A menaquinone is a Vitamin K compound with a methyl-substituted quinone ring system and an unsaturated side chain built from isoprene-derived units. The family includes menaquinone-4; the number in MK-4 counts side-chain units, not the number of Vitamin K molecules. The side-chain chemistry distinguishes menaquinones from phylloquinone.[ref]
PubChem Compound database by National Center for Biotechnology Information. Menaquinone entries; compare menaquinone-4 and phylloquinone structures
Menaquinone-4 need not arrive in a tissue as the finished compound. In 2010, reducing UBIAD1 expression in human cells decreased conversion of isotope-labelled Vitamin K derivatives to labelled menaquinone-4; expressing UBIAD1 in insect cells supplied complementary evidence for the biosynthetic enzyme. The product was checked by nuclear magnetic resonance.[ref]
Identification of UBIAD1 as a novel human menaquinone-4 biosynthetic enzyme by Kimie Nakagawa and others. Abstract and Figures 2–3; knockdown, heterologous expression and deuterium NMR
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