Human Factor Va₁ and Factor Va₂:  Properties in the Procoagulant and Anticoagulant Pathways

Abstract

Human plasma factor V is heterogeneous and yields two forms of activated factor V that bind with low (factor Va1) and high affinity (factor Va2) to phospholipids. The properties of factor Va1 and factor Va2 in the anticoagulant and procoagulant pathways were evaluated by comparing their sensitivity for inactivation by APC and their ability to act as cofactor in prothrombin activation. At low phospholipid concentrations and on membranes containing low amounts of phosphatidylserine (PS), factor Va1 was inactivated by APC at 15-fold lower rates than factor Va2, both in the absence and in the presence of protein S. At high phospholipid concentrations and on membranes with more than 15 mol % PS, factor Va1 and factor Va2 were inactivated with equal efficiency. Differences between cofactor activities of factor Va1 and factor Va2 in prothrombin activation were only observed on membranes with less than 7.5 mol % PS. Due to the different phospholipid requirements of APC-catalyzed factor Va inactivation and of expression of factor Va cofactor activity in prothrombin activation, the thrombin-forming capacity of factor V1 was 7-fold higher than that of factor V2 in a reaction system containing factor Xa, prothrombin, APC, protein S, vesicles with a phospholipid composition resembling that of activated platelets, and traces of thrombin to initiate prothrombin activation. This shows that in the process of generation, expression, and down-regulation of factor Va cofactor activity on physiological membranes, the overall procoagulant activity of factor V1 can considerably exceed that of factor V2.

Soluble Phosphatidylserine Triggers Assembly in Solution of a Prothrombin-activating Complex in the Absence of a Membrane Surface

“…As a result, intense effort has been devoted to discerning the role of this cofactor in thrombin generation. The picture that emerges is that factor V a binds tightly to acidic lipid membranes (K d Ϸ2 nM) (16) and then tightly binds factor X a already bound to that surface (K d Ϸ1 nM) (15), making it effectively an anchor to assemble the prothrombinase at the low concentrations of factor X a and membranes expected to be found in vivo (17). Because the interaction between factors V a and X a in solution is much weaker (K d Ϸ1-3 M) (18) than that reported on a membrane (15), it has been speculated that either the membrane alters these two proteins so as to enhance their interaction or the presence of both proteins in the reduced dimensionality of the membrane surface leads to tighter association than would be seen in three dimensions (15).…”

Coagulation factor V mediates inhibition of tissue factor signaling by activated protein C in mice

“…Potential candidate mechanisms that do not involve the degradation of fVa or fVIIIa but are consistent with known functions of pS and fV include (1) the destabilization of the signaling-competent conformation of the TF complex secondary to physical interactions of B domain-containing fVa with fXa 49 and of pS with TF and fXa 50 , and/or (2) the recruitment of TF pathway inhibitor a (TFPIa) to the TF-fVIIa-fXa complex via interaction of pS with the TFPIa K3 domain or of the fV B domain with the carboxyterminal fragment of TFPIa (reviewed in Wood et al 51 ). It also remains to be clarified whether other naturally occurring fV variants such as fV Liverpool (I359T), 52 fV Nara (W1920R), 53 fV released from platelets, 54 or fV present in homozygous carriers of the fV R2 haplotype 11,[55][56][57] would exhibit similarly defective cofactor function for the inhibition of TF signaling by aPC as does fV Leiden.…”

Mechanism of Factor Va Inactivation by Plasmin

“…Since these are encompassed by the 50-kDa fragment and span the 40-kDa fragment, both of which dissociated from aPL due to chelation, an explanation is difficult to provide without further study. One possibility is related to the two forms of FVa that have been identified, designated FVa 1 and FVa 2 , that differ in the COOH terminus of FVaL (51,52). The difference has been suggested to be due to alternative glycosylation at Asp-2181 within the C2 domain (49,53,54) leading to a weaker association between FVa 1 and aPL (51,52,55).…”

“…One possibility is related to the two forms of FVa that have been identified, designated FVa 1 and FVa 2 , that differ in the COOH terminus of FVaL (51,52). The difference has been suggested to be due to alternative glycosylation at Asp-2181 within the C2 domain (49,53,54) leading to a weaker association between FVa 1 and aPL (51,52,55). Since we found that the 48-kDa fragment associated strongly with aPL, it is possible that the difference in the electrophoretic mobility between the 50-and 48-kDa species is due to derivation from FVa 1 and FVa 2 , respectively.…”