Synthesis, Function, and Inactivation of Blood Coagulation Factor V.

Blood coagulation factor V (FV) is a 330 kDa plasma glycoprotein. Activated FV (FVa) is a non-enzymatic cofactor to activated Factor X in the activation of prothrombin that occurs during coagulation. To investigate the molecular mechanisms of the quantitative FV deficiency associated with the FV R2 haplotype (a common genetic variant in several populations), four missense mutations identified in the R2 haplotype allele were analysed by in vitro expression studies. The aparagine to glycine mutation at position 2194 was found to play a key role in the impaired secretion of the mutant FV by interfering with its transport from the endoplasmic reticulum to the Golgi complex. FVa consists of a heavy (HC) and a light chain (LC) associated in a divalent metal-ion dependent complex. Calcium ions bind to FVa, and we constructed a recombinant FV mutant to abolish calcium binding to a potential calcium-binding site in FVa in order to study the specific role of these residues in the expression of FVa activity. Following activation, the mutant rapidly lost FVa activity, and the HC and LC dissociated in agreement with what is expected of a FV molecule that cannot bind calcium ions. We assessed the effect of heparin on FVa inactivation catalysed by activated protein C (APC). Heparin specifically inhibited the rapid phase of FVa inactivation (Arg506 cleavage). The inhibition was most likely caused by heparin binding to an electropositive cluster of amino acids in APC which normally interacts with a complementary electronegative region close to Arg506 in FVa