Structural Insights into Zn2+ and Ca2+ binding to Human Protein Z and Their Impact on Membrane Association

Calcium ions (Ca 2+ ) are essential for the proper folding and stabilization of the γ carboxy glutamic acid-rich (Gla) domain in Vitamin K Dependent (VKD) coagulation proteins enabling their binding to phospholipid membranes. Zinc ions (Zn 2+ ), the second most abundant plasma cation, have been shown to bind and regulate the activity of certain coagulation proteins can also bind and modulate the activity of several coagulation proteins through structural alterations. However, their role in membrane-protein interactions remains poorly understood. In this study, we investigated how Ca 2+ and Zn 2+ ; influence the structure and function of human protein Z (PZ), a cofactor of protein Z–dependent protease inhibitor (ZPI) that inhibits clotting factor Xa on phospholipid surfaces. Zn 2+ was found to substitute for Ca 2+ in the Gla domain of PZ and promote its membrane binding by inducing conformational changes like Ca 2+ . Additionally, a Zn 2+ located in the epidermal growth factor1 (EGF1) domain of PZ appeared to synergize with Ca 2+ to enhance the membrane affinity of PZ. Furthermore, both Ca 2+ and Zn 2+ were suggested to destabilize PZ–ZPI complex, thereby modulating the cofactor activity of PZ, a finding that warrants further experimental investigations. Collectively, this study provides the first structural and functional characterization of Zn 2+ bound PZ, offering new insights into metal ion driven structural dynamics of PZ and their implications in coagulation regulation.