Vitamin K (K from "Koagulations-Vitamin" in German and Scandinavian languages) denotes a group of lipophilic, hydrophobic vitamins that are needed for the posttranslational modification of certain proteins, mostly required for blood coagulation, but also a number of other proteins that chelate calcium ion, and are involved in bone and other tissue metabolism. They are 2-methyl-1,4-naphthoquinone derivatives.
There are three synthetic forms of vitamin K, vitamins K3, K4, and K5, which are used in many areas including the pet food industry (vitamin K3) and to inhibit fungal growth (vitamin K5)