Vitamin K is not a single compound, it consists of a group of essential fat soluble vitamins. Vitamin K family is divided into vitamin K1, a single molecule (phylloquinone) and a group of vitamin K2 molecule (menaquinones). Vitamin K2 exists in many forms, the most common forms are synthetic menaquinone-4 (MK-4) and natural or synthetic menaquinone-7 (MK-7).

All vitamin K has a "quinone" ring. However, their saturation and the number of side chain connections of hydrocarbon atoms are different, which are called isoprene like residues. These differences were especially significant in vitamin K2 group, which including menaquinones with different length of side chain. The name of a specific menaquinone is marked by the length of its side chain (for example, MK-4 indicates that its side chain contains four isoprenoids), and the length affects their ability to reach different tissues in the body. In other words, the longer the side chain, the better the absorption, and it stays in blood for longer time.