Structure of Ca⁺²-free Gla diomain shed light on membrane binding of blood coagulation proteins

Reversible membrane binding of γ-carboxyglutamic acid (Gla)-containing coagulation factors requires Ca²⁺-binding to 10–12 Gla residues. Here we describe the solution structure of the Ca²⁺-free Gla-EGF domain pair of factor X which reveals a striking difference between the Ca²⁺-free and Ca²⁺-loaded forms. In the Ca²⁺-free form Gla residues are exposed to solvent and Phe 4, Leu 5 and Val 8 form a hydrophobic cluster in the interior of the domain. In the Ca²⁺-loaded form Gla residues ligate Ca2²⁺ in the core of the domain pushing the side-chains of the three hydrophobic residues into the solvent. We propose that the Ca²⁺-induced exposure of hydrophobic side chains is crucial for membrane binding of Gla-containing coagulation proteins.