Abstract
The acid–base and redox properties of the menaquinones MK-4, MK-7, and MK-9 (vitamin K2) have been studied in DMPC monolayers on mercury electrodes. The monolayers were prepared by adhesion-spreading of menaquinone-spiked DMPC liposomes on a stationary mercury drop electrode. All three menaquinones possess {text{p}}K_{{text{a}}} constants outside the experimentally accessible range, i.e., they are higher than about 12. The standard potentials of MK-4, MK-7, and MK-9 in the DMPC monolayers are very similar, i.e., 0.351, 0.326, and 0.330 V (corresponding to the biochemical standard potentials − 0.063, − 0.088, and − 0.085 V).Graphic abstract
Highlights
A very recent review of the electrochemistry of vitamins highlights the importance of solid thermodynamic data, and it shows that no reliable data concerning Vitamin
Vitamin K, in its hydroquinone state, functions as an exclusive coenzyme of γ-glutamyl carboxylase (GGCX, EC 4.1.1.90), which catalyzes the post-translational γ-carboxylation of a number of vitamin K-dependent proteins (Kleuser 2018)
GGCX is only activated by vitamin K in its hydromenaquinone state, which in this reaction is converted to the inactive vitamin K epoxide (Oldenburg et al 2008)
Summary
A very recent review of the electrochemistry of vitamins highlights the importance of solid thermodynamic data, and it shows that no reliable data concerning VitaminK are available (Lovander et al 2018). Keywords Menaquinones · Acidity constants · Standard potentials · Lipid monolayer · DMPC · Electrochemistry Standard potentials and acidity constants of compounds dissolved in water are strictly defined for all involved species in the solvated (hydrated) state.
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