Vitamin K is the fat-soluble vitamin that is the essential cofactor for the gamma-carboxylation of glutamate (Glu) residues in a family of proteins that are called the vitamin K-dependent proteins — including the clotting factors II (prothrombin), VII, IX, and X, the anticoagulant proteins C and S, and the matrix proteins osteocalcin and matrix Gla protein (MGP). The gamma-carboxylation of glutamate is a post-translational modification that converts the glutamate residues in these proteins to gamma-carboxyglutamate (Gla) residues, which are the active form of these proteins that can bind calcium and that can participate in the calcium-dependent processes of blood clotting, anticoagulation, and bone mineralisation. Without adequate vitamin K, the gamma-carboxylation of these proteins is impaired, the clotting factors are less functional (producing bleeding disorders), and the osteocalcin is less able to bind calcium and to promote the mineralisation of bone (producing a bone mineralisation defect that contributes to osteoporosis). This vitamin K-dependent gamma-carboxylation is one of the most important post-translational modifications in human physiology, and its dysfunction is implicated in both bleeding disorders and in the bone disease that is associated with osteoporosis and with vascular calcification.
The Gamma-Carboxylation Reaction
The gamma-carboxylation of glutamate residues is catalysed by the vitamin K-dependent gamma-glutamyl carboxylase enzyme, which uses vitamin K hydroquinone (the reduced form of vitamin K) as a cofactor and which requires molecular oxygen, carbon dioxide, and the gamma-glutamyl carboxylase enzyme itself as substrates. During the carboxylation reaction, vitamin K hydroquinone is oxidised to vitamin K epoxide, which must then be reduced back to vitamin K hydroquinone by the vitamin K epoxide reductase (VKOR) enzyme. The VKOR enzyme is the target of the warfarin-type anticoagulant drugs (including warfarin and acenocoumarol), which inhibit VKOR and thereby prevent the recycling of vitamin K, impairing the gamma-carboxylation of the clotting factors and producing an anticoagulant effect. This warfarin-sensitive vitamin K cycle is the foundation of the anticoagulant effect of warfarin, and it explains why vitamin K supplementation reverses the effect of warfarin — by providing a source of vitamin K that can bypass the inhibited VKOR and restore the gamma-carboxylation of the clotting factors.
The clinical importance of the gamma-carboxylation reaction is most clearly seen in the conditions that result from impaired gamma-carboxylation — including the bleeding disorders that result from vitamin K deficiency, from warfarin use, or from genetic mutations in the gamma-carboxylase gene or in the VKOR gene. The newborn haemorrhagic disease, which results from the inadequate transfer of maternal vitamin K across the placenta and from the immature liver function of the newborn, is one of the most important clinical manifestations of vitamin K deficiency — it presents in the first week of life with intracranial haemorrhage, GI bleeding, or umbilical stump bleeding, and it is preventable by the administration of vitamin K at birth. The warfarin-induced bleeding disorders are managed by the administration of vitamin K (for mild over-anticoagulation) or by the administration of fresh frozen plasma or prothrombin complex concentrate (for severe bleeding in the context of excessive warfarin anticoagulation).
Vitamin K and Bone Health
Vitamin K is also essential for the function of osteocalcin — the vitamin K-dependent matrix protein that is synthesised by osteoblasts and that is one of the most abundant non-collagenous proteins in the bone matrix. Osteocalcin is secreted by osteoblasts during the mineralisation phase of bone formation, and its gamma-carboxylated glutamate residues allow it to bind calcium and to incorporate into the hydroxyapatite crystals of the bone mineral. The undercarboxylated osteocalcin (ucOC, the form that is present when vitamin K is deficient) cannot bind calcium effectively, and it accumulates in the blood as a marker of vitamin K deficiency and as a predictor of hip fracture risk in older adults. Studies in postmenopausal women have shown that vitamin K supplementation (at 100-500mcg of vitamin K2 daily) reduces the levels of undercarboxylated osteocalcin, increases the levels of carboxylated osteocalcin, and is associated with a reduced rate of bone loss and a reduced risk of hip fracture in some studies.
Practical Application
For general vitamin K supplementation, the evidence-based approach is to supplement with vitamin K2 (menaquinone-7, MK-7, the most bioavailable and the most effective form of vitamin K2) at 100-200mcg daily, alongside adequate vitamin D (for calcium absorption and bone health) and alongside calcium (for bone mineralisation). Vitamin K1 (phylloquinone) is the form that is found in green leafy vegetables and is less bioavailable than K2. Vitamin K2 MK-7 is preferred for supplementation because it has a longer half-life and is more effective at carboxylating osteocalcin than the shorter-chain K2 forms (MK-4). For comprehensive bone health support, vitamin K2 pairs well with calcium (which is the primary mineral component of bone), with vitamin D (which enhances calcium absorption and which works synergistically with vitamin K2 for the activation of osteocalcin), with strontium (which has independent effects on bone through the calcium-sensing receptor), with silicon (which supports the collagen matrix of bone), and with the weight-bearing exercise programme (which is the most effective non-pharmacological intervention for the maintenance of bone mass).
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