinone (vitamin K1) possesses a phytyl side chain, which consists of four isoprene units, and one of them is unsaturated. Phylloquinone is found primarily in plants in association with chlorophyll, whereas menaquinone (vitamin K2) is principally synthesized by bacteria. Menaquinone contains side chains of varying length, for most the part of a polymer of repeating unsaturated 5‐carbon prenyl units. Depending on the microorganism by which the chain is synthesized, the chain length generally ranges from 4 to 13 prenyl units. Menaquinones are classified according to the number of prenyl units. The number of units is given in a suffix (‐*n*), that is, menaquinone‐*n* and often abbreviated as MK‐*n* [2, 6, 7]. Some bacteria produce isoprenologues in which one or more of the prenyl units are saturated. The additional hydrogen atoms are indicated with the prefix dihydro‐, tetrahydro‐, and so on and are abbreviated MK‐*n*(H2), MK‐*n*(H4), etc. [8].

Vitamin K is fat soluble. The melting points of menaquinones vary from 35°C to 62°C depend‐ ing on the length of the multiprenyl side chain. Menaquinones are stable to heat and air but are very sensitive to alkali and ultraviolet (UV) irradiation [9].
