= Vitamin K
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<Vitamin K> is a family of fat-soluble compounds with a shared quinone framework. It includes <phylloquinone>, conventionally called vitaminK1, and <menaquinones>, grouped as vitaminK2. The family supports an enzymatic modification required for several biologically active proteins, including proteins involved in <coagulation>.<Vitamin K Fact Sheet for Health Professionals by the Office of Dietary Supplements>[Introduction: compound family and biochemical role]{ref}
An important role is supporting conversion of selected <glutamate residues> in proteins into <gamma-carboxyglutamate residues>. The protein sequence and the completed chemical modification are different kinds of information. Producing a polypeptide of the expected length does not establish that all its required modifications have occurred.<Vitamin K Fact Sheet for Health Professionals by the Office of Dietary Supplements>[Introduction: vitamin-K-dependent carboxylase and prothrombin]{ref}
The chemistry became concrete in a 1974 <prothrombin> study: researchers isolated a peptide from its calcium-binding region and used <nuclear magnetic resonance> and <mass spectrometry> to identify modified glutamic acid. This linked vitamin dependence to a molecular structure that could be measured.<Vitamin K dependent modifications of glutamic acid residues in prothrombin by J. Stenflo and others>[Abstract: isolated peptide and gamma-carboxyglutamic-acid identification]{ref}
For a mechanistic example, consider two preparations containing the same amount of a vitamin-K-dependent protein. If one is insufficiently modified, equal protein abundance need not mean equal activity in an assay requiring the completed binding region. The example distinguishes nutrient-supported chemical maturation from simply supplying more structural protein. VitaminK is involved in several physiological systems; the name should not be reduced to a synonym for a single clotting factor.
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