In these tissues, menadiol (reduced vitamin K3) reacts with geranylgeranyl pyrophosphate to form MK4 (vitamin K hydroquinone), the form of the vitamin required as cofactor for gamma-carboxylation of protein glutamate residues (Hirota et al. 2013). The gamma-carboxylation reactions, annotated elsewhere in Reactome as a part of protein metabolism, convert MK4 to its epoxide form, which is inactive as a cofactor. Two related enzymes, VKORC1 and VKORCL1, can each catalyze the reduction of MK4 epoxide to active MK4. VKORC1 activity is essential for normal operation of the blood clotting cascade and for osteocalcin function (Ferron et al. 2015). A physiological function for VKORCL1 has not yet been definitively established (Hammed et al. 2013; Tie et al. 2014).
Suhara, Y, Tsugawa, N, Okano, T, Wada, A, Kamao, M, Uchino, Y, Sawada, N, Nakagawa, K, Tanaka, K, Takeuchi, A, Hirota, Y, Okitsu, T
Shearer, MJ, Newman, P
Booth, SL, Shearer, MJ, Fu, X
Wu, SM, Stafford, DW, Sanchez-Vega, B, Lanir, N, Solera, J, Brenner, B
Matagrin, B, Lattard, V, Prouillac, C, Spohn, G, Hammed, A, Benoit, E
Oury, F, Germain, A, Lacombe, J, Ferron, M, Karsenty, G
Stafford, DW, Jin, DY, Tie, JK
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