Factor VIII and the coagulation cascade. (A) The blood coagulation cascade consists of 2 pathways (extrinsic and intrinsic) that are initiated by the exposure of tissue factor (TF) or phosphatidylserine groups (PS) of activated platelet membranes to circulating protein factors, respectively. fVIII is a plasma glycoprotein that acts as an initiator and regulator of the intrinsic pathway. Upon proteolytic activation by either fXa or thrombin, fVIIIa dissociates from VWF, associates with the fIXa serine protease, and directs the localization of the resulting complex to the membrane surface of activated platelets via an interaction with its C-terminal C2 domain (structure in inset). The membrane-bound complex between fVIIIa and fIXa complex functions to proteolytically activate fX, which then activates thrombin (fII). (B) Domain structure of fVIII. fVIII is synthesized as a single polypeptide chain of 2332 residues. Based on sequence homology, fVIII has the domain structure A1-A2-B-A3-C1-C2. Linker regions between domains are denoted with lowercase letters (“a1,” etc). The location of domain boundaries and primary sites of proteolytic processing during secretion and activation are denoted by residue numbers. The circulating fVIII heterodimer is associated with VWF primarily through interactions with the “a3” acidic region at the light chain N-terminus and with the C2 domain. Various proteases interact with the activated heterotrimer at positions denoted at the bottom panel. Membrane association is primarily accomplished through the C2 domain; its deletion completely abrogates binding of fVIII to platelet surfaces.