The mechanism of action of vitamin K.

Vitamin K is the blood-clotting vitamin. The mechanism of action of vitamin K is discussed in terms of a new carbanion model that mimics the proton abstraction from the gamma position of protein-bound glutamate. This is the essential step leading to carboxylation and activation of the blood-clotting proteins. The model comprises an oxygenation that is coupled to carbon-carbon bond formation, as is the oxygenation of vitamin K hydroquinone to vitamin K oxide. The model hypothesis is also supported by the mechanism of inhibition of the carboxylase by HCN, which acts as an acid-base inhibitor rather than a metal-complexing inhibitor. The new model postulates a dioxetane intermediate that explains the presence of a second atom of 18O (from 18O2) incorporated into vitamin K oxide in the course of the enzymatic carboxylation. Finally, the chemistry developed here has been used to define the active site of vitamin K hydroquinone as the carbon-carbon bond adjacent to the methyl group.