Membrane binding module of the prothrombin-prothrombinase complex. The Gla domains of prothrombin and fXa, along with the C1 and C2 domains of fVa, are colored as in Figure 1 and are shown in the same orientation as Figure 2A, that is, as seen from the plane of the membrane. The module features the Gla domain of fXa close, but not in contact, with the C1 domain of fVa, which is positioned relative to the C2 domain as seen in free fVa.³⁵ The Gla domain of prothrombin, conversely, is widely separated from the Gla domain of fXa and the C2 domain of fVa. The module has an overall triangular arrangement with sides of up to 90 Å (Gla domain of prothrombin from Gla domain of fXa), 70 Å (Gla domain of fXa from C2 domain of fVa), and 110 Å (Gla domain of prothrombin from C2 domain of fVa). Residues likely involved in direct interaction with the membrane are highlighted (blue).