Any of several related fat-soluble compounds, including vitamins K1 (phylloquinone) and K2 (menaquinone), that act as coenzymes in the gamma-carboxylation of glutamate amino acid residues in certain proteins. Such a reaction is essential for the function of various proteins involved in blood coagulation (e.g. prothrombin and certain other coagulation factors) and bone metabolism. Deficiency of vitamin K, which leads to extensive bleeding, is rare because a form of the vitamin is manufactured by intestinal bacteria. Green vegetables and egg yolk are good sources.