What characteristic defines vitamin K?

Vitamin K is primarily defined by its crucial role in the carboxylation of specific glutamate residues on several coagulation factors, including factors II (prothrombin), VII, IX, and X. This post-translational modification is essential for the activation of these proteins, which are vital for proper blood clotting. When vitamin K facilitates the carboxylation process, it allows these coagulation factors to bind calcium ions, an essential step for their function in the coagulation cascade.

The other options do not accurately represent key characteristics of vitamin K. While vitamin K does play a role in hemostasis, it does not directly relate to fibrinolysis, which is the process of breaking down clots. Heparin therapy mainly enhances the activity of antithrombin, impacting thrombin and factor Xa but does not enhance vitamin K's activity. Lastly, vitamin K is not synthesized by endothelial cells; instead, it is primarily obtained through dietary sources and can also be produced by gut bacteria. Therefore, the defining characteristic of vitamin K is its role in carboxylation of glutamate residues in coagulation factors, which ultimately supports effective blood coagulation.