The Dynamic Motion and Delicate Control Of Vkor Catalysis

Vitamin K epoxide reductase (VKOR) is an intramembrane enzyme required for blood coagulation and is the target of the anticoagulant warfarin. The reduction of vitamin K epoxide is coupled with disulfide-bond formation at the active site of VKOR. To regenerate the active site, VKOR is reduced by protein partners that transfer electrons to VKOR. Here we report two crystal structures of a bacterial VKOR homolog with its reducing partner captured in different conformational states. These structures reveal a short helix at the hydrophobic active site of VKOR that undergoes stretching and compressing motions. Motions of this “horizontal helix” promote electron transfer to the VKOR active site by regulating the positions of two cysteines in an adjacent loop. Compression of the helix also separates the “loop cysteines” to prevent backward electron transfer. During these motions, hydrophobicity at the active site is maintained to facilitate vitamin K reduction. Biochemical experiments support the structural model and suggest that several warfarin-resistant mutations act by changing the conformation of the horizontal helix. Taken together, these structures provide a comprehensive understanding of electron transfer and vitamin K reduction catalyzed by VKOR.